GERNSBACK SPECIALTY SERIES...Great for special effects and remote fire starting. CAUTION REQUIRED!...

GERNSBACK SPECIALTY SERIES r:,=3 49604

$3.95 U.S.$4.50 CANADA

Moles 111 Headphones

Receiver

el Scrambler

CA AdapterAf

Test Bench Amp

DVM for Your PC

Portable Barometer

Relative Humidity Gauge

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2 YEAR WARRANTY

STANDARD SERIESS-1325 25MHz $349S-1340 40MHz $495S-1365 60MHz $849

Features:

M High Luminance 6" CRT 1mV Sensitivity XY OperationIM Voltage, Time, + Frequency differences displayedon CRT thru the use of cursors (S-1365 only) Plus much, much more

TV SyncII 2 - x1, x10 Probes Complete Schematic

D19 dal Capacitance Meter

11111

$58.959 Ranges

1p420.000uld5% basic acct'.

Big pray Zero control w/ Case

CM -155011

by Blanco

12A DC Power SupplyB+K 1686

$169.953 14v @ 12A

Fully regulated & protectedSeparate volt & current meters

with current limiting . low ripple

ELENCO OSCILLOSCOPES

DELUXE SERIESS-1330 25MHz $449S-1345 40MHz $575S-1360 60MHz $775

Features:

Delayed Sweep M Dual time base Automatic Beam Finder II Illuminated internal Z Axis Modulation gradicule

Built-in Component Test Plus all the features of the 'affordable" series

L.*

Big 1' Display

Digital LCR MeterLC -1801

$125Measures

Coils 1uH-200HCaps Ip1.200u1

Res 01-20M

by Elencoa ® .T.!2

DigitalMultimeter

DVM-638

$39.95II Functions with

Case

Quad Power Supply XP -580by Elenco

$79.95 2-212VV0 @ @.5AA

3

@ IA2A

-55VV @@.5A

Fully regulated and short circuit protected

Sweep/Function Generatorwith Freq. Counter

$239Elenco

GF-8026Int/Ext operation

Sine, Square, Triangle. Pulse, Ramp.2 to 2MHz. Freq Counter .1-10MHz

Learn to Build and ProgramComputers with this Kit

Includes All Pails, Assembly and Lesson Manual

Model

MM -8000

$129.00by Elenco

Sarong from scratch you build a complete system. CurMao -Master trainer teaches you to wine into RAMs.ROMs and run a 0065 microprooessor, which usessimilar machine language as IBM PC.

Hitachi Compact Series ScopesV-212 20MHz Dual Trace $399V-525 - 50MHz, Cursors $995V-523 - 50MHz, Delayed Sweep $949V-522 50MHz, DC Offset $895V-422 - 40MHz, DC Offset $795V-222 - 20MHz, DC Offset $649V-660 - 80MHz, Dual Trace $1,149V -665A 60MHz,DT, w/cursor $1,325V-1060 - 100MHz, Dual Trace $1,395V -1065A - 100MHz. DT, w/cursor $1,649V-1085 - 100MHz, OT, w/cursor $1,995V -1100A 100MHz, Quad Trace $2,495V-1150 - 150MHz, Quad Trace $2,595

B&K OSCILLOSCOPES2120 - 20MHz Dual Trace $3892125 20MHz Delayed Sweep $5391541B - 40MHz Dual Trace $6952160 - 60MHz Dual Trace, Delayed Sweep,

Dual Time Base $9492190 - 100MHz Three Trace Dual Time Base,

Delayed Sweep $1,3952522A - 20MHz / 20MS/s Storage $875

FLUKE MULTIMETEilSScopemeters (All Models Available Call)Model 93 $1,225.00 70 SeriesModel 95 $1,549.00Model 97 $1.795.0010 SeriesModel 10 $62.95Model 12 $79.95

Mode! 7011Model 7711Model 791180 SeriesModel 87

$65.00$149.00$169.00

$289.00

goo 1 B&K 390

$1393-3/4 Digit DMM

Bargraph9 Functions

IncludingTemp. Freq

Rubber Boot

Audio GeneratorElenco GF-800

$5920Hz 15001z

Sine SquareWaves

Handheld

Elenco Wide BandSignal Generators

SG -9000 $119RF Freq 100K-50MHz AM Modula-

tion of 1KHz Variable RF output

SG -9500 WI Digital Display &

150MHz built -In counter $239

Dual -Display LCR MeterStat Functions

B+K 878

$239.95Auto Manual Range

Many Featuresw. 0 Factor

High Accuracy

2MHz Function Generator

B+K 3011B $219.95LED Display, Sine, Square, Triangle, Ramp& Pulse Waves. TTL & CMOS

TELEPHONE PRODUCT TESTER

B+K 1045A $499.95Provides basic operation tests for corded& cordless telephones, answeringmachines and automatic dialers.

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Digital Multimeter Kitwith Training Course

ElencoM -2665K

$49.95Fun & Easy

to Build

Full Function 34 Ranges, IncludesCapacitance,Transistor Diode Testing20Amp AC/DC, Extra Large Display, IdealSchool Protect M-2661 (assembled) $55

Multi -Function CounterElenco F-1200

2GHz

$229

Measures Frequency, Period, Totalize8 LED digits, Crystal oven oscillator,

.5ppm accuracy

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AMAZING Electronic andScientific Products

MysteryLevitatingDevice!Remember War of fife World? Objects float in air and move to the

touch. Defies gravity, amazing gift. conversation piece, magic trick or

great science project.

ANTIK Easy to Assemble Kit !Plans $19.50

Laser Ray Gun

Advanced project produces a burst of light energy capable of burning

holes in most materials. Hand-held device uses rechargeable batteries.

500 joules of flash energy excite either a neodynium glass, yag or other

suitable 3' laser rod. This is a dangerous CLASS IV project (individual

parts/assemblies available).

LAGUNI Plans $20.00

LAGUN1K Kit / Plans Price on Request

Extended PlayTelephoneRecording SystemREADY TO USEI Autornatcally controls and records on our X-4

extended play recorder, taping both sides of a telephone conversation.

Intended for order entry verification. Check your local laws as some

states may require an alerting beeper.

TAP2OX Ready to Use System $12930

Shocker Force Field/A Vehicle Electrifies

Neat lithe device allows you to make hand and shock balls, shod

wands and electrify objects, charge capadtas. Great payback for

those wise gm who have wronged you!

SHKIKM Easy to Assemble Electronic Kit $24.50

Blaster PulserPocket -sized wand produces 100,000 watts of power for personal

defense, field and lab use, etc. BLS3 Plans $10.00

BLS3K Kit / Plans $69.50

Homing / Tracking TransmitterBeeper device, 3 mile range.

HOD1 Plans _...........$10.00 11001K Kit/ Plans $49.50

Listen Through Walls, FloorsHighly sensitive stethoscope mkt

STETHI Plans_......$8.00 STETH1K Kit/Plans $44.50

UNLIMITED

NEW!

INFINITY TRANSMITTER++Room Monitor I Phone Line GrabberALL NEW! The Ultimate In Home or Office Security & Safety!Simple to Use! Call your home or office phone, push a secret tone on

your telephone keypad to access either: A. On premises sounds and

voices; or B. Existing telephone conversation with break in capability for

emergency messages. CAUTION: Before assembly or use, check

legalities with your state Attorney Generars office as you may require

'beepers" or other 3rd party alerts.

TELEGRABI Plans Only . $10.00

TELEGRAB1K KIt I Plans $99.50

Ultrasonic Blaster ciLaboratory source of acoustical shock

waves. Blow holes in metal, produce

'odd' steam, atomize liquids. Many

cleaning uses for PC boards, jewelry, -4coins, small parts, etc (ULB1 Plans $10.00 ULBIK Klt/Plans.....$61.50

100,000V Intimidator /Shock Wand ModuleBuild an electrical device that is affective up to 20 feet. Maybe

endosed for handheld, portable field or laboratory applications.

ITM2KM Easy -to -Assemble Electronics Kit $4930ITM2 Plans only, credit -able to kit $10.00

Ion Ray GunProjects charged ions that induce shocks in people and objects without

any connection! Great science project as well as a high tech party

prank. 10G3 Plans 11.00

10G3K Kit/Plans $6930

Invisible PainField GeneratorShirt pocket size electronic

device produces time variant

complex shod waves of intense directional acoustic energy, capable of

warding off aggressive animals, etc.

IPG7 Plans ........... IPG7K Kit/Plans _349.50IPG70 Assembled $74.50

1000 Ft++Potato CannonNOT A TOY. Uses electronic or piezoignition. CAUTION REQUIRED!POT1 Plans

(Dangerous Product) $10.00

FireBall GunShoots flaming ball - two shot capacityGreat for special effects and remotefire starting. CAUTION REQUIRED!FIREBALL Plans (Dangerous Product) $10.00 Rh. flk.

TV lit FM Joker / JammerShirt pocket device allows you to totally control and remotely disrupt N

or radio reception. Great gag to play on family a friends. Discretion

required. EJKIKM Easy to Assemble Electronic Kit $24.50

Visible Beam LaserHigh brightness red HeNe laser visible for miles.

Produce your own light show! Projects a visible beam of red life dearly

visible in most circumstances. Can be used to intimidate by projection

of a red dot on target subject. Also may be used to listen in' using our

laser window bounce method h1LLIS1 below. Easy to &id module

makes A working visible laser!

LAST KM Kit w/1mw Laser Tube, Class I. .$69.50

LAS3KM Kit w/2.5mw Laser Tube, Class IIIA $99.50

"Laser Bounce" Listener SystemAllows you to hear sounds from an area via a lite beam reflected from a

window a other similar objects. System uses our ready -to -use LATR1

Laser Terminator gun site as the transmitter. The receiver section is

suppled as an easy -to -build kit, including our cushioned HS10

headsets. WST2 Plans $20.00

LLISTIK Kit of Both Transmitter and Receiver $19930

WST20 Assemble with Laser Gun Site $29930

3mw Visible Red Pocket LaserUtilizes our touch power control!

VRUKM Kit I Plans ....... _ .... _ .... ......._....$74.50

ElectronicHypnotism P

Puts subjects under control using highly effective electronic stimuli.

Intended for parties and entertainment but must be used with olutign,

Includes valuable text book reference and plans.EH2 Plans and Text Book $19.50

Pocket Sized Night ViewerUses Low Level Starlightto See in the Dark! Low Cost

litra-Hi Lite Amplification!

Auto Brightness Control

Limited Amount Available

Made in USA Nght surveillance Animal studies, etc.

Can be used to fly an airplane or drive a car!PKV7 Plans $15.00PKV7K Easy to Assemble Kit. Price on RequestPKV70 Ready to Use Price on Request

3 Mi FMWireless MicrophoneSubminiature! Crystal dear, ultra sensitive pickup transmits voices and

sounds to FM radio. Excellent for security, monitoring of children or

invalids. Become the neighborhood disk jodey, or go 'under cover'

using our sunglasses FM radio (see catalog). FMV1 Plans ... $7.00

FMV1K Kit and Plans $39.50SUGL10 Sunglasses with built In FM Radio $29.50

Telephone Transmitter - 3 MiAutomatically transmits both sides of a telephone conversation to an FM

radio. Tunable Frequency Undetectable on Phone Easy to Build

and Use Up to 3 tAle Range Only transmits during phone use.

VWPM7 Plans $7.00

VWPM7K KIVPians $39.50

Dept PEM16, Box 716, Amherst, NH 03031

Phone: 603-6734730 FAX 603-672-5406MC, VISA, COD, Cheeks ac opted Rims* add $5.00 Shill:Ong S Handling

CATALOGWith many

more items!Free with Orderor send $1 PAH

Order by Mailor by 24 Hour

Orders -Only Phone

300-221-1705 1

2

9 -Band Shortwave ReceiverSee page 19

Add a DVM to Your PCSee page 46

1"""'""71444 watts&441

4 PCliTABLE BAIIOMIE7ER

Portable BarometerSee page 51

Design Your Own LoudspeakersSee page 79

Telephone ScramblerSee page 75

Popular FALLElectronics®

1994Contents

HIGH -VOLTAGE ELECTRONICS

The Wimshurst Machine 57The ultimate static -spark machine and you can build it

Voltage Doublers 61An easy and inexpensive way to experiment with highvoltages

Make Your Own High -Voltage Capacitors 65Take on a big charge

MEASURING THE WEATHER

Portable Barometer 51

Relative Humidity Gauge 84

Build an Anemometer 98Keep up with the wind with this inexpensive device

FABULOUS PROJECTS

Sense small changes in barometric pressure

Keep an eye on the humidity with this build-iy gadget

Nine -Band Shortwave Receiver 19Tune in the world with this classic regenerative receiver

Microwave Motion Detector 27A security system for your home or car that works on thesame principles as police radar

Computer -Controlled A/B Switch 32Gadget makes it easy to switch any pair of AC or DC signals,including RS -232 signals

Test Bench Amplifier 38Add this handy little amplifier to your test -station arsenal

Call Blocker 40Use this circuit and the 'phone company's "Caller ID" serviceto spare yourself from unwanted calls

As a service to readers, Popular Electronics Fall 1994 Electronics Hobbyists Handbook publishes available plans orinformation relating to newsworthy products, techniques and scientific and technological developments. Because ofpossible variances in the quality and condition of materials and workmanship used oy readers, we disclaim anyresponsibility for the safe and proper functioning of reader -built projects based upon or from plans or information publishedin this magazine.

ELECTRONICSHOBBYISTShandbookAdd a DVM to Your PC 46

Turns your PC into a digital voltmeter for automated testing, torecord readings over time, and more

How to Split a Power Supply 49A nifty way to get a dual supply from a single -ended source

Electrical Analyzer for Your Car 72Avoid inexpensive road -service calls with a monitor that alertsyou to any of several problems

Build a Telephone Scrambler 75Foil the snoopers and keep your conversations private

Design Your Own Loudspeakers 79Three simple programs that will help you design your ownsystem right away

Remote Control Relay Station 89Your IR signals will turn corners or overcome extra -long andwell lit rooms

SCA Adapter 91

Hear the hidden broadcasts on the FM band

Wireless IR Headphones 94Cut the tether between you and your hi-fi

DEPARTMENTS

Editorial-Discover our secret 4

New Products-All that's fit to buy 6

Cartoon Page-A couple of laughs 10

Electronics Library-Tomes for technicians, etc. 12

Free Information Card 32A

Classified Advertising 111

Advertiser's Index 111

Advertising Sales Offices 112

Since some of the equipment and circuitry described in Popular Electronics Fall 1994 Electronics HobbyistsHandbook may relate to or be covered by U.S. patents, we disclaim any liability for the infringement of such patents by themaking, using, or selling of any such equipment or circuitry, and suggests that anyone interested in such projects consult apatent attorney.Popular Electronics Fall 1994 Electronics Hobbyists Handbook is published semi-annually by Gernsback Publica-tions Inc_ All rights reserved. Printed in U.S.A. Single copy price $3.95. Canadian G.S.T.Registration No. 125166280.Canada $4.50 Gernsback Publications Inc. 1994

Microwave Motion DetectorSee page 27

ELECTRICAJ. SYSTEM ANALYZER

Ilk-BATTERY OK

WEAK BATTERY

tip- NOT CHARGING

IV INSUFFICIENTCHARGING

IYF OVERCHARGING

Electrical Analyzer CarSee page 72

Computer -Controlled A/BSwitch

See page 32

Test Bench AmplifierSee page 38 3

4

ELECTRONICSHOBBYISTShandbook

Larry Steckler1:111.. (1..1

-rrd,l 0,41 P111,11,11,

Julian S. Martin

POPULAR ELECTRONICSEDITORIAL DEPARTMENT

Carl Laron

Robert A. Young.1,01

John J. Yacono

Teri Scaduto1,1111,

Evelyn Rose

Marc SpiwakA1kl

PRODUCTION DEPARTMENTRuby M. Yee

p,M1111 1,111 1)11,1

Karen S. BrownPr odii4 Item Memel gc,

Kathy CampbellI on ,Issivitril

ART DEPARTMENT

Andre Duzant1,? Om, tot

Russell C. Truelson

Jacqueline P. CheeseboroUtr udel 11,4 for

Michele Torrillolir

BUSINESS AND EDITORIAL OFFICES

Gernsback Publications. Inc.500-B Bi-County Blvd.

Farmingdale, NY 117351-516-293-3000

Fax: 1-516-293-3115

President: Larry Steckler

Advertising Sales Offices listed on page 112

Cover byLoewy Design

Oak Ridge, TN

Illustration byJohn Edwards

Composition byMates Graphics

Since some of the equipment and circuitry described in1994 ELECTRONICS HOBBYIST HANDBOOK may relateto or be covered by U.S. patents, 1994 ELECTRONICSHOBBYIST HANDBOOK disclaims any liability for the in-fringement of such patents by the making, using, or selling ofany such equipment or circuitry, and suggests that anyoneinterested in such projects consult a patent attorney

Editorial

We are getting to be a habit. When Electronics Hobbyists Handbookwas first published a few years ago, our magazine appeared once ayear on the newsstand. Now we publish twice a year, Spring and Fall.Considering the fact that as many as 500 new magazines are publishedeach year, you can imagine that the fatality rate of new magazines isvery high. We made it this far because our readers accept the magazinefor its contents.

The Electronics Hobbyists Handbook is not supported by any societyor association, whether they be business, trade or consumer groups.Nor do we look to sell subscriptions to underwrite the magazine. Sothen, what is the secret of our success?

You are our big secret. You, the builders of countless projects, whoare constantly looking for new and better electronic gadgets to build tosatisfy your recreation desires. You are hobbyists to the core, and wefound you and give you the do-it-yourself projects that make your sparetime activities enjoyable. When we fail you, you'll leave us. That is notgoing to happen because the editorial staff will continue to present toyou projects that you want to build. We get feedback from suppliers thattell us which projects are hot! You tell us directly when you write to us,and we find out what you like by building projects ourselves and beingjust like you.

The editorial staff sends its personal thanks to the many readers whosupport this magazine and we look forward to as many years of serviceto our readers as we have fun doing it! Thank you for being out there.

Julian S. Martin,Editor

Don't Despair...REPAIR.Here's how to troubleshoot andrepair your electronics successfully!

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Mail to WEKA Publishing, 97 Indian Field Rd.,Greenwich, CT 06830 400082

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NEW PRODUCTSVideo

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6

The broadcast -qualityTitleMaker 2000 video titler, anupgraded version of Videonics'popular TitleMaker, offers a hostof new features. IntInded tobring an added level of con-venience and flexibility to homemoviemakers who want to give

their productions professional -looking titles and specialeffects, the TitleMaker 2000features "preview output," anadditional video output that al-lows a second monitor to beused to create new pages ormodify the titles. Other newfeatures include more than 90font and size combinations, in-cluding script fonts; rapid pageaccess without scrolling; ad-vanced scrolling, which allows atitle to scroll in from one direc-tion and out from another; and aset of independent pages thatcan be saved for different jobsor users. Like the original unit,the TitleMaker 2000 offersbackgrounds, letters, outlines,and borders that can be createdwith over a million colors or awide range of patterns, includ-ing animated patterns. Thekeyboard has accented charac-ters for more than 16 languagesand special characters.

The TitleMaker 2000 has asuggested retail price of $599.for further information, contactVideonics, 1370 Dell Avenue,

Campbell, CA 95008-6604; Tel.708-866-8300; Fax:408-866-4859.

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SOLDERING IRON/TORCH KITPhilips ECG J-500 solderingiron,torch is now available in a10 -piece kit complete with carry-ing case, four different tips, and

other accessories. The com-pact, lightweight2handheld toolfeatures a see-through refillablebutane tank, a temperature con-trol for accurate adjustment oftip temperature, and a built-inigniter. The J-500KT kit includesthe J-500, a solder tip, a blow-torch tip, a hot -knife tip, a heat -blower tip, a metal safety stand,a cleaning pad, spare flints, anda half -ounce of 60/40 rosin -coresolder, all packaged in a heavy-duty, fitted carrying case. Addi-tional tips are availableoptionally. Applications includeelectrical 'electronic circuit re-pair, light -gauge welding,jewelry and eyeglass -frame re-pair, model building, crafts, andthawing frozen locks.

The J-500KT soldering iron/torch kit has a suggested retailprice of $35.50 and is availablefrom participating Philips ECGdistributors in the United Statesand Canada. For more informa-tion, contact Philips ECG, 38State St., Seneca Falls, NY13148-0730; Tel. 315-568-5875.


COAXIAL -CABLECHECKERDesigned for testing coaxial ca-bles in common use for networkand audio/video applications,Paladin Tool's Coax -Check 1560provides a simple pass -or -fail

analysis for RG58, 59, and 62coaxial BNC-type cables. Ittests the continuity of both theconductor and the braidedshield, and identifies shorts re-sulting from contact betweenthe conductor and the shield. Agreen LED indicates a "pass"diagnosis. One of a group of redLED's lights to indicate the rea-son for failure. The testercomes in a high -impact case,complete with a 9 -volt batteryinstalled.

The Coax -Check 1560 has asuggested retail price of $35.67.For additional information, con-tact Paladin Tools, 3543 OldConejo Road, Suite 101, New-bury Park, CA 91320; Tel.800-272-8665: Fax:800-272-5257.


WEATHERWIZARD IIYou can monitor indoor andoutdoor weather at the touch ofa button with Davis Instruments'Weather Wizard II. The sophisti-cated, professional -qualityweather station measures insidetemperature from 32'F to 140 F,outside temperature from

50"F to 140''F, records highand low temperatures, monitorswind direction and wind speed,records high wind speed, calcu-lates wind chill to -- 134', andrecords low wind chill. All highsand lows are recorded with timeand date. Alarms can be set to

Enter A World Of Excitement with a Subscription to

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Now you can subscribe to the magazine that plugs you into theexciting world of electronics. With every issue of Popular Elec-tronics you'll find a wide variety of electronics projects you canbuild and enjoy.

Popular Electronics brings you informative new product and lit-erature listings, feature articles on test equipment and tools-alldesigned to keep you tuned in to the latest developments in elec-tronics. So if you love to build fascinating electronics, just fill outthe subscription form below to subscribe to Popular Electronics....It's a power -house of fun for the electronics enthusiast.

EXCITING MONTHLY FEATURES LIKE:

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MORE NEW PRODUCTSsound when temperature, windspeed, or wind chill fall above orbelow preset levels.

The Weather Wizard ll's func-tions can be easily expandedwith optional add-ons. The RainCollector allows you to measuredaily and accumulated rainfall.Weatherlink allows you to linkthe Weather Wizard II to anIBM-compatible PC or Macin-tosh to process, analyze, andstore weather data.

The Weather Wizard II, com-plete with an anemometer with

40 feet of cable, an externaltemperature sensor with 25 feetof cable, a junction box witheight feet of cable, an AC -poweradapter, and detailed instruc-tions, costs $195. For moreinformation, contact Davis In-struments, 3465 Diablo Avenue,Hayward, CA 94545; Tel.800-678-3669 or 510-732-9229;Fax: 510-732-9188.


900 -MHz TELEPHONEAccording to BEL-Tronics, itsMicro 900 cordless phone sys-tem features the industry'slightest handset in a multi -hand-set 900 -MHz model. Operatingon the 900 -MHz frequency, thesystem provides exceptionalclarity and up to three times therange of traditional cordlessphones. It is also virtually free ofinterference from electricmotors, fluorescent lights, andother cordless phones. The 7.4 -ounce handset is paired witheither the stand-up model 900M

or the lie -down model 900LXbase. A complete Micro 900system consists of three basicelements: a base -and -handsetunit, up to three 900MR remoteextension-and-recharger units,and a 900BP battery pack.Each base -and -handset unithas the capacity for up to threeremote extension handsets, andcan work independently of theextensions; the extensions re-quire a base unit to operate.The extension rechargers canbe plugged into any wall outletin the home with no need foradditional telephone jacks.

The 900M base/handsetcosts $499,95; the 900LX base/handset costs $529.95; each900MR handset/recharger costs$349.95; and the 900BP batterypack costs $27.95. For furtherinformation, contact BEL-TronicsLimited, 8100 Sagi Parkway,Covington, GA 30209; Tel.800-341-1401.


FRONT -AND -REARLASER DETECTORBecause police laser -monitoringdevices can clock both ap-proaching and recedingvehicles, BEL-Tronics' SuperWideband with Laser Plus se-ries of integrated radar/laserdetectors feature rear laser de-tection. No added attachmentsare required for rear detection.Besides Dual Tracking Laser(DTL) system, the new detec-tors also feature a wide field ofview for off -axis laser detectionand advanced super-widebandKa radar detection to pick upsignals from 33.4 to 36 GHz.

Each model in the series fea-

tures dual -patentedFundamental Mixer Technology(FMT), which uses a fundamen-tal -mixer response to detecteach radar frequency and pro-vide maximum sensitivity to allradar bands. Each also featuresadvanced signal -processingtechniques that automaticallyeliminate common sources offalsing for both radar and laser,a combination power/volumecontrol, instant -on audio and vi-sual alerts, single -setting CTYmode, and memory retention forall mode selections. The top -of -the -line 645STi-P, picturedhere, also offers "Shadow Tech-nology" for complete immunityto the Interceptor VG -2 or anyother radar -detector detector.

Prices for the Super Wide -

band with Laser Plus series ofradar/laser detectors range from$279.95 to $499.95. For moreinformation, contact BEL-TronicsLimited, 8100 Sagi Parkway,Covington, GA 30209; Tel:404-787-6500 or 800-828-8804;Fax: 404-784-9898.


HOME -STUDYELECTRONICSENGINEERINGTECHNOLOGY DEGREEA leading electronics home -study school, the Cleveland In-stitute of Electronics (CIE), hasorganized World College, whichoffers a Bachelor's Degree inElectronics Engineering Tech-nology (BEET). Theindependent -study programconcentrates on electronicstechnology and also includescourses on computer engineer-ing, mathematics, basicscience, communications, so-

cial science, and humanities.World College, which is locatedin Virginia Beach, has beenauthorized to enroll students inthe BEET program by the Vir-ginia Council of HigherEducation, and is currentlyawaiting the Council's approvalto grant the BEET degree.World College is also accreditedby the Accrediting Commissionof the National Home StudyCouncil.

For additional information,contact World College, 1776East 17th Street, Cleveland, OH44114; Tel: 800-243-6446.


32 -SWITCH REED -RELAY CARDA versatile PC -based switchingsolution from AccuSys, the 32 -Switch Reed Relay Card, allowsPC's to control real -world ap-plications by smoothly switchingany signal. Offering simple -to -use and completely configura-ble low power management ofanalog or digital signals, thecard can be used in such di-verse applications as flightsimulators, home automation, ir-rigation systems, modemswitching, and even model -trainoperation. An expanded optionsconnector allows more switchconfigurations than other switchcards, including conditional andmulti -conditional switching com-binations. New circuitry allowsstatus checks of any relay witha simple software inquiry. Anoptional Windows application,

"Win -Switch," offers simple con-trol and relay scheduling with aneasy graphic interface. A broadselection of addressing optionsallow the user to populate anyISA-slot compatible system withas many cards as there areslots available. That allows asingle PC to control hundreds ofsignals. If an application re-quires a lot of specially wiredcards, mini-daughterboards areavailable optionally for easy rep-lication of special relay

interconnections. An optional B-side connector, which providesdiscrete access to both sides ofall relays, is also available.

The 32 -Switch Card, includ-ing software primitives inAssembler, BASIC, C, Fortran,Cobol, FoxPro, and dBase,costs $395. The B -Side con-nector costs $29, and "Win -Switch" costs $89.95. For moreinformation, contact AccuSys,Inc., Information Group, 3695Kings Row, Reno, NV 89503;Tel: 702-746-1111.


INSTANT HOME -THEATER SOUNDPresenting a unified approachto home entertainment, Sony'sSA-VA3 is a complete Dolby Pro

Logic Surround Sound audiosystem that can be set up in tenminutes. The SA-VA3 includesfront and rear speakers, built-inamplifier, and a Dolby surround -sound processor. The systemfeatures four amplified ANspeakers-two specifically de-signed for the front and twooptimized for the rear surround -sound channel: an integratedcenter channel, built-in DolbyPro Logic, five surround -soundmodes; and a super woofer. Thecomplete system adds dramaticimpact to any audio track formovies, music videos, or videogames.

The SA-VA3 home -entertain-ment sound system has asuggested retail price of $950.For additional information, con-tact Sony, 1 Sony Drive, ParkRidge, NJ 07656.


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CORDLESS PHONE/ANSWERING SYSTEMSanyo's CAS -170 cordless tele-phone/digital TAD providesnoise -free, tape -free con-venience. With all -digitalrecording, an integrated chipstores up to 18 minutes worth of

ncoming messages, and two60 -second outgoing messages.The record -memo function al-lows the user to record amessage for a family memberor friend without calling :n. Atwo -digit LED call counter letsthe user know exactly howmany messages have been re-ceived. The answering machinealso features a time day stamp,two-way recording from thebase or the handset, extension -phone disconnect, rapid mes-sage erasing, announce -onlymode, and 100 user -selectedsecurity codes. The phone fea-tures Compander IV noisereduction, which compressesthe audio signal at the base andamplifies it at the headset,

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CONVERTIBLEHEADPHONESJasco8 Model 455 super -bassconvertible headphones featurea unique design that allows thebasic headphones to be trans-formed to over -ear design withthe easy addition of convertibleadaptors. The Model 455 alsofeatures an adjustable paddedheadband for a comfortable fit.Other features include a singleside cord with a gold-plated "fl-

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MONITOR ANTI -GLAREFILTERSThe Secure -View anti -glare filterfrom Kantek'Spectrum com-bines 99% glare reduction withan innovative design that re-stricts the peripheral view ofmaterial on the screen. Thatprovides a practical means ofcontrolling access to sensitiveinformation in a crowded officeenvironment. The filter is madeof high -quality, optically pureglass that contains multiple thin-

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ELECTRONICS LIBRARY

A BeginnersGuide to TTL

Digital ICsby R. A. Penfold

12

Logic circuits are now part ofeveryday life, and TTL logic IC'sare widely regarded as standarddigital devices, used in a widevariety of applications. Becausemany fundamental concepts ofdigital design seem abstractand far removed from practicalapplications, getting started withlogic circuits can be difficult.This book covers the basic the-ory of digital electronics and theuse of TTL IC's, but keeps inmind the real -world applica-tions. Along with the basic

A Beginners Guideto

TTL Digital ICs

concepts of logic circuits, thebook covers the functions ofgates, inverters, and other logicbuilding blocks; TTL IC charac-teristics and their use in circuitdesign; oscillators and mono -stables; flipflops, binarydividers, and binary counters;and decoders, data latches, andthree -state buffers. The empha-sis is on the practical side of thesubject matter, and all the cir-cuits presented are based onreal TTL IC's.

A Beginners Guide to TTLDigital ICs (order numberBP332) is available for $6.50plus $2.50 shipping and han-dling from ElectronicsTechnology Today Inc., P.O. Box240, Massapequa Park, NY11762-0240.


BECOMING ANELECTRONICSTECHNICIAN:Securing Your High -TechFutureby Ronald A. Reis

Aimed at would-be electronicstechnicians, this book offers acomplete picture of what thecareer is like, what preparationand training is required, andhow to secure that first elec-tronics -technician position. In aconversational style, the bookthoroughly discusses what itmeans to be an electronicstechnician in today's high-techworld. It explores the history

BECOMING ANELECTRONICSTECHNICIAN

and background of the field, anddescribes the general occupa-tions and people who make upthe industry. The specifics of theelectronics technician's job areclearly outlined. The book in-cludes discussions of what ittakes to make it through therequired course work to obtain acertificate, an associates de-gree, or a four-year degree.Particular attention is paid to thenon -technical, non -electronicskills necessary to completingthe course work-reading, lis-tening, calculation, and writingskills. The book also suggests awealth of activities and intereststhat students can explore ontheir own time to supplementand expand their knowledge ofelectronics.

Becoming an ElectronicsTechnician: Securing Your High -Tech Future costs $10 and ispublished by Macmillan Publish-

ing Company, 445 HutchinsonAvenue, Columbus, OH 43235.


UPGRADE YOURCOMPUTER PRINTERAND SAVE A BUNDLEby Horace W. LaBadie, Jr

This book provides computerusers with the practical guid-ance they need to give their oldprinters the power and flexibility

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of today's best commercial mod-els at the lowest possible price.It describes how to take advan-tage of inexpensive hardwareand software upgrade optionsfor all types of computerprinters, including dot matrix,ink jet, daisy wheel, and thermalfrom all major manufacturers.The step-by-step, illustrated in-structions show readers how toget near -letter quality from adraft printer by adding RAMchips to expand the input buffer;ROM chips for font or emulationchanges; printer spoolers/buff-ers; printer -sharing andnetworking boxes; interfaceconverters; HP PCL, AdobePostScript, and PostScript emu-lators; and various softwareenhancements.

Upgrade Your ComputerPrinter and Save a Bundle costs$19.95 and is published byWindcrest/McGraw-Hill, BlueRidge Summit, PA 17294-0850;Tel: 800-233-1128; Fax:717-794-2103.


WORDPERFECT 6EXPLAINEDby P.R.M. Oliver and N.Kantaris

Intended to help PC users get ahandle on WordPerfect asquickly as possible, this bookprovides practical, hands-onroutines for first-time word -pro-cessor users as well as thosewho are switching from anotherprogram. To that end, eachchapter is a self-contained tu-torial unit that builds upon theinformation presented in pre-vious chapters. Readers canfollow the book from beginningto end, or select only thosechapters that they need. Anemphasis is placed on themost -often used features of theprogram.

The book explains the hard-ware requirements and theinstallation process before famil-iarizing the reader with theWordPerfect environment and

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Does your VCR have a "Head Cold?"Probably not! However, through constant playing and using of degrading dryor wet cleaners, the output of your video tapes has slowly diminished to anunacceptable level and the VCR plays as if it has a head cold! The culprit ismost likely clogged and dirty video and/or audio heads.The 3M Black Watchn" Head Cleaner Videocassette uses a patented mag-netic tape -based cleaning formation to remove head clogging debris. No for-eign substances such as cloth, plastics or messy liquids and no harsh abra-sive materials are present. The cleaner's usable life is 400 cleanings or more!

It's easy to use. Place the 3M Black Watch"A Head Cleaner Videocassettein the VCR and press the Play button. A pre-recorded message appears clear-ly on your screen and an audible tone is heard, telling you that the cleaningprocess is now completed. No guess work, you never over clean!3M Black WatchTM Head Cleaner Videocassette SHIS VHj $19.95

Once your VCR's head cold is cured, and the unit plays like new, considerusing the finest videocassette you can buy-the 3M Black WatchTM T120 HiPro VHS 4410 Videocassette. The 4410 is the highest performing video-cassette available today for use with all standard format VHS recording hard-ware!

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ELECTRONICS LIBRARYCONTINUED

Editor-that are included withthe WordPerfect 6 package.

WordPerfect 6 Explained(order number BP351) is avail-able for $6.95 plus $2.50shipping and handling fromElectronics Technology TodayInc., P.O. Box 240, MassapequaPark, NY 11762-0240.


ELECTRICALCONTRACTOR:Start and Run a Money -Making Businessby Dan Ramsey

This book provides apprenticeand journeyman electricianswith the knowledge they need tomake it on their own without abusiness degree. It offers soundadvice on how to start or ex-pand a self -owned electrical -contracting company. The book

ELECTRICALCONTRACTORStart and Run aMoney -MakingBusiness

Dan Ramsey

begins by describing the latestopportunities for residential,commercial, and industrial elec-tricians, including the newesttechnologies that make elec-trical contracting moreproductive and profitable than inthe past. The book goes on toaddress all the issues thatwould-be contractors are likelyto face. Those include choosinga location and obtaining a li-cense; obtaining financing;reducing overhead; estimatingjobs and setting prices; hiringand managing employees; leas-ing versus buying equipment;finding and keeping customers;minimizing risk; marketing andadvertising; and communicating

with clients, employees, part-ners, and government officials.The book includes practicalworksheets that make it easy toestimate jobs and costs, keeptrack of income and expenses,and bid on jobs.

Electrical Contractor: Startand Run a Money -Making Busi-ness costs $17.95 and ispublished by Tab Books Inc.,Blue Ridge Summit, PA17294-0850; Tel:1-800-233-1128.


LOOKING GOOD INPRINT:A Guide to Basic Designfor Desktop Publishingby Roger C. Parker

There's more to successfuldesktop publishing than learn-ing to use the software. Gooddesign is a basic element of anyeffective printed material, be it anewsletter, an ad, a manual, ora letter. Aimed at any computer -literate but "graphically chal-lenged" desktop publisher-regardless of the type of hard-ware and software being used-this book teaches the elementsof good design. As it guides thereader through the maze ofboxes, grids, screens, fonts, clipart, and other graphic elements,it offers guidelines, advice, anda wealth of ideas for creatingprofessional materials.

The third edition containsnew information on adding im-pact and accent with color in anew chapter with 24 pages offour-color illustrations. It alsoexplains how to choose, place,

LOOKING(;001) IN

PRINT

crop, and retouch photos; workwith service bureaus; and vastresources of ready-made artthat are available to desktoppublishers for a minimal cost.

Looking Good in Print, ThirdEdition costs $24.95 and ispublished by Ventana Press, P.0. Box 2468, Chapel Hill, NC27515; Tel: 919-942-0220; Fax:919-942-1140.


BASIC ELECTRONICCOMMUNICATIONSFOR THE FCC GENERALRADIOTELEPHONEOPERATOR'S LICENSEEXAMby Victor F C. Veley

This comprehensive studyguide and reference work pro-vides all the informationnecessary to pass the FCC

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General Radiotelephone Oper-ator's License (GROL) exam.More than just a sampling ofexam questions, the book pro-vides detailed answers andexplanations that help aspiringtechnicians fully understand theprinciples of electronic commu-nications. Starting with thebasics of alternating and directcurrent, the book progresseslogically through the operationand use of diodes and powersupplies, transistors and othersemiconductor devices, AM andFM transmitters and receivers,transmission lines and anten-nas, microwaves and radar,numbering systems, and digitalconcepts. The Ship Radar En-

dorsement (Element 8) is alsothoroughly covered.

Basic Electronic Communica-tions for the FCC GeneralRadiotelephone Operator's Li-cense Exam costs $24.95 andis published by Tab Books Inc.,Blue Ridge Summit, PA17294-0850; Tel:1-800-233-1128.


MS-DOS 6 EXPLAINEDby N. Kantaris & P.R.M. Oliver

To use a PC effectively, youmust have a thorough workingknowledge of its operating sys-tem. This book helps readersgain such an understanding ofMS-DOS 6. For those who areupgrading. the differences be-tween versions 5 and 6 areexplained wherever possible.Written for busy, non -expert PCusers, each self-containedchapter builds upon what wastaught in previous chapters.

The book explains the enhan-cements over previous versions,how the operating system isstructured, how to use directo-ries and subdirectories toeffectively structure the harddrive, how to use the DOS Shellgraphical interface, how to man-age disk files, and how to usethe MS-DOS editor to fully con-figure the system. In addition, itcovers how to increase memoryusing MemMaker, how to in-crease speed, how to writebatch files to automate systemoperation, and how to write and

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BP2 4 8-TESTEQUIPMENT CON-STRUCTION$5.95. Details con-struction of simple, in-expensive, but ex-tremely useful testequipment. AF Gen,Test Bench Ampl, Au-dio Millivoltmeter, Tran-sistor Tester and sixmore.

BP311-AN INTRODUCTION TOSCANNERS AND SCANNING ....$7.95. Radio scanners have openeda realm of exciting radio listening.Understand radio wave propagation,types of transmissions, antennas,band assignments-the straightdope on what to hear and where tohear it! Comes complete with index,glossary of important terminology.

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BP298-A CONCISE INTRODUCTION TO THEMACINTOSH SYSTEM AND FINDER... 46.26. If youhave one of the popular Macintosh range of computers,this book is designed to help you get the most from it.Although the Mac's WIMP user interface is designed tobe easy to use, much of it only becomes clear when it isexplained in simple terms. All Macintosh computers arecovered including the new "Classic" range.

MIAOW BP245-DIGITALAUDIO PROJECTS

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BP303-UNDERSTANDINGPC SOFTWARE $8.95. Thisbook will help you understand thebasics of various types of businesssoftware in common use. Types ofsoftware covered include word pro-cessors, spelling checkers, graph-ics programs, desktop publishing,databases, spreadsheets and util-ities.

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BP251-COMPUTERHOBBYISTS HANDBOOK ... $4.95. A wrapup of ev-erything the computer hob-byist needs to know in oneeasy to use volume. Pro-vides a range of useful ref-erence material in a singlesource.

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BP257-INTRO TOAMATEUR RADIO.... $6.95. AmateurRadio is a unique andfascinating hobby. Thisbook gives the new-comer a comprehen-sive and easy to under-stand guide to thesubject.

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CI BP256-INTRO TOLOUDSPEAKERSAND ENCLOSUREDESIGN $5.95. Weexplore the variety ofenclosure and speakerdesigns in use today sothe reader can under-stand the principles in-volved.

BP249-MORE AD-VANCED TEST EQUIP-MENT CONSTRUCTION.... $6.95. Eleven moretest equipment con-struction projects. They in-clude a digital voltmeter,capacitance meter, currenttracer, etc.

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PCP115-ELECTRONIC PROJECTSFOR HOME SECURITY .... $10.00.25projects ranging from a single -door pro-tection circuit that can be completed in anhour or two, to a sophisticated multi -chan-nel security system. Each project is de-scribed in detail with circuit diagrams,explanations of how it works, instructionsfor building and testing, and how to adaptcircuits to meet special requirements.

0 PPM-ADVANCED ELECTRONIC SECURITY PROJECTS...45.95. Includes apassive infra -red detector, a fiber-optic loop alarm, computer -based alarms and anunusual form of ultrasonic intruder detector.

BP235-POWER SELECTOR GUIDE .... $10.00 Complete guide to semicon-ductor power devices. More than 1000 power handling devices are included. Theyare tabulated in alpha -numeric sequence, by technical specs Includes powerdiodes. Thyristors, Triacs, Power Transistors and FET's.

0 BP234-TRANSISTOR SELECTOR GUIDE $10.00. Companion volume toBP235. Book covers more than 1400 JEDEC, JIS, and brand -specific devices. Alsocontains listing by case type, end electronic parameters. Includes Darlingtontransistors, high-vollege devices, high -current devices. high power devices.

0 OMIT-PRACTICAL ELECTRONIC BUILDING BLOCKS-8*okt $5.75.Oscillators, Timers, Noise Generators. Rectifiers, Compwators, Triggers and more.

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BP/79-ELECTRONIC CIRCUITSFOR THE COMPUTER CONTROL OFROBOTS $7.50. Data and circuits forinterfacing the computer to the robot'smotors and sensors.

0 BP239--GETTING THE MOST FROM YOUR MULTIMETER $5.96. Coversbasics of analog and digital meters. Methods of component testing includestransistors, thyristors, resistors, capacitors and other active and passive devices.

BP97-IC PROJECTS FOR BEGINNERS.....55.50. Power supplies, radio andaudio circuits, oscillators, timers, switches, and more. If you can use a soldering ironyou can build these devices.

CI RADIO -100 RADIO HOOKUPS $3.00. Reprint of 1924 booklet presents radiocircuits of the era including regenerative, neutrodyne. reflex 8 more.

BP42-SIMPLE LED CIRCUITIL....$5.50. A large selection of simple applicationfor this simple electronic component.

BP122-AUDIO AMPLIFIER CONSTRUCTION .... $5.75. Construction detailsfor preamps and power amplifiers up through a 100 -watt DC -coupled FET amplifier.

C1 DPW-CRYSTAL SET CONSTRUCTION $5.50. Everything you need to knowabout building crystal radio receivers.

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BP255-INTERNATIONAL RADIOSTATIONS GUIDE .... $7.95. Providesthe casual listened, amateur radio DXerand the professional radio monitor with anessential reference work designed toguide him or her around the more thanever complex radio bands.

ELECTRONIC TECHNOLOGY TODAY INC.P.O. Box 240, Massapequa Park, NY 11762-0240

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implement multiple configura-tions. The book describes theuse of Microsoft Utilities such asDoubleSpace, MsBackup, andAnti Virus. A summary of allDOS commands, illustrated withexamples, is provided in the lastchapter.

MS-DOS 6 Explained (ordernumber BP341) is available for$7.95 plus $2.50 shipping andhandling from Electronics Tech-nology Today Inc., P.O. Box240, Massapequa Park, NY11762-0240.


CONTEMPORARYLOGIC DESIGNby Randy H. Katz

This textbook offers an inno-vative approach to logic designthat combines a superior intro-duction to design fundamentalswith an in-depth look at today's

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Contemporary Logic Design

costs $67.75 and is publishedby The Benjamin/CummingsPublishing Company, Inc., 390Bridge Parkway, Redwood City,CA 94065; Tel: 800 -950 -BOOK.


BUILD YOUR OWNHOME THEATERby Robert Wolenik

Home theater has changed theway we watch TV. Viewers areturning to large -screen sets, ac -

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companied by surround -soundaudio systems. If you've beenthinking of updating your exist-ing audioivideo system orbuilding a complete home the-ater from the ground up, thisbook can help you do it withoutspending a fortune.

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The book also shows how tofit all the electronics equipmentinto your home. Beginning withselecting an appropriate room,the book explains how to acous-tically insulate it for the bestpossible performance, how toarrange speakers for true sur-

round sound, how to shop foran entertainment center, how tochoose colors and lighting thatcomplement the system, andhow to arrange furniture to max-imize viewing pleasure. Numer-ous illustrations as well as pho-tographs of real -life home the-aters illustrate the concepts pre-sented in the text.

Build Your Own Home The-ater costs $16.95 and ispublished by Sams Publishing,201 West 103rd Street, Indi-anapolis, IN 46290;Tel: 317-581-3500;Fax: 317-581-3500.


POWER UP!How to Make BatteryAdapters for Portable &MilitaryRadios, and otherMilitary & Non -MilitaryElectronicsby Dave Strom

Readily available on the surplusmarket, military radios are rug-ged, versatile, well -designed,and easy to operate. It's nowonder that the surplus sets areused by amateurs, NationalGuard units, experimenters,

federal agencies, and survivalgroups. The only problem withthem is that almost every typeof portable and mobile militaryset requires a different, usuallyhard -to -find, battery, designedparticularly for that piece ofequipment.

This book solves the problem

of powering surplus military ra-dios and electronic equipment.It shows how to easily makebattery adapters for the manypopular units, allowing the useof standard commercially -avail-able batteries. The fullyillustrated book contains de-tailed instructions and clearwiring diagrams that show ex-actly how to make adapters thatcan be used to power a widevariety of popular portable andmobile military radios. In addi-tion, the book covers batteryadapters for strobes, nightscopes, radiac sets, detection/intrusion sets, and field tele-phones.

Power Up! is available for$13.95 plus $4 shipping andhandling ($5 to Canada) fromCRB Research Books. Inc.,PO. Box 56, Commack, NY11725; Tel: 800-656-0056 incontinental U.S., or516-543-9169. (New York resi-dents please add $1.53 statesales tax.)


A CONCISEINTRODUCTION TOMICROSOFT WORKSby N. Kantor's and P.R.M.Oliver

Written with the busy, non -ex-pert person in mind, this bookaims to help the beginner cometo grips with Microsoft Works forMS-DOS in the shortest andmost effective way. Fully up-dated to cover the improve-ments included in version 3.0,the book has an underlyingstructure that allows the readerto build upon the informationpresented in each chapter. Ex-perienced Works users neednot read the book cover -to -cover, but can start from any ofthe self-contained sections.

The book explains how to usethe word processor to type, edit,print, and save documents. Itdescribes how Works can beused to build up simplespreadsheet examples and thento edit, save, print, and retrieve

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Graduate with anAssociateDegree from CE!CIE is the best educationalvalue you can receive if youwant to learn aboutelectronics, and earn a goodincome with that knowledge.CIE's reputation as the worldleader in home studyelectronics is based solelyon the success of ourgraduates. And we'veearned our reputation withan unconditional commit-ment to provide our studentswith the very best electronicstraining.

Just ask any of the150,000 -plus graduates ofthe Cleveland Institute ofElectronics who are workingin high -paying positions withaerospace, computer,medical, automotive andcommunications firmsthroughout the world.They'll tell you successdidn't come easy...but itdid come...thanks to theirCIE training. And today, acareer in electronics offersmore rewards than everbefore.

CIE'S COMMITTED TOBEING THE BEST...IN ONEAREA...ELECTRONICS.CIE isn't another be -everything -to -everyoneschool. CIE teaches onlyone subject and we believewe're the best at what wedo. Also, CIE is accreditedby the National Home StudyCouncil. And with more than1,000 graduates each,year,we're the largest home studyschool specializing exclu-sively in electronics. CIE hasbeen training career -mindedstudents for nearly sixtyyears and we're the best atour subject...ELECTRONICS...ITS THE ONLY SUBJECTWE TEACH!

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LEARN BY DOING...WITHSTATE-OF-THE-ARTEQUIPMENT ANDTRAINING.CIE pioneered thefirst ElectronicsLaboratory

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PERSONALIZEDTRAINING....TO MATCHYOUR BACKGROUND.While some of our studentshave a working knowledge ofelectronics others are juststarting out. That's why CIEhas developed twelve careercourses and an A.A.S.Degree program to choosefrom. So, even if you're notsure which electronics careeris best for you, CIE can getyou started with core lessonsapplicable to all areas in

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electronics. And every CIECourse earns credit towardsthe completion of yourAssociate in Applied ScienceDegree. So you can worktoward your degree in stagesor as fast as you wish. In fact,CIE is the only school thatactually rewards you for faststudy, which can save youmoney.

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them. It shows readers how togenerate, edit, and print bothsingle and multiple graphs. Thebook covers setting up, sorting,and searching through adatabase -management system,and how to use it to create top-quality printed reports. To sim-plify long, repetitive tasks, thebook explains how to createsimple macros and how to

lIA ConciseIntroduction to

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customize the program to meetyour own needs.

A Concise Introduction to Mi-crosoft Works (order numberBP294) is available for $6.95plus $2.50 shipping and han-dling from ElectronicsTechnology Today Inc., P.O. Box240, Massapequa Park, NY11762-0240.


ZEN AND THE ART OFTHE INTERNET:A Beginner's Guide,Third Editionby Brendan P Kehoe

If you are considering joiningthe more than 15 -million peopleworldwide who are already con-nected to the Internet, this bookcan help you get started. In astraightforward, easy -to -readstyle, the book makes learningabout and traveling through theInternet easier for beginners.No previous knowledge of theInternet is assumed, and thebook works for any type of

computer operating system.

This third edition of the firstuser's guide to the Internet hasbeen updated and expanded toreflect the rapidly changingtechnology and services. Newtopics covered include Gopher,WAIS; InterNIC; Internet BBS's;

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Zen and the Art of the Inter-net: A Beginner's Guide, ThirdEdition is available for $23.95from PTR Prentice Hall/Neo-data, P 0. Box 11073, DesMoines, IA 50381-1073; Tel:515-284-6751; Fax:515-284-2607.


ELECTRONIC HOBBYAND AMATEUR RADIOKITS CATALOGfrom Ramsey Electronics

This 20 -page catalog containsan assortment of exciting, edu-cational kits for the electronicshobbyist or student. The prod-ucts range from fast and easy

"Mini-Kits"-including a univer-sal timer, a voice -activatedswitch, and a light -beam com-municator-to VHF/UHF/FMtransceiver kits. Other projectsinclude QRP transmitters, activeantennas, shortwave gear, a mi-crowave motion detector, a radiodirection finder, a TV transmitter,and an FM broadcast -band re-ceiver. The catalog also offers

test equipment, antennas, andradio accessories.

The Electronic Hobby andAmateur Radio Kits Catalog isfree upon request from RamseyElectronics, Inc., 793 CanningParkway, Victor, NY 14564;Tel: 800-446-2295;Fax: 716-924-4555.


SOUTHPAWELECTRONICSCATALOG 931from Southpaw Electronics

Electronics hobbyists and tech-nicians alike will find a wide

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variety of components and partsin this 22 -page catalog. In-cluded are LED's, switches,piezo products, capacitors, tran-sistors, replacement semi-conductors, resistors, rectifiers,diodes, transformers, and more.Special buys are offered onseveral products, includingquartz crystals, oscillators,shorting jumpers, and a jumper -cable assembly. The companyoffers a "frequent buyer bonusplan" that awards buyers with abonus point (worth five cents onfuture purchases) for each dol-lar of merchandise ordered.Certain products-includinggrab-bag specials of assortedparts-earn double or triplebonus points.

Catalog 931 is free upon re-quest from SouthpawElectronics, P 0. Box 886, NewHyde Park, NY 11040-0311;Tel: 800-851-8870;Fax: 516-775-5091.


GREAT RADIO READS!from Tiare Publications

Tiare's 1994 catalog is filled withbooks that will help you get themost from your radio hobby, nomatter what form it takes. Thecatalog features books coveringeverything from amateur -radioQSL'ing to scanning your locallaw -enforcement agencies totuning in radio signals from out-er space. Other books canshow you how to work morestations, log more stations, geta better response from yourQSL'ing efforts, and run yourham station more efficiently. Anew division of Tiare, LimeLightBooks, offers general non-fic-tion titles for the radioenthusiast.

Great Radio Reads! (Catalog#8) is available for $1 fromTare Publications, P 0. Box493, Lake Geneva, WI 53147;Tel: 800-420-0579 (8AM to 6PMCentral Time).


Nine -Band Shortwave ReceiverTune in on the world with this nine -band solid-state version of

the classic regenerative receiver

Although once extensively usedin military and maritime ap-plications, today the re-

generative receiver is generallyviewed as a hobby or toy circuit. How-ever, the redesigned regenerative re-ceiver described in this article is muchmore than a toy or a historical curi-osity. The World Band RegenerativeRadio (which hereafter will be re-ferred to as the WBR) can receive allnine shortwave broadcast bands, al-lowing you to listen to numerous for-eign, English -language stations -such as the BBC, Radio Netherlands,Radio Canada, Radio China, and Ja-pan Radio, If so desired, the numberof bands can easily be extendedbeyond the initial nine provided onthe basic receiver. It can also receivesingle -side band (SSB) and continu-ous -wave (CW) code signals.

Unlike traditional regenerative re-ceivers, the WBR has a calibrated fre-quency dial and uses a short whipantenna. The unit also has an outputthat can be fed to a frequency coun-ter to provide a digital frequencyreadout. Also, because there are nocustom parts (including inductors),the WBR is easy to build.

BY LYLE RUSSELL WILLIAMS

Regenerative Radio. There arethree favorable characteristics of re-generative radios: For a one -tube ortransistor device, it has extraordinarysensitivity; for a one resonant -circuitdevice, it has extraordinary selectivity;and unlike a single -conversion super -het, the regenerative radio does notproduce unwanted image signals.

A schematic diagram of the tradi-tional tube -based regenerative re-ceiver is shown in Fig. 1. That circuit isessentially an RF oscillator with varia-ble feedback (that is connected to anantenna) with a resonant circuit com-posed of variable -capacitor C1 (thetuning control) and inductor L1. Feed-back is provided through a secondcoil (L2), called a tickler coil, that islocated on the same coil form as L1and is wired into the tube's plate cir-cuit. The amount of positive or re-generative feedback is controlled bya variable resistor that's connectedacross the tickler.

The quality (Q) factor of the reso-nant circuit without feedback is lessthan 50 when loaded by the antennaand the vacuum tube. When thefeedback of the oscillator is adjustedso that it is just below the point of os-

ciliation, the Q of the resonant circuitincreases to around 1000. That in-crease in Q greatly amplifies the sig-nal and narrows the bandwidth of theresonant circuit.

The tube is biased so that plate de-tection of an AM signal can takeplace. Thus, RF amplification and AMdetection are accomplished with asingle vacuum tube. The 3- 30-pF trim-mer capacitor (C2) that is connectedin series with the antenna is used toreduce the loading effect of the an-tenna. In some cases, antenna load-ing can be large enough to preventoscillation. The regenerative principleis sometimes used in different parts ofsuperhet radios: The second detector,intermediate -frequency (IF) amplifier,or the radio -frequency (RF) amplifierstage can be regenerative. In the lat-ter case, the stage is called a Q multi-plier.

A review of approximately 40 re-generative -receiver designs revealedsome interesting characteristics of thetraditional circuits. There are over adozen distinct types of RF oscillators(such as Colpitts, Hartley, Pierce, etc.),any of which could be used as thebasis of a regenerative radio. But for 19

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Fig. 2. As shown by this block diagram, the WBR is composed of severalsubassemblies: An active antenna, an amplifier (with a regeneration control and aband -switching circuitry), an AM detector, a power amplifier, and some form of outputdevice (internal speaker, external speaker, or headphones), plus a multi -voltage powersupply.

some reason, nearly all of the old de-signs used the basic Armstrong circuitshown in Fig. 1.

Tradition must have played a role inthe strong preference for the Arm-strong circuit. It was well known thatusing a pentode tube in a re-generative receiver was superior tothe triode design. However, nearly allof the old circuits were of the triodetype. Almost none of the old circuitshad a volume control. The volume of

the receiver had to be reduced bybacking off the regeneration control,which had the undesirable effect ofwidening the bandwidth of the circuit.Fortunately, those and other undesira-ble characteristics of the traditionalregenerative receiver have beenovercome in this modern, solid-statedesign.

The WBR Receiver. A block di-agram of the WBR is shown in Fig. 2.

The receiver is composed of severalsubassemblies: An active antenna, anamplifier (with a regeneration controland band -switching circuitry), an AMdetector, a power amplifier, andsome form of output device (internalspeaker, external speaker, or phones),plus a multi -voltage power supply.

The schematic diagram of the WBRis shown in Fig. 3. In that solid-stateversion of the regenerative receiver, adual -gate MOSFET, 04 (which is anal-ogous to a pentode vacuum tube), isused as the regenerative amplifier.The dual -gate MOSFET is configuredas a Colpitts oscillator, rather than anArmstrong type. The MOSFETs feed-back from source to gate is providedvia R18 and C33.

The circuit contains several stan-dard fixed -value inductors, each ofwhich is connected in parallel with asmall variable capacitor. Those L/Cpairs along with a SP127 rotary switch(S2) are used for band selection. Plug-in coils were very popular for bandswitching in the past, but this rotary -switch arrangement is cheaper andmore convenient.

Regenerative amplification andAM detection are performed by twoseparate transistors, Q4 and Q5. TheAM detector (which is called an infi-nite impedance detector) has theadvantage of not loading the RFstage appreciably. Because D3 is op-erated with a slight forward bias, ei-ther a germanium or silicon diodecan be used in that position. The DCvoltage across R24 (the volume con-trol) should be about 0.1 volts.

Regeneration is controlled by vary-ing the voltage applied to gate 2 of04. That changes the transconduc-tance of the transistor. The circuit alsocontains fine and course regenera-tion controls that allow delicate ad-justments.

The traditional regenerative re-ceiver required at least an indoorlong-wire antenna for proper recep-tion. The WBR, on the other hand, usesan active -antenna circuit, consistingof transistors 01, 02, and Q3. Thanks tothat circuit, under most conditions, ashort whip antenna is adequate.

The active -antenna circuit con-nects to gate 2 of Q4, while the reso-nant circuit is connected to gate 1.The resonant circuit is isolated fromthe antenna and tuning is indepen-dent of antenna length, allowing a

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calibrated dial to be used on the tun-ing capacitor. The level of regenera-tion as set by R16 and R17 is

unaffected by the antenna, and sothe WBR does not require an antennatrimmer.

Components C12 -C14, C18, L3, L4,R6, R7, and R14 form a high-pass filter(which has a cutoff frequency of 3.4MHz) that prevents local broadcaststations from overloading the circuit. Ifoperated in a rural area away fromstrong stations, that filter can be dis-abled. (The high-pass filter may haveto be modified if bands below 3.4 MHzare added to the WBR.)

Many traditional regenerative re-ceivers had tuning ranges as large as10 -MHz per band. But in our circuit,using a low -value tuning capacitor al-lows the circuit to tune only the short-wave broadcast bands-which varyin width from 350 kHz to 700 kHz. Nar-rowing the range makes the receivereasier to tune. It is not necessary to use

a reduction drive on the tuning ca-pacitor. A simple knob with a pointer isadequate.

Although a standard 12 -position ro-tary switch was used for S2, only 9 posi-tions are needed. However, the threeadditional switch positions can beused to extend the range of the WBR.The extra positions could be used forthe ham bands, VVVVV signals at 5 MHzand 20 MHz, or some other segmentof the shortwave spectrum. In order toadd the extra band, the band induc-tances must be determined. The in-ductances required for the extrabands, are given by:

L = 0/46.96 x 10-5)]2

where L is the desired inductance inhenrys, and f the is the center fre-quency of the desired band in hertz.The equation assumes a mean tuningcapacitance of 123 pE To receive theVVWV time signal at 5 MHz, the re-quired inductor would be 8.25 µH.

6 INCHES

Resistor R20 is used to sample the RFsignal in the regenerative amplifierwithout affecting its operation. Thesignal is amplified by Q6 and Q7, andfed to J4 and J5 for application to anexternal frequency counter if desired.

Construction. Except for the powersupply, the controls, the connectors,and the internal speaker, the entirecircuit was assembled a 5- x 6 -inch,single -sided, printed -circuit board. Atemplate for the printed -circuit layoutis shown in Fig. 4. Once you haveetched your board and obtained theparts listed in the Parts List, con-struction can begin.

Guided by Fig. 5, begin assembly byinstalling the passive components (re-sistors and capacitors), making surethat the polarized capacitors are cor-rectly oriented. After installing the pas-sive components, install the semicon-ductors; start with the diodes, followedby the transistors, and then the IC's.

Fig. 4. This full-sized printed -circuit template can he used to etched your own board.

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Fig. 5. When assembling the WBR's printed -circuit board, make sure that thepolarized capacitors are correctly oriented. Also, when installing the semiconductors,pay particular attention to the orientation of Q4, and be careful that you do notconfuse the 2N3904 general-purpose transistors with the low -power (78Lxx series)voltage regulators, both which are housed in the same TO -92 style case.

TABLE 1-FREQUENCY LIMITS OF THE INTERNATIONAL BAND

Band(Meters)

BandLimits(MHz)

FrequencyReceived

(MHz)

L/CBand

Trimmers

49 5.95-6.20 5.90-6.26 L5/C23

41 7.10-7.30 7.00-7.40 L6/C24

31 9.50-9.90 9.40-10.0 L6/C25

25 11.65- 12.05 11.37-12.1 L8/C26

21 13.60 -13.8 13.44 -14.23 L9/C27

19 15.10-15.6 14.96-15.88 L10/C28

16 17.55-17.9 17.44 -18.46 L11/C29

13 21.45-21.85 21.21-22.40 L12/C30

11 25.67-26.10 25.10-26.32 L13/C31

Pay particular attention to the orien-tation of Q4 (the dual -gate MOSFET).Be careful when installing the 2N3904general-purpose transistors and thelow -power (78Lxx-series) voltage reg-

ulators; those units are housed in thesame TO -92 style case, so don't getthem confused.

Note from the parts -placement di-agram that there are three unused

R16FINE

R24VOL:

REGEN

pad groupings in the area of theband -determining capacitor/induc-tor pairs. Those pads (as mentionedearlier) are provided for those whowant to extend the band coverage ofthe WBR.

Once all of the on -board compo-nents have been installed, check yourwork for constructions errors-coldsolder joints, solder bridges (and othershorts), misoriented or misplacedcomponents, etc. When you are satis-fied that there are no errors, put theboard to the side momentarily andgo on to the next phase of con-struction.

Power Supply. The next thing thatyou will need is a power source for theWBR. There is, of course, nothing crit-ical about the circuit's power source,and no printed -circuit layout is pro-vided for that portion of the project.Instead, an external power supply,

PARTS LIST FOR THE WORLD -BAND REGENERATIVE RADIO

2

I

SEMICONDUCTORSU1 -78L12 12 -volt, 100-mA, voltage

regulator. integrated circuitU2-7808 8 -volt I -amp or 78L08 8 -

volt 100-mA, voltage regulator,integrated circuit

U3 -78L05 5 -volt, 100-mA, voltageregulator. integrated circuit

U4-7812 12 -volt 1 -amp, voltageregulator, integrated circuit

U5-LM386 low -voltage audio -poweramplifier, integrated circuit

QI-MPF102, VHF JFETQ2. Q3. Q5 -Q7 -2N3904 general-

purpose NPN silicon transistorQ4-40673, MPFI2I, MPFI31, or

NTE222, or similar dual -gateMOSFET

BRI-DB101 1 -amp, 50-PIV, bridgerectifier

DI-IN4001 I -amp, 50 -Ply, rectifierdiode

D2 -1N34 general-purposegermanium diode

D3 -1N914 general-purpose silicondiode

RESISTORS(All fixed resistors are 1/4 -watt. 5%

carbon units, unless noted.)RI. R5 -22(X) -ohmR2 -220 -ohmR3-4.7-megohmR4-2.2-megohmR6 -22,000 -ohmR7 -10,0(X) -ohmR8, RI3, R28 -1000 -ohmR9 -150,(X0 -ohmR10 -47.0(X) -ohmRII-3300-ohmR12 -470 -ohmR14- 4700-ohms1/4-watt, 1%, metal-

filmR 15 -15,000 -ohmR16---1(X).0(X)-ohm panel -mount

linear potentiometerRI7-5000-ohm panel -mount linear

potentiometer

R18 -82.5 -ohm, 1/4 -watt, 1%, metal -film

R19 402 ohm. 1/4 -watt, 1%, metal -film

R20 -51 -ohm 1/4 -watt, 1%. metal -filmR2I-810-ohm, 1/4 -watt, 1%, metal -

filmR22-10-megohmR23 -560,000 -ohmR24 -5000 -ohm, panel -mount, audio

potentiometer with SPST switchR25 -10 -ohmR26, R29 -499 -ohm 1/4 -watt, 1%,

metal -filmR27 -3900 -ohm

CAPACITORSCI, C2, C9. C45 -470 -RE 16-

WVDC, electrolyticC3, C5, Co. CIO, C15 -C17.

C20 -C22, C35, C36. C38 -C41.C46, C48, C49-0.IRE ceramic -disc

C4, C7, C19, C37. C42. C4722-µF, I6-WVDC, electrolytic

C8 -4700 -RE 16-W VDC,electrolytic

CII-100-pE silver micaC12-6.8-pF, silver micaC13-300-pF, silver micaC14-56-pE silver micaCI8-10-pF, silver micaC23-C31-3.5-20-pF, miniature

ceramic trimmerC32-3-I8-pF panel -mount air

variableC33-330-pF, silver micaC34-120-pE silver micaC43-IOU-µF, 16-WVDC.

electrolyticC14 0.05 -RE ceramic -disc

INDUCTORSLI, L2, L14- 47(1 W. Miller

9250-473)L3-8.2-01 (J.W. Miller 9310-34)L4-43 RH (J.W. Miller 9210-58) or

two 22-RH Ii. W Miller 9310-44),

see textL5-5.6-RH (J. W. Miller 9310-30)L6-3.9-01 (1 W. Miller 9310-26)L7-2.2-1111 (J. W. Miller 9310-20)L8-1.5-0-1 (J. W. Miller 931(1-16)L9-L0-µH (J. W. Miller 9310-12)L10-0.82-0 (J. W Miller 9310-10)LII-0.56-RH (J. W. Miller 9310-07

or 9230-14)LI2-0.33-0 (J. W. Miller 9310-04

or 9230-080L13- 0.I8 -µH (J. W. Miller 9230-02)L15-56-01 (J. W. Miller 9250-563)FBI-General-purpose ferrite head

ADDITIONAL PARTS ANDMATERIALS

JI-2.1-mm DC power jack (RadioShack 274-1565)

12-V8 -inch panel -mount, closed-circuit stereo lack

J3-RCA jackJ4. J5-Individual pinlacksSI--SPST switch (part of R24)S2-SPI2T rotary switchSPKRI-4-8-ohm, miniature speaker

PL1-2.1-mm power plug (RadioShack 274-1567)

ANT--30-inch whip antenna (RadioShack 270-1401)

T1--24-volt. 300-mA, center -tappedtranslOrmer

Printed -circuit materials,117 -volt AC power plug with linecord, four small control knobs, onelarge pointer knob, wire, solder.etc.

The t011owing is available fromDan's Small Parts and Kits. 1935 S.3rd W. #I, Missoula. MT 59801;Phone,'Fax 1-406-543-2872. Printedcircuit hoard 513.00 plus 52.50shipping and handling. Cicuit hoardplus complete kit of all components(excluding power supply. cabinet.knobs and hardware) 559.95 plus53.75 shipping and handling.

comprised of two chokes, a filter ca-pacitor, and a bridge rectifier werewired together (see Fig. 3) on a smallsection of perfboard. The perfboardand a 24 -volt transformer were thenwire together and mounted to ablock of wood. An AC line cord wasthen connected to the input of thesupply and a suitable plug con-nected to its output. Doing things thatway helps to keep power -line noiseout of the receiver.

Of course, a standard plug-in 12 -volt, 500-mA or so power adaptor canbe used as the power source as well; if

24 you decide to go that route, connect

two filter chokes between the supplyand the receiver.

The Enclosure. The authors pro-totype was housed in a large alumi-num enclosure measuring aboutx 53/s x 6 ''/.1 inches. Prepare the en-closure by drilling holes in suitable lo-cations for the controls, and mountingholes for the printed -circuit boardand the unit's internal speaker.

The author's front -panel layout in-cludes the band, volume, the fine andcoarse regeneration controls, and aphone jack. The rest of the off -boardcomponents-the counter, auxiliary

speaker, and the power and antennajacks are mounted to the rear panelof the prototype.

After deburring the holes, dry -trans-fer lettering can be used to labeleach control, switch, and connector.Once labeled, mount the off -boardcomponents to their respective posi-tions on the enclosure and, using shortlengths of insulated wire, connect theoff -board components to the appro-priate points on the printed -circuitboard. The connections to the volumecontrol (R24) and the counter jacksshould be made through small -gauge shielded cable,

Fig. 6. The author started out with a blank tuning -dial pattern like this one, and addedthe appropriate frequency markings to the dial during the calibration procedure.

Fig. 7. Using a marker generator, the author first located and marked the 100 -kHzpoints on the tuning dial, then the 50 -kHz points, followed by 25 -kHz points. This iswhat his dial looked like when completed. The dial shown here is strictly for reference.Each unit is different and must be calibrated according to the instructions given in thetext to compensate for the tolerances of the actual components used in the set.

Down The Dial. The tuning -dial pat-tern for the WBR was made by taking ablank pattern (like that shown in Fig.6), and gluing it to a piece of card-board using white paper glue (avail-able from art -supply stores).

Once the glue dries, the excesscardboard can be cut away with asingle -edge razor blade or X-actoknife. Cut the screw and the capaci-tor -shaft holes in the dial face andmount the dial on the aluminum en-closure with four 4-40 screws and nuts.The mounting nut for the variable ca-pacitor goes outside the dial card-board and helps hold the dial to theenclosure, Frequency markings on

the dial will have to be added afterconstruction is completed.

The dial pointer was made from alarge paper clip. The paper clip wasstraightened out and filed downslightly. A hole was drilled in the side ofa large control knob with a #66 drillbit (the size used to drill printed -circuitboards). The filed -down paper clipwas then force fitted into the hole inthe knob, and the knob was mountedon the shaft of the tuning capacitor.

Once the knob is mounted, set thetuning capacitor to maximum ca-pacitance and place the pointer sothat it points to the far left (log scale 0)position on the dial. After that, mount

the unit's internal speaker to the insidesurface of the enclosure. The connec-tions between the internal speakerwires and the printed -circuit boardshould be made with short lengths oftwisted -pair wire and kept close to themetal enclosure and away from theantenna.

The antenna was mounted to theback of the cabinet with two plasticbrackets that were intended to an-chor the bottom of window blinds. Theantenna feedthrough was made bydrilling holes to mount an SO -239coax receptacle. Instead of mountingan SO -239, however, a piece of sheetplastic was mounted over the cutoutand a hole was drilled in the center ofthe plastic. A wire from the antennaconnection on the printed -circuitboard was then passed through thehole in the plastic to the antenna ter-mination on the outside of the en-closure. That type of feedthrough hasa lot less capacitance than a coaxconnector.Calibration. Calibration should bedone before the outside cover isplaced on the enclosure. The first stepis to set the band limits. A frequencycounter is helpful for that purpose. If acounter is available, connect it to thecounter terminals, J4 and J5. Connectthe power supply to the receiver andturn it on. Set R24 for maximum vol-ume and set R16 (FINE REGEN) to its cen-ter position. Set S2 to 49 meters. TurnR17 (COARSE REGEN) control until the WBR

oscillates (heard as a rushing sound, ahum, or whistle from the speaker) andthe counter shows a steady reading.From Table 1, note that the band limitsfor 49 meters are 5.95 MHz to 6.20MHz. Adjust C23 until the receivercovers that range as C32 is rotatedfrom maximum to minimum capaci-tance-a range of 5.9 to 6.26 MHzwas obtained in the author's receiver.

Referring to Table 1, switch S2 to ahigher frequency band and adjustR17 (COARSE REGEN) for continuous os-cillation if necessary. While rotatingC32 from minimum to maximum ca-pacitance, adjust the appropriatetrimmer capacitor so that the receivertunes the international band. Onsome of the bands, the tuning capac-itor will tune a wider frequency rangethan needed.

Trimmer capacitors C23 throughC31 should have enough range to al-low the proper band limits for each 25

band to be set. But if the frequencyrange is too high, a small fixed capac-itor can be soldered across the ap-propriate inductor (L5 through L13). Ifthe frequency range is too low, theinductor for that band will have to bechanged to the next lowest value.

The dial should be calibrated whenthe receiver is completed and no fur-ther changes are expected. Eachband should be calibrated when thefewest radio stations are on the air.The lower frequency bands are clearduring the daytime, and the higherfrequency bands are clear at night.

A marker generator capable ofgenerating squarewaves of 100 kHz,50 kHz, and 25 kHz that was built fromplans found in the 1988 ARRL Hand-book was used to calibrate the dial.Use a frequency counter to find theapproximate location of the first multi-ple of 100 kHz on the dial. Start byturning on the WBR and allowing itwarm up for at least 30 minutes. Ro-tate R17 until the WBR oscillates. Backoff R16 until oscillation stops; then ad-vanced it until the set almost oscil-lates. Set the marker generator to 100kHz and turn it on.

Loosely couple the output of themarker generator to the antenna ofthe WBR. A wire from the output of themarker generator placed near theWBR's antenna should be adequate.A signal that is strong enough to bereceived but weak enough that theregeneration control can be set to itsmost sensitive position is desired. TurnC32 (TUNE) slightly until a weak signal isheard as a rushing sound or a hum. Ifno signal is heard, increase the coup-ling to the marker generator and tryagain. When a signal is heard, turn themarker generator off and on to verify

8 the origin of the signal. Using a pencil,a lightly mark the point on the dial.

Move the tuning capacitor up the dialco and mark the next 100 -kHz point.

When all the 100 -kHz points areco marked, switch the marker generator

to 50 kHz and mark points betweenr.) the 100 -kHz points. Make those points

distinguishable from the 100 -kHzcr

points. Once the 50 kHz points have`..'1 been marked, switch the marker gen-

erator to 25 kHz and add those points.O) When all the points for all bands are

on the dial, remove the dial from theLi_z radio, write over the dial markings in

ink and add the frequency numbers.26 Erase any pencil markings left visible.

Spray the completed dial with clearlacquer or cover with a clear plasticsheet. The finished dial should looksomething like the one in Fig. 7. If thereceiver drifts out of calibration, adjustthe appropriate trimmer capacitor(C23-C31) until the stations appear inthe correct place for each band.

Using the WBR. A regenerative ra-dio does not operate like a superhet.It requires more adjustment to prop-erly receive a given station. That ispart of the charm of this vintage cir-cuit. Once the technique is learned,the receiver can be easily operated.

The narrowest bandwidth and high-est sensitivity is obtained when the re-generation controj is set just below thepoint of oscillation. The optimum set-ting of the regeneration controlchanges as the receiver is tuned todifferent frequencies. As the tuningcapacitor is tuned up in frequency(less capacitance), the regenerationcontrol has to be backed off to pre-vent oscillation.

Here is the author's attractit0v laid outprototype of the WBR. Note theprofessional -looking lettering thatadorns the front panel.

Turn the radio on, select an appro-priate band, extend the antenna to itsmaximum length, turn the volume(R24) all the way up, rotate C32 to theminimum capacitance (highest fre-quency) position, set R16 to its centerposition, rotate R17 until the receiveroscillates and back off until the set juststops oscillating.

Rotate C32 to tune down the scaleuntil a station is heard. Adjust R16 formaximum sensitivity; back off R24 ifthe station is too loud. When a dif-

ferent station is tuned, it will be neces-sary to readjust R16 for optimumreception. Potentiometer R17 shouldneed readjusting only when the set isswitched to a different band.

To use a frequency counter, con-nect the counter to J4 and J5 of theWBR. Very strong stations will provideenough signal at the counter outputto produce an accurate station -fre-quency reading. To use the counterfor weaker stations, tune in a stationand turn up the regeneration controluntil the receiver oscillates, Going intooscillation causes the frequency todrop a few kHz. Tune up the bandslightly until the oscillator produces abeat note against the station. AdjustC32 for minimum frequency (pitch) ofthe beat note. Then read the frequen-cy on the counter. If the band is con-gested, this procedure cannot beused but the approximate frequencycan still be determined.

If the sensitivity is too low for a givensituation, add a few feet of wire to thewhip. If an indoor or outdoor long-wireantenna is available, wrap two turnsof the insulated wire around the whipto create a loose couple. Directlyconnecting a long-wire antenna tothe whip will usually result in the re-ceiver being overloaded.

When the WBR was being de-veloped, I tuned in a ham operator on40 -meter LSB (lower sideband) fromMiami who was describing to otherhams the conditions as hurricane An-drew moved onto the Florida coast. Afew days later, the same hurricane hitnear my location in Louisiana.

For receiving LSB, locate the stationusing the same procedure as used forAM stations. Then turn up the fine re-generation control until the receiveroscillates continuously. Turn the tuningcapacitor slightly up in frequencythen slowly tune down until the oper-ator's voice is intelligible. Changingthe fine regeneration control slightlymay be helpful for obtaining a goodsignal. The required tuning is delicate.

Receiving USB is the same exceptthat the tuning capacitor must betuned up the band until the speech isintelligible. CW (Morse code) stationswill be heard in the vicinity of amateurvoice stations. Like SSB they are re-ceived with the radio oscillating.Often several code stations will beheard at once, but the audio tone ofeach will be different.

Have you ever wondered howmicrowave signals are used todetect movement within a de-

fined area? Well, here's your chanceto find out! The Microwave Motion De-tector described in this article can beused to sense speed or any movingbody-a person, animal, car, or bicy-cle! using the Doppler effect.

Doppler theory states that whensound, light, or even radio waves areemitted or reflected by a moving ob-ject, the frequency of the waves willbe different from those emitted or re-flected when the object is stationary.

Circuit Description. The MicrowaveMotion Detector is nothing more thana combination transmitter/receivercircuit that's built around four tran-sistors, and a quad op -amp. A sche-matic diagram of the MicrowaveMotion Detector is shown in Fig. 1. Inthat circuit, Q3 (a 2SC2570 high -fre-quency NPN silicon transistor) is con-figured as a free -running (astable) RFoscillator that is designed to operateat the low end of the microwaveband (approximately 1.0 GHz).

The microwave signal developedby the oscillator follows two paths. In

Build ThisMicrowave

MotionDetectorBY FRED BLECHMAN

Essentially what happens is that as theobject moves toward you, the fre-quency emitted by or reflected off ofthe object increases. The opposite istrue as the object moves away fromyou; the frequency decreases.

A similar effect can be seen in ev-eryday life: Have you ever noticedhow a train whistle, racing car engine,or car horn appears to alter in fre-quency as the vehicle approaches,passes, and moves away? The per-ceived change in frequency iscaused by the waves being eithercompressed or stretched as theyleave the moving object. Compress-ing the waves increases the per-ceived frequency, while stretching thewaves reduces it.

Doppler -effect based systems areused to clock speeding cars, missilevelocity, and even to detect movingair. In fact, Doppler radar systems areused at some of the larger airports todetect dangerous cross winds, calledwindshear, which have been blamedfor countless takeoff and landing ac-cidents.

one path, the microwave signal is ap-plied to D1, a 1SS99 Schottky barrierdiode, which is designed for opera-tion at low microwave frequencies. Atthe same time, the microwave signalis applied to ANTI and is radiated out-ward in an omni-directional pattern,filling the surrounding area with mi-crowave signals.

The radiated microwave signals,upon contacting an object within thecovered area, are reflected back tothe antenna. The frequency of the re-flected signal depends on the ob-ject's direction of movement. (Themotion detector doesn't care whichdirection is taken by the object; it isonly interested in the frequency shiftthat results because of that move-ment.) The antenna, in turn, picks upthe reflected signal, and feeds it to D1,where it is mixed with the originallyradiated signal, to produce both asum and difference signal at Dl'scathode.

The sum frequency is bypassed toground via capacitor C8, leaving onlythe difference frequency. The dif-

ference, which can range from 10 to40 Hz depending upon the speed ofthe moving object, is applied to theinput of U1 -c (1/4 of an LM324 quad op -amp) at pin 10. The output of U1 -c isfed to sensitivity control R8, which al-lows you to adjust the Doppler -shiftdifference signal. The Doppler -shiftsignal is applied to U1 -d (another 1/4 ofthe LM324 quad op -amp), which, inthis instance, is configured as a band-pass filter.

The output of the bandpass filter(U1 -d) is fed to U1 -a (another op -amp;this one configured as a buffer), and

This security -systemadd-on foryour home

or carworks onthe same

principles aspoliceradar.

output to diode D2, which rectifies thepositive half cycle of the input wave-form, producing a series of positive -going pulses. The rectified output ofD2 charges C17 (to some unspecifiedvalue), during the positive alternationof the signal.

At the same time, a voltage equalto the charge on C17 is applied to pin5 of U1 -b, which is wired as a com-parator. During the negative half cy-cle, the charge on 017 is bled toground through R17 and U1 -a. Com-ponents C17 and R17 determine the"on" time of U1 -b. Larger values of ei-ther C17 or R17 result in a longer ontime for U1 -b.

Each time that the voltage at pin 5of U1 -b exceeds the reference volt-age on pin 6, the output of U1 -b at pin7 swings positive. The pin -7 output isapplied to the gate terminal of Q4 (aBS170 N -channel TMOS FET) andcauses it to turn on and off. Each timethat Q4 turns on, a ground path(though Q4) for LED1 is completed.causing it to turn on and off, as theinput signal dictates. Transistor Q4 can 27

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22pF

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032SC2570

R2 C191000 I

C3 I = Q1

2N3904

2200 ANTI

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47K

C62.2pF

R7 C71K 2.2pF

-4/4/0.--S ETCHED STRIP LINE

+9-15VSOURCE

D1

1339901

R1547052

J1

0 1

14

5

LED10

0C20

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C8.01

04B8170

D G S

88170

BOTTOM VIEW

1/4 LM32410

R6 t1K S -9-

R1

4700

C1

C4 10220

022N3904

C210

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R2110K

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R9R1 1MEG1K

C13 $ R13.*470K

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C9

CW fR810K

10

R124.7K

C1410

C12.1

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1MEG

C1010

K --

R1410K

C15 C16.01 1

1E -.1(-

R17100KVA U1 -a

1/4 LM324

00D2

1N914

2

-Mr-R1610K

Fig. I. The Microwave Motion Detector is essentially a combination transmitter/receiver circuit built around four transistors, and a quad op -amp.

PARTS LIST FOR THE MICROWAVE MOTION DETECTOR

SEMICONDUCTORSUI-LM324 quad op -amp, integrated

circuitQI, Q2 -2N3904 general-purpose

NPN silicon transistorQ3-2SC2570 high -frequency, NPN

silicon transistor (or NECNE02132E).

Q4-BS170P-ND N -Channel,enhancement, TMOS FET (Digi-Key)

DI-ISS99 Schottky barrier diode(NEC). See text.

D2 -1N914 or IN4148 general-purpose, small -signal, silicondiode

LEDI-Green light -emitting diode

RESISTORS(All fixed resistors are 1/2 -watt. 5c4 -

units.)RI, R15 -470 -ohmR2 -220 -ohmR3 -47,0(X) -ohmR4. RI4, R16. RI9, R21 -10,000 -

ohmR5, R17, R18 -100,000 -ohm

R6, R7, RII, R20 -1,000 -ohmR8--I0,000-ohm PC -mount 1/2 -watt

potentiometer (see text)R9. R10-1-megohmRI2-4,700-ohmR13 -470,000 -ohm

CAPACITORSCl, C2, C9, CIO, C13, C14 -10-µF.

16-WVDC, electrolyticC3, C8. C1.5.

ceramic -discC4-220-tif, 16-WVDC, electrolyticC5-22-pF, ceramic -discC6, C7-2.2-pF, ceramic -discC 1 I . C12. C20- O.1 -µF, ceramic -discC16. C17 -1.0-µF, 16-WVDC,

electrolyticC19 -1000-µF, I6-WVDC,

electrolytic


11-Molex WM 3303 6 -pin femalePC board connector (Digi-Key)

Printed -circuit materials, enclosure,

9-15 volt DC power source, solder,hardware. etc.

Note: The following items areavailable from Ramsey Electronics,lnc.(793 Canning Parkway, Victor.NY 14564: Tel. 716-924-4560): Acomplete kit of parts for theMicrowave Motion Detector(MD-3BP), including printed -circuit board (but not the case orcontrol knobs)-$19.95; an etchedand drilled printed -circuit boardonly (MD-3PCBP)-$10.00, aSpecial Parts Kit (MD-3SPKBP)containing all semiconductors, 128,and 11-$12.50: a custom case andknob set (CMD-BP)-$14.95.optional Siren Kit (SM-3BP)including printed -circuit board andmanual- $3.95; optional UniversalTimer Kit (UT-5BP) includingprinted -circuit hoard and manual-$7.95. Please add $3 for ordersunder $20, plus $4.95 postagehandling. New York residents.please add appropriate sales tax.

4 INCHES

Fig. 2. The project was assembled on a printed -circuit measuring about 4 X 43/4

inches. A template of that printed -circuit board layout is shown here full-size.

be safely operated at voltage levelsas high as 50 volts DC and currentlevels of 250 mA. Capacitor C20 isincluded in the circuit to provide tran-sient suppression for stability.

The output of the Microwave Mo-tion Detector (via pins 5 and 6 of J1)can be used to drive any number ofsignaling devices, timers, counters,etc. More on that later.

Power for the circuit is suppliedthrough J1 (a multi -conductor jack)from an external source of 9-15 volts.The external source voltage is regu-lated by Q1 and Q2. That transistorregulator circuit also provides the ref-erence voltage used by comparatorU1 -b. In the regulator portion of thecircuit, the reverse -biased base -emit-ter junction of Q2 behaves like a low -power Zener diode, with a break-down voltage in the 6- to 8 -volt range.Capacitor C19 is used to filter the ex-ternal power source to assure a cleanand smooth source of current.

Construction. The Microwave Mo-tion Detector was assembled on aprinted -circuit measuring about 4 x43/4 inches. Because of the criticalnature of microwave -circuit wiring,where lead lengths and ground con-nections can effect circuit operation,don't try building this project withoutusing the printed -circuit board layout.And don't be alarmed when lookingat the printed -circuit board pads thatconnect the anode of D1 and the an-tenna to what looks like ground. Thatshort etched stripline," is carefully de-signed to function as a small inductorat the selected microwave frequency,forming part of the resonant tank cir-cuit for the microwave oscillator (03)!

Figure 2 shows a full-size templateof the Microwave Motion Detector'sprinted -circuit board layout. You canetch your own board from the tem-plate provided and gather your ownparts, or you can order the board onlyor a kit (which includes an etched and

pre -drilled, printed circuit board andall the components, but no case) fromthe supplier listed in the Parts List.

Although most of the parts for thisproject are commonly availablethrough conventional electronic -component suppliers, some of themmay be difficult to find. A source forthe more difficult -to -find parts is indi-cated in the Parts List. The difficult -to -find parts (such as Q3, D1, and R8) areavailable in a "Special Parts IGt" fromthe listed source. If you opt to gatheryour own parts, or you plan to usewhat you have on hand, keep in mindthat the printed -circuit board layoutwas designed to accommodate thespecial mounting or specific dimen-sions of J1 and R8.

In any event, once you have theprinted -circuit board and all of theparts, assemble the Microwave Mo-tion Detector according to the parts -placement diagram shown Fig. 3.When assembling the Microwave Mo-tion Detector, take special care thatpolarity -sensitive components (elec-trolytic capacitors, diodes, transistors,and the integrated circuit) are in-stalled in the right direction.

Begin construction by installing thepassive components Oumper wires,resistors, and capacitors) first; followthat by installing the active compo-nents (diodes, transistors, and the IC).Once the active components havebeen installed, check your work forthe usual construction errors: cold sol-der joints, misplaced or misorientedcomponents, solder bridges (a mag-nifying glass helps here), etc.

Once you've determined that thecircuit has been correctly assembled,it's time to consider the enclosure thatwill house the Microwave Motion De-tector. The circuit can be housed inany enclosure of your own choosing.However, if you prefer, an optionalcase and knob kit is available from thesupplier listed in the Parts List. The stur-dy black plastic instrument case issupplied with neatly lettered front andrear panels, knobs, rubber feet andmounting screws.

If you choose a case other than theone available from the listed supplier,it will be necessary to drill holes in thefront panel of the enclosure to ac-commodate the shaft of R8 and LED1'slens. Once that's taken care of, it willbe necessary to make a cutout in therear panel for the output connector, 29

V)

0

rn

7-)

lL

30

ANTI

R33

-C6- -R2-C1

CDC5

DI1 47

R7

-R6-

C13+ -R11- 1 R9

C1-R13-- I

-+ -R12-C12 1I0

C14

-C15-

C101

-C19-

1 J1 6

+-01

0 aJ

1C18

R20 -R4--R21-

U1

-R5-C9 I -R16--R14 - +

C16

Li

-R19-

-C20-

-R18--

C2I

1

-R17--104-

D2 +1c17

LED1 ff

Fig. 3. Once you have the printed -circuit board and all of the parts, assemble theMicrowave Motion Detector according to this parts -placement diagram. Note whenassembling the circuit, take special care that the polarized components are properlyoriented.

J1, and a small hole in the top of theenclosure for the antenna. Onceproperly prepared, the cabinet's frontand rear panels can be labeled usingdry -transfer lettering.

Testing and Tune Up. Testing theMicrowave Motion Detector requiresnothing more than a stable source ofdirect current of from 9 to 15 volts.There should be no tendency for thevoltage to drop when under load.While the Detector uses only about 15mA on standby, it draws about 30 mAwhen triggered. That difference isenough to affect the operation of thecircuit when using a typical small 9-voff transistor radio battery-even analkaline unit. The effect is that oncetriggered, the Detector output keepsretriggering itself due to the on -off

2

MOTION3-

NC

DETECTOR _NC

5

MOTIONDETECTOR

2

NC4-NC5

6

Fig. 4. The detector can easily be testedby using a transistor radio as asignaling device. Simply wire the twodevices together as shown here. and turnon the radio. When the detector istriggered. power will be applied to theradio, turning it on.

voltage change. For the same reason,use short leads when feeding powerto the circuit through J1. Connect+ 9. 15 volts DC to pin 1 of J1, and

connect pin 2 of J1 to the ground ter-minal of the DC supply.

Rotate R8 fully counter -clockwiseto the minimum -sensitivity position -and the LED extinguish. Slowly rotateR8 clockwise until the LED lights. Backoff R8 until the LED just goes out. Nowmove your hand near the antenna;you should see the LED light up. Mov-ing either toward or away from theDetector should trigger it. The LEDshould stay lit for only about one sec-ond. Adjust R8 for the desired sen-sitivity.

Figure 4 shows how the Motion De-tector can be wired to use a transistorradio as a signaling device; other,more practical applications will bediscussed later on, but this set-up isuseful for testing. The positive terminal

T

BATTERY

TRANSISTORRADIO

+3-12V

MOTIONDETECTOR

A

D

E

OUTPUTUT -5 A --0 SEE UT -5 MANUAL

TIMER FOR CONNECTIONS

2 R

10K4-3 NC

4- NC2N3908

+3-12V

10K

B

Fig. 5. The output of the detector can used to trigger a timer (as shown in A). or todrive siren (as shown in B). The timer and siren circuits can be obtained in kit formfrom the supplier listed in the Parts List.

SM-3SIREN

SPEAKER

MOTIONDETECTOR

2

-3 NC

5

1

2

3MOTION

DETECTOR

5

NC

R

2.2K

NC

NC

W4R

10K

2.2K

10KR

TW'30,TP62,MJE700,ETC.

a2N3906

A

B

+5- 28V

1K

+5- 28V

0 OUTPUT IS HIGHON DETECT

OUTPUT IS LOWON DETECT

0

2N 3055,TIP29,NUE800,ETC.

Fig. 6. The output of the motion detector can be used to trigger a transistor drivercircuit, like those shown here. The one in A is designed to deliver a positive outputvoltage to the circuit that follows, while the one in B is designed to complete theground -return path for that same circuit.

MOTIONDETECTOR

D

1N4002

+5-28V

I.RELAY ENERGIZED

ON DETECT

Fig. 7. The Microwave MotionDetector's output can also be used totrigger a relay.

of a separate power source (a 9 -voltbattery in this case) is connected tothe positive supply input of a transistorradio, and the negative terminal ofthe radio going to pin 5 of J1. Pin 6 ofJ1 is connected to the negative sideof the 9 -volt battery.

With the radio switched on, it shouldplay whenever the Motion Detector istriggered. Note that trying to use theMotion Detector power supply to alsopower your external load will proba-bly result in unwanted retriggering. Ifyou find that your load triggers thedetector, try moving it further away.The detector is extremely sensitive toits surroundings.

The control (pin 5 of J1) switches toground through 04 when motion isdetected. Do not apply an AC volt-age to the circuit and do not attemptto "sink" more than 250 mA at pin 5.

Modifications. If you find that thedetector seems to constantly retrig-

ger at the medium- or high -sensitivitysettings, try adding a 0.14LF non -po-larized capacitor across pins 5 and 6of J1.

The antenna may be tweaked tomaximize the detector's range by sim-ply clipping or adding 1/4 -inch lengthsof wire to the antenna, while checkingfor a range improvement. Typically,the detector should have enoughsensitivity to pick up an adult walkingat a normal pace from 10 to 12 feetaway in a clear room. Note that roomcharacteristics can affect range.

You may wish to shape the pickuppattern of the Microwave Motion De-tector. That's fairly easy to do. Try plac-ing a 12- x 12 -inch metal plate about4 inches behind the Microwave Mo-tion Detector's antenna. That sharp-ens the detection beam forward ofthe plate and reduces it behind theplate. You may wish to experimentwith different sizes of plates placed atdifferent distances from the antenna.

Interface Circuits. The MicrowaveMotion Detector's output transistor,04, provides only a short switched"on" time when motion is sensed. Youhave probably asked the question,how do I make it do something otherthan just make the LED blink?

For instance, you might want to acti-vate an alarm for a period of timeafter the motion is detected. That'seasily accomplished using a solid-state timer, such as the optional UT -5Universal Timer -Oscillator (based on

the popular 555 oscillator/timer) of-fered in the Parts List. The UT -5 can bewired as either a timer or an oscillator.Figure 5A shows how the two circuit'sare wired together.

When the UT -5 is wired as a timer, avariable resistor and several suppliedcapacitors allow you to set "on" timesof from several seconds to severalminutes with a single trigger pulse. Theoutput of the UT -5 can be used to trig-ger a relay or alarm siren.

If you'd prefer to have the Micro-wave Motion Detector drive a sirendirectly, one is offered (the SM-3 PoliceSiren) in the Parts List. The connectionsfor the siren and the Motion Detectorare shown in Fig. 5B. The Siren soundswith an upward wail whenever thePNP transistor turns on, connecting pin4 to the positive supply voltage.

In that circuit, a 10k pull-up resistor istied to the base of the transistor, hold-ing it at cutoff. Whenever the Detectoris triggered, the base of the PNP tran-sistor is pulled to ground, allowing thetransistor to conduct. That places apositive voltage on pin 4, causing thesiren to sound. When the Detectorturns off, positive voltage is removedfrom pin 4 of the siren, turning it off. Thetotal sounding time of the siren isroughly twice the detector's on time.

Figures 6A and 6B show typical tran-sistor driver circuits triggered by theMotion Detector in a similar fashion.The circuit in Fig. 6A is designed todeliver a positive output voltage tothe circuit that follows. The circuit inFig. 6B, with the aid of a second tran-sistor (which is used as a inverter), isdesigned to complete the ground -re-turn path for the circuit that follows.

Figure 7 shows how the MicrowaveMotion Detector can be used to trig-ger a relay. If that scheme is used, justbe sure to note the limits of 50 volts DCor 250 mA at pin 5 of J1. Of course, if ahigher capacity is required, the smallrelay of the circuit in Fig. 7 can beused to trigger one that's capable ofhandling the heavier load.

Another application for the motiondetector might be to connect a taperecorder with a recorded message(or perhaps the sound of a barkingdog) to the circuit using the schemeoutlined back in Fig. 4. Then with therecorder turned on and set to theplayback mode, when the motiondetector senses movement, the re-corder will play the message. III 31

Build a1

/VI NO

Computer -Controlled A/B SwitchOur simple computer -controlled A/B switch makes it easy to

switch any pair of AC or DC signals, including RS -232 signals.

It would seem that there are twogeneral types of computerowners: power users with up-to-

the-minute hardware and those thatget by with slightly older equipment.However, both types eventually endup with a stockpile of hardware; ad-vanced users quickly amass hard-ware because they want to, whilemodest users accumulate wares be-cause they have to upgrade just to

oO remain moderately current.coo Once you end up with enough sur-z

plus hardware, old and/or new, it,u2 seems pretty natural to want to con -;1? nect at least some of it (terminals,

ococo modems, printers) together, if for noI other reason than the versatility thatcoO such a mini -network can provide. Thatzo usually leads to the purchase of acrt., couple of NB switches to permit you

Li,t`21 to configure new set ups on the fly.

4Unfortunately, simple NB switches

ol must be directly operated by the user.= That is pretty inconvenient if you'reLL working from a remote terminal. Be-

sides, your computer should "know"32 what resources you need and auto -

BY JOHN YACONO AND MARC SPIWAK

matically provide them for you-shouldn't it? After all, let's say that youhave two computers or terminalssharing a modem. It would be nice ifthe NB switch between them auto-matically switched to the computerthat's presently running communica-tions software. An NB switch like thatcould even be used in a similar way toallow two computers to share a serialprinter.

The Computer -Controlled A/BSwitch presented in this article is sucha device. In fact, it's one of the mostversatile communication devices thatyou'll find. It can direct just about anypair of AC and/or DC signals. With it,power users can use one to multiplexinput to a data -acquisition board,and more common folks can redirectaudio signals between their home -entertainment components. Furtherstill, just about anything possible with adouble -pole, double -throw switchrated for 350 volts at 120-mA AC or200-mA DC can be done with our NBSwitch.

The device was originally designed

to allow a computer to disconnectone device on a three -wire RS -232communications link and switch toanother device on the line. (Specifi-cally, the unit would interrupt a com-munication between two modems sothe computer could communicatewith one of them). However, after theprototype was working, we found thatby re -wiring the "contacts and poles"of the switch, its operation could bemodified for use in many other ap-plications. In this article we'll show youhow to build, configure, and (if neces-sary) write software to 'NB switch" al-most any pair of signals.

Circuit Operation. The NB Switchcircuit is the electronic equivalent ofthe double -pole double -throw switchshown in Fig. 1. Refer back to that fig-ure (the labels on the poles and con-tacts) from time to time as we discussthe actual NB Switch circuit.

The Switch's schematic diagram isshown in Fig. 2. Note that key points inthe schematic bear the same labels

(Continued on page 34)

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POLE 1 0

crS)03

POLE 2 o

O

_01

DPDT SWITCH

o Al

0 B1

o A2

0 B2

Fig. 1. The A/B switch is very similar toa standard double -pole, double -throwswitch, like the one shown here.

POLE 1 0

Al 0

as the poles and contacts in Fig. 1. Thecircuit's active components arenothing more than a pair of diodesthat steer current through the inputs offour special optocouplers. The op-tocouplers are specially designed fortelecom (standard slang for "tele-communications") applications andconform to the RS -232C standard.

POLE 2 o

A2 0

U1

LCA110

01 1_1N4148 41

U3LCA110

U2LCA110

12

D2Y 1N4148

U4LCA110

o B1

o B2

CONTROL(TO DB25 PIN 20 0

OR TO DB9 PIN 4)

Fig. 2. The A/B switch is very simple, as you can see. Four special telecom switches(U1 -U4) are wired together to form a double -pole, double -throw switch.

POLE 1(+) o

Al(-

POLE 2(+) o

U1

LCA110U3

LCA110

A

D1

1N4148

;

A20

U2LCA110

2

D2Z1N4148

1

U4

LCA110

B1o(_)

B2o )

CONTROL(TO DB25 PIN 20 0

OR TO DB9 PIN 4)

Fig. 3. The version of the A/B switch shown back in Fig. 2 is fine for AC signals, but it

34 can be enhanced for DC operation by rewiring the optocoupler outputs as shown here.

As you can see, each diode is con-nected in series with (and in between)the inputs of two optocouplers. Alsonote that the two series -connectedoptocoupler-diode-optocoupler legsof the circuit are connected back-to-back (i.e., with opposite polarity) be-tween the control input and ground.In that configuration, U1 and U2 willturn on whenever the control voltageis positive, and U3 and U4 will turn onwhen the control voltage is negative.When any one of the optocouplers ison, its output pins short to simulateswitching action.

The circuit wired as shown in Fig. 2 isfine for AC signals, but it can be modi-fied for enhanced DC performance.The necessary modifications areshown in Fig. 3. The revised circuit op-erates Just like the AC circuit, but wiringthe optocoupler outputs as shownpermits them to handle more currentand yields faster switching action.However, you must obey the polarityindicators to wire the device property.

If desired, one pole of the circuitcan be wired to handle AC with theother pole configured for enhancedDC operation. For example, U1 and U3can be wired for AC, as shown back inFig. 2, while U2 and U4 can be wiredfor DC as in Fig. 3, or vice versa,

One benefit of using optocouplersin an application like this is the isola-tion provided between the devicegenerating the control signal and thesignal lines. The incidental isolationwill protect the controlling devicefrom harmful voltages (perhaps dueto a nearby lightning strike) on thesignal line up to 3750 volts! Since thecontrolling device will likely be a com-puter or terminal, that sort of protec-tion is very nice indeed.

Our Application. Take a look at theversion of the A/B Switch circuit shownin Fig. 4. Note that pins 2 and 3 of J3(the connector to modem 2) are con-nected to the poles of the switch. Thatway, the circuit can switch J3 (thepoles) between J1 (which you canthink of as position A) and J2 (or posi-tion B).

The switch receives its control signalfrom pin 20 of J1, which supports thedata -terminal ready (DTR) line on thecomputer. (The reason that we usedthe DTR line will be explained a littlelater.) The DTR line causes one pair ofoptocouplers to turn on when

02

o -

0

U1

LCA110

15

0

3

16

40-17

0

0

5

18

6o --19

0 7

20

o -.-_821

9

22

10

23

011

24

0

0 12

25

13

J1

DB25-F

TO

COMPUTER

1401

1N4148

D2

1N4148

titi

U2LCA110

4

J2

DB25-F 13 >.TO 25 o

MODEM 12.01 24

11

23 0

U3

LCA110

9

10

220

0

210

08-0

20 TO

7 ° MODEM

19o 2-1-

1.-0 .013 ."*.%18 25

5.--0 .012

17 24_0 40

_1_0 0it16 23_o __-__03 10o15 22-0 02 _09-0

14

1

-00

t..."..-

4

U4

LCA110

J3

DB25-F

21-0

-§-o-o20..--6.--07 0-L 19

68 o

17

00

4

16

3

15

2

14

0

0

0

0

00

Fig. 4. Our application circuit for the AIB switch (shown here) allows a computer tointerrupt one modem to talk with another.

positive, and the other pair to turn onwhen negative (remember, RS -232ports use bipolar signals).

If we wanted one of the modems tohave control of the switch instead ofthe computer, we could have usedpin 20 of J2 or J3. While that's not whatwe wanted, you should keep that inmind when sizing up your own needs.

Wired as shown, the state of pin 20from the computer controls theswitch's "position." That allows thecomputer to disconnect modem 2from modem 1 and get its full atten-tion.

If you wish to use the circuit for com-puter communications, please keepin mind that the switch should con-nect transmit lines to receive lines,and vice versa. In other words, theswitch should not attempt to connecttwo transmit pins together, or two re-ceive pins together. See the docu-mentation for your peripherals todetermine their pin functions. By the

way, if you make a wiring mistake,don't worry; RS -232 devices are de-signed so that they cannot bedamaged by such mistakes.

If you will be using DB9 connectorsin place of the DB25's we used, youshould again check the peripheral'smanual to determine the identity ofthe transmit and receive pins. How-ever, we can tell you right off the batthat pin 4 on that connector is for theDTR line, and pin 5 is ground.

Automatic and Programmed Con-trol. The DTR line was chosen to con-trol the switch because that linechanges states as soon as any stan-dard communications software isready to run. That makes the action ofthe switch automatic: if the controllingcomputer runs communications soft-ware, the A/B Switch will change stateswhen the program toggles the DTRline.

For less -automated control of the

LISTING 1

PUSH

PUSH

MOVIN

XOR

OUT

POP

POP

RETA

PUSH

PUSHMOVIN

XOR

OUT

POP

POP

RETB

PUSH

PUSH

MOV

IN

XOR

OUT

POP

POP

RET

C

PUSHPUSHMOVIN

XOR

OUT

POP

POP

RET

0

AXDX

DX, 03FCAL, DXAL, 02DX, ALDX

AX

AXDX

DX, 02FCAL, DXAL, 02DX, ALDX

AX

AXDX

DX, 03ECAL, DXAL, 02DX, ALDX

AX

AXDX

DX, 02ECAL. DXAL, 02DX, ALDX

AX

switch, you could run a program likeone of the ones shown in Listing 1,which are available from the authors(see the Parts List for ordering informa-tion). The four programs shown are(structurally speaking) identical. Theydiffer only in the communications portthey affect. The program shown in Aworks for COM1, the one in B forCOM2, C affects COM3, and D is forCOM4. It should go without sayingthat the program of choice must runon the controlling computer or termi-nal.

Since the programs are almostidentical, we'll just discuss the one in A(If you are not familiar with accessingports, see "Programming Serial Ports,"in the August, 1993 issue of PopularElectronics for more information). Thefirst two instructions simply preservethe contents of the AX and DX regis- 35

36

ters (special-purpose memory loca-tions in the CPU) by "pushing themonto the stack" (storing them in an-other special location). The third in-struction places the hexadecimalnumber "3FC" into the DX register. Thatnumber is the address of the byte thatcontrols the DTR line. In the next step,the value stored in the address in DX(to be literal, the value stored at ad-dress 3FC in hexadecimal) is placed inthe lower half of the AX register (thelower half is denoted "AL").

We now seek to toggle the bit thatcontrols the DTR line. That would bethe least -significant bit of the numberthat was taken from address 3FC,which is now in AL. To toggle that bit,

PARTS LIST FOR THECOMPUTER -CONTROLLED

A/B SWITCH

1.11-tr-1- -1,CA110 opto-MOS telecom1NN itch

1)1, D2 -IN -114X general-purposesmall -signal silicon diode

Printed -circuit hoard. DB9 and orDB "5 connectors as needed (sectext i, ribbon or other suitable4:ahle. IC sockets, ease andmountn hardware, solder_ etc.

you simply take the exclusive xorz of 1and the value in AL, which is per-formed by the next instruction. Nowthat the value of AL has been appro-priately altered, it replaces the oldvalue in 3FC as soon as the OUT in-struction is performed. The DX and AXregisters are then restored to their ini-

tial values by "popping them off thestack" (retrieving them from the spe-cial location mentioned before) andthe program terminates. The state ofthe DTR line will change each time thisprogram is run, toggling the A/BSwitch.

Determining Your Needs. Table 1has been provided mainly to assistyou in configuring your switch forRS -232 applications, although it doeshave limited use for other applica-tions. In particular, the table will helpyou configure the connector wiring.Let's discuss how to fill-in the table.

Start by mentally designating thedevice being switched as the "pole"device, one of the other devices asunit "A" and the remaining one as unit

TABLE 1-CONFIGURATION CHART

DeviceRequired

Connectors(DB9 or DB25)

GroundPins

(5 or 7)

Control Pin(One Entry

Only)

ReceivePin

Numbers

TransmitPin

Numbers

Pole

A

B

Ell --17/H INCHES

Fig. 5. You can use this foil pattern tobuild your own A/B switch, or order theboard, kits, etc. from the listed supplier.

"B." Now examine each peripheral inturn to determine the connector it re-quires (DB9 or DB25) and fill-in the firstblank column of Table 1. The con-nectors you list there should be onyour shopping list for the project.

In the ground -pin column, place a5 in each row with a DB9 connector,

Al o-

POLE 2 0-

A2 o --

and/or a 7 for each DB25 connector.That column now indicates the ap-propriate ground pin for each con-nector. Determine which device (thepole, A, or B) will have control of theswitch. In the row for that device underthe control -pin column, place a 20 ifthe device uses a DB25 connector, ora 4 if the device uses a DB9 connector.That tells you which pin on which con-nector carries the control signal.Leave the other two spaces in thatcolumn blank or cross them out as theother connectors will not generate acontrol signal.

For the last two columns, you'll needto check the documentation for yourperipherals. You must find out whichpin (pin 2 or pin 3) is used for transmis-sion and which is for reception oneach device. Fill the last columns withthose pin numbers accordingly. Ingeneral, computers use pin 2 for

GROUNDS(TO PIN 7 ON DB25 AND/OR

TO PIN 5 ON DB9)

U1

D1

U2

U3

02

-14--

U4

-o B1

---0 POLE 1

---0 B2

0CONTROL

(TO PIN 20 ON DB25 ORTO PIN 4 ON DB9)

Fig. 6. Follow this parts placement diagram to stuff the board for AC use (such fortrue RS -232 applications or audio projects).

Al (-)

POLE 2(+)

A2 (-)

GROUNDS(TO PIN 7 ON DB25 AND/OR

TO PIN 5 ON DB9)

J U1

D1

U2

0CONTROL

(TO PIN 20 ON DB25 6RTO PIN 4 ON DB9)

Fig. 7. To use the switch for DC operation, this method of stuffing the board will yielda device that is faster and capable of handling higher current.

AtakkamatiiiHere's what the inside of the A/B switch looks like. Note the compactness of thedesign.

transmit and pin 3 for receive whenthey have a DB25 connector, and justthe opposite when they have a DB9connector. Most modems are entirelythe opposite: they use pin 3 for trans-mit and pin 2 for receive over a DB25connector, and vice versa for a DB9connector.

Construction. Now that we under-stand how the NB switch works, itstime to build one. A printed -circuit

board should be used. Not only willusing a printed -circuit board save youtime and lessen the chance of wiringerrors, it will also eliminate the pos-sibility of signals interfering with oneanother. If you want to make your ownprinted -circuit board, you can use thefoil pattern shown in Fig. 5. Otherwise,printed -circuit boards, kits, and fullyassembled units are available fromthe source mentioned in the Parts List.

It as we talked about before, you

want to use the NB switch for IC (likeRS -232 and audio applications), fol-low the parts -placement diagram inFig. 6, and wire your connectors usingFig. 1 as a guide. However, to use theswitch for DC operation, the config-uration shown in Fig. 7 will yield a de-vice that is faster and capable ofhandling higher current. (Please ob-serve the proper polarity of the out-puts for DC use.) As mentioned earlier,you can even wire half the unit for ACand the other half for DC signals.

For strict RS -232 applications, youshould follow the parts -placement di-agram in Fig. 6 to stuff the board (thatillustration omits the jumpers of Fig. 7).Connect the grounds and the controlline guided by Table 1. Connect thetransmit and receive lines from thepole device to poles 1 and 2, respec-tively. Connect the receive and trans-mit pins of the A device to Al and A2,respectively (again guided by the ta-ble). Also use the table to help youconnect the receive and transmit pinsof the B device to B1 and B2, respec-tively, and the wiring is complete. Thefinished board can now be installed inany case you like.

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If you experiment with audiocircuits or are involved withaudio -equipment repair, the

Test Bench Amplifier discribed inthis article may be just what youneed. At the heart of the circuit isa high -efficiency, audio poweramplifier, coupled with a fewsupport components. The circuitincludes options for either or bothinternal and external powersources.

The Test Bench Amplifier canbe used to boost low-level signalsenough to drive a speaker, or itcan be coupled with an AC volt-meter (with the speaker muted)for adjusting a circuit for peakoutput. The Test Bench Amplifiercan also be used as a high -fre-quency RF-signal tracer; whenused in that capacity, it rendersAM signals audible. For experi-menters, it can be used as a gen-eral-purpose power op -ampwith a 300 kHz bandwidth.

About the Circuit. Figure 1 shows aschematic diagram of the Test BenchAmplifier (which is designed as a"building block" to allow it to easily beincorporated into a myriad of otherprojects). The circuit is built around anLM386 low -voltage, audio power am-plifier (U1), which is easy to use, stable,and requires very few support com-ponents. The LM386 can deliver up to0.4 watts with harmonic distortion be-low 1%. It can be powered from asupply voltage of anywhere from 4 to12 volts, and has a quiescent currentdrain of 3 mA, making it ideally suitedto battery -powered projects.

The minimum voltage gain of theLM386 is internally set at 20 (26 dB);however, by adding C2 (a 10-lif ca-pacitor connected across pins 1 and8 of U1, shunting an internal resistor),

j2 the voltage gain is increased to 200(46 dB).

o For some applications needing ad-justable gain, a 5k potentiometer

U added in series with C2 will smoothlyset the voltage gain between 20-200.

cc

Capacitor C1 is used as an audio-.

input coupler, which provides DC -volt -4 age isolation from the audio source.cr) Potentiometer R3 serves as a volume

control and sets the input impedanceti.t of the amplifier at 10k. The LM386 has

an input impedance of 50k, If your38 application requires a higher input

Test BenchAmplifierAdd this handy little amplifier

to your troubleshooting!

design -station arsenal

BY LUTHER STROUD

impedance, select a larger value forR3.

A corresponding value resistorshould be inserted in the circuit be-tween U1 pin 2 and ground that re-flects the resistance in the U1, pin -3circuit. It isn't needed if R3 is below25k. The series circuit of R1 and C5

J4

EXTERNAL12V DC

D1 SUPPLY1N34

J1

AUDIO RF C1

IN

0--S2

C8

270pF

'SEE TEXT

Fig. I. The Test Bench Amplifier tvi hich is designed US a "building block") is builtaround an LM386 low -voltage. audio -power amplifier WI I.

10

-714+R5

470KR3

10K

R4 -

provide a bit of bass boost toovercome the limited frequencyresponse of small speakers. Youmay want to shift the valuessomewhat to tailor the sound toyour speaker or leave the partsoff altogether. Don't leave off thesnubber network of C6 and R2,which keeps the LM386 stable bycompensating for the induc-tance of the speaker. Leaving outthat network can result in dis-torted sound, excessive currentdrain, and RF interference.

Capacitor C3 provides by-passing of the internal bias net-work in U1's input -amplifier stage.Capacitor C7 couples the audiosignal at U1 pin 5 to the speaker,while providing DC -path isola-tion. The pin -5 voltage without anaudio signal is at 1/2 the DC supply.Components D1, C8, and R5 forma wideband radio -frequency(RF), amplitude -modulated (AM)detector circuit. Almost any AMsignal applied to the project's in-

put when S2 is in the RF (open) posi-tion will be converted to an audiosignal that can be heard from thespeaker. Note that D1 is a germaniumdiode: a silicon type, such as a 1N914or similar unit, will not work here.

When S2 is closed, shorting D1. au-dio signals pass freely. Opening S3mutes the loudspeaker audio, thusisolating the amplifier so that it can beused alone or with a level -indicatingdevice; SPKR1 can be used as a testspeaker since it is isolated from theother circuitry.

Getting It Together. To minimize cir-

2212

- S1POWER

C4

10

LM388

1N4001

ON

D2 B1

9V

+1 1 n_

SPK11612

R1 OUT IN10K

S3

SPKR

C5 I J202 SPKR

OUT

J3AUDIO

R2 OUT10l2

C7470

tation of U1. If you follow the board -stuffing sequence of the lowest profilecomponents first (resistors and U1, fol-lowed by the capacitors, and finishingwith C7 and R3), the assembly will pro-ceed easily.

The terminals of R3 (the large vol-ume -control potentiometer) and thewiring that connects the off -boardcomponents to the main chassis aresoldered to the oversized pads dis-tributed around the edge of the cir-cuit board. Solder a short wire fromground to the case of R3 to help pre-vent noise from entering the input cir-cuitry through the body of the control.As you assemble your project, be sure

41-8°115/18 INCHES ----101

Fig. 2. The author's prototype wasassembled on a printed -circuit board,measuring about 11/8 by 1/5/16 inches. Atemplate of the printed -circuit layout isshown here.

EXTERNAL12V DCSUPPLY

D2

R4

Fig. 3. Assemble your printed circuit board according to this parts -placementdiagram. When assembling the board, be mindful of the orientation of the electrolyticcapacitors, the diodes, and Ul.

cult size and to cut down on stray in-put/output coupling, the author'sprototype was is assembled on aprinted -circuit board measuringabout 13/8 by 115/18 inches. A templateof the printed -circuit layout is shown inFig. 2. You can etch your own boardfrom the artwork provided or order aboard from the vendor listed in theParts List.

Figure 3 shows a parts -placementdiagram for the author's printed -cir-cuit layout. When assembling theboard, be careful with the polarity ofthe electrolytic capacitors and orien-

to keep all leads short. The input leads(preferably of shielded cable) shouldbe isolated from the output andpower wiring to reduce the chance offeedback problems.

The front panel of the author'smodel is metal so only one groundconnection was needed for all of theinput/output connectors. If your ver-sion is plastic, be sure to add the ap-propriate ground connections. Again,note that D1 is a germanium diode; aheat sink should be placed on thediode's leads, about 1/4 inch awayfrom the body of the unit, when sol-

PARTS LIST FOR THETEST BENCH AMPLIFIER

SEMICONDUCTORSU I-LM386 low -voltage, audio -

power amplifier, integrated circuitDl -1N34 general-purpose

germanium signal diodeD2-IN4001 1 -amp. 50-PIV. rectifier

diode

RESISTORSAll fixed resistors are 1/4 -watt, 5%

units.)R1 -100/0 -ohmR2 -10 -ohmR3 -10.00() -ohm potentiometerR4 -22 -ohm. 1/2 -watt (see text)R5 470,0(X) -ohm

CAPACITORSl6-WVDC, radial -

lead electrolyticCS 0.02-p.F, ceramic -disc or MylarC6 -0.05-µF. ceramic -disc or MylarC7-470-pk, 16-WVDC, radial -lead

electrolyticC8 220-pF, ceramic -disc


SI-SPST-miniature toggle switchS2. S3-SPDT miniature toggle

switchSPKRI-4-inch, 16 -ohm, round

speaker11-13-RCA phono jack14-Miniature phone jackPrinted -circuit materials. enclosure.

battery holder and connector oroptional AC adaptor (see text).knob, wire, solder, hardware. etc.

Note: The following is available fromPershing Technical Services. P.O.Box 1951, Fort Worth, Texas 76101:A partial kit of parts, consisting ofthe printed -circuit board, all board -mounted components, and thebattery connector for $9.50. Priceincludes shipping cost. Texasresidents, please add appropriatesales tax.

dering to avoid possible damagefrom excessive heat.

A Word on Power. The LM386 con-sumes very little power at idle, in-creasing to about 75 mA when drivento full volume. An input signal of about10 millivolts is sufficient to drive thespeaker to full volume. The LM386'slow power consumption makes it suit-able for battery power.

The internal resistance of the bat -(Continued on page 110) 39

Build TheCall Blocker t

Use this circuit and thetelephone company's "CallerID" service to spare yourselffrom unwanted calls.

BY TERRY J. WEEDER

Have you ever been annoyedby a phone that constantlyrings off the hook while you

were trying to devote your attentionelsewhere. To make matters worse, itseems that most of those calls arefrom people you did not wish to speakwith. You can, and at times probablyhave, broken down and turned off thebell on your phone just to get a littlepeace and quiet. The only problemwith that solution is that you are thencompletely cut off from the outsideworld and even important calls fromfriends or family can't get through.

There are a number of companiesthat are now offering a device that willdisplay the number of the calling par-ty for you to view prior to picking upthe telephone (called "Caller ID").That is nice, but even if you can deter-mine that you don't want to answerthe call, your peace has alreadybeen invaded by the ringing of thetelephone and the act of getting upto see whose number is displayed onthe screen.

There is a better solution. The Caller0z Block described in this article is a de -a= vice that connects between your tele-O phone and the telephone wall jack

and prevents the phone from ringing

ococo unless the telephone number of thei person calling matches one of the0O numbers you have entered into its

memory. For maximum versatility, ancc

15 answering machine can be placed`i' on the same line to handle those calls

uj. that do not qualify to ring the phone;a> those can be dealt with later on ata,

your own convenience. The unit canL.L.zr' be used in conjunction with a Caller -

ID display unit40 A list of up to 62 different telephone

numbers can be easily entered intothe non-volatile memory using atone -dialed phone without changingthe initial hook-up of the Caller Blockunit. The memory is broken up into twoseparate directories with an externalswitch used to select which one will beactive. One of the directories can beused to hold a large list of numbersfrom people whom you wish to allowto ring your phone on a daily basis,while the other directory can be usedfor a more selective list of the mostimportant callers for those times whenyou want more privacy. Because theCaller Block's memory is stored in anEEPROM, all telephone numbers pro-grammed into memory will stay thereeven if the power is interrupted.

About Caller ID. With the recentimplementation of the service knownas CND (Calling Number Delivery) onmost major phone networks, it is nowpossible to receive data about thecalling party prior to picking up thephone. That data is what is used bythe many caller ID units now hittingthe market, and it is what makes thiscircuit possible. CND is a subscriberfeature that transmits data (date,time, number) about the calling partyduring the silent period between thefirst and second ring. You must sub-scribe to the service or no data will bereceived, but once you have sub-scribed, you will receive the data onall calls regardless of whether the call-er subscribes or not. Call your localphone company to get more infor-mation about getting CND on yourline.

The date and time information issent on all calls, but the actual calling

number will only be available fromparties who are calling from withinyour area. At the present time 'yourarea" consists of local calls that ori-ginate from where CND is available,but will soon be set up to include long-distance calls made from whereverCND is available. As more and moretelephone companies upgrade theirequipment, the list of callers outside"your area" will continue to decrease.

The actual data sent over the tele-phone line is a burst of modem tonesusing 1200 Hz as a logical 1 (mark) and2200 Hz as a logical 0 (space), Thedata stream begins with a "Channel -Seizure" signal that consists of a blockof 300 alternating mark/space bitsused by the receiver as an indicationthat data will soon follow. That is fol-lowed immediately by 180 mark bits;that allows time for the receiver to setup for the first word. Each data word is8 -bits long (least significant bit trans-mitted first), and is preceded by a startbit (space) and followed by a stop bit(mark). In addition, up to 10 mark bitsmay be added between words.

The portion of the message thatcontains the calling number is pre-ceded by a "type code" of 04(he%).The first word to follow is a "length"word that indicates the number of thewords in the message. Following thelength word, the date, time (in 24 -hour format), and the telephonenumber is sent in ASCII format. Finally,a "checksum" word is sent. For exam-ple, 01-15-94 2:30pm 513-555-1212would be sent as follows:

<type > <length >01159414305135551212

<checksum>

I FIRST RING 0.5I 2 SECONDS ISECI

RING DETECT (12)

101 DATA

0.5 SECOND RING

ISECI 2 SECONDS I

CARRIER DET (13)

DATA OUT (15)

POWER UP (7)

DATA

Fig. I. The timing relationships between the data sent over the phone line (top trace),and the output pins of U2 (Motorola's new calling line identification receiver chip) areshown here.

Circuit Theory. The circuit is madepossible by U2, Motorola's new Call-ing -Line Identification Receiver chip(part no. MC145447), which receivesthe modem -like tones from the tele-phone line and decodes them. Figure1 shows the timing relationship be-tween the data on the phone line(top), and the various output pins ofthat IC.

When a ring pulse is detected onthe phone line, the RING DETECT pin (pin12) goes low. About Y2 second afterthe end of the ring, the channel -sei-zure signal (shown as 0101) causes theCARRIER -DETECT pin (pin 13) to go low. The

"1" shown after the "0101" representsthe series of marks that are sent toallow the microprocessor to set up toreceive data, The DATA -OUT pin (pin 15)goes low when receiving the very firststart bit, which is the space followingthe series of marks, and continues tooutput the decoded data, serially, atTTL levels. The POWER -UP pin (pin 7) was

designed for use in battery -operatedcircuits to conserve power and is notused here.

The schematic diagram for theCaller Block is shown in Fig. 2. Themodem -like tones are coupledthrough C3, C4, R1, and R2 to pins 1and 2 of U2. The metal -oxide varistor(MOV1) protects the circuit from high -voltage spikes, such as those pro-duced by lightning strikes. The ringpulses are coupled to the RING -DETECTpins (pins 3 and 4) of U2 via C1, C2,BR1, and R3-R5. The relay (K1) is usedto connect/disconnect the telephonefrom the phone line. The optoisolator(U5) monitors the current flow through

the telephone and pulls the voltageon R10 low when the phone is lifted offhook. Transistor Q2 is used as a switchto turn LED1 on and off, which indi-cates when the circuit is in the pro-gramming mode.

Power is taken from a 9-VDC walladapter and is regulated by U6, a low -current 5 -volt regulator, and filteredby capacitors C13-C15. To allow thetelephone's keypad to be used, cur-rent is supplied to the telephonethrough R9 when the circuit is in theprogramming mode.

The heart of the circuit is U1, aPIC16C55 EPROM -based 8 -bit CMOSmicrocontroller manufactured by Mi-crochip. That microcontroller has one4 -bit and two 8 -bit I/O ports, with eachI/O pin being configured separatelyas either an input or output throughsoftware commands. The chip has512 x 12 bits of EPROM memory tohold the operating program and 32x 8 bits of data RAM used for workingregisters. A block diagram of U1's op-erating program is shown in Fig. 3. Forthose who wish to program their ownPIC, the .ASM and .OBJ code files areavailable in the PE Library on thismagazine's BBS (516-293-2283, 8N1,up to 9600 bps), or a pre-pro-grammed PIC16C55 microcontrollercan be purchased from the sourcementioned in the Parts List. A non -pro-grammed chip along with a PIC pro-grammer can be obtained from Digi-Key (701 Brooks Ave. South, FO, Box 677,Thief River Falls, MN 56701-0677) andother sources.

The output pins of U2, as referred toin Fig. 1, are connected to U1 through

the upper half of port B. U1 monitorsthe RING and CARRIER -DETECT pins and

when a call is detected, U1 reads thecalling party's number from U2, whichis output serially through pin 15, andstores it in an internal register. The tele-phone number is then compared tothe group of numbers stored in U4, a93LC56 2K serial EEPROM also man-ufactured by Microchip. The 93LC56uses a 4 -line (CHIP SELECT CLOCK, DATA IN,

and DATA OUT) interface, which is con-nected to port A of U1. After a high isdetected on the CHIP -SELECT pin, data isthen transferred to and from the93LC56 on the positive transition ofthe CLOCK pin. Each read or write func-tion is preceded by a start bit, an op -code (identifying the function to beperformed, i.e. read, write, etc.) an 8 -bit address, and the 8 bits of data thatare being written to, or read from, thataddress. Immediately preceding andfollowing all write operations, the mi-crocontroller (U1) sends instructions tothe 93LC56 that enables/disables thewrite function, thereby protecting thedata afterwards.

When entering the programmingmode, the telephone is disconnectedfrom the phone line by the relay (K1)and is connected to the analog input(pin 7) of U3, a DTMF receiver man-ufactured by Motorola. Integrated -circuit U3 is used to decode the tonesemitted by the telephone and placeits 4 -bit word equivalent on the lowerhalf of U1's port B. As each number isentered by the telephone, U1 readsthe 4 -bit word on port B and writes it tothe EEPROM (U4) using port A. If thejumper (JU1) is installed, the micro -controller is configured to omit allarea codes (both in programmingand receiving), thereby simplifyingthe programming operation and al-lowing more numbers to be enteredinto memory.

Construction. The whole circuit, ex-cept for the switches and LED, fits nice-ly on a double -sided printed circuitboard. The artwork is provided in Fig. 4for those who wish to etch their ownboard, or you may purchase a pre -etched and drilled PCB from thesource mentioned in the Parts List.Once you have obtained your board,identify the component side of the PCboard (which is marked), then refer tothe parts -placement diagram shownin Fig. 5 to begin construction. 41

TOTELEPHONE 4

WALL JACK

TOTELEPHONE

TO9V

ADAPTER

,J1RED0

J2

GRN0

J3RED0

C3470pF

C4R2

470pF 10KA

CI0.2

MOV1130 VRMS

BRI1A

200PIV

itC20.2

0

J4GRN0

+5V

K t

5V

DI1N41411

R9750

+5V

01

2N4401

R11

10K

R1010K

IAAR1

10K

R3470K

R418K

-1. R5VA

- 15K

+5V C6 +5V

0.1

1

2

3

AAAR6

11 16

U2UC145447

10 6

13

R82K

R72K

12

15

9

7

270KCS 1.

0.22

aa

D5 ,D6

D7 ,

Ex)

N01 1

,D2D3 13

12

10

7

+5V

C10 0.1

28

18

19

115

PS2506-1

0

U6TILOS

+51/

S1

BYPASS

21

R13R12 10K10K

20

+5V

C15T0.1

C1347

C14T 0.1

S2

DIRECTORYCry 0----

XTAL13.58MHz0 /---

C12 C I 1

15pF 150

U1

PiC16095

VDDRTCC

MCLR

RCO

RC1

RC3

RC2

OSC2

OSC1

R80RB1

RB2RB3

RB4RB5

RE16

RB7

RAO

RA1

RA2RA3

RCS

RC4

VSS

10

11

12 D2

DO

01,

13

14

15

16

D3

D4,D5

D617 D7

6 PO

P1

8 P29 P3

23

C9.01

D1

D2

D4

V00

GT

EN

08 XEN

0

XIN

ANO GND

U3MC145436

+5V

4

5

3

C70 1

PO 1

CSP1 2

CLKP2 3

DIP3 4

02

VCC

NU

ORG

VSS114

931056

5V

1]6

2N4401

22

27 4

Fig. 2. Microcontroller (UI) reads the calling party's telephone number from U2,compares it to the list of user -entered numbers stored in U4 and closes the relay (KI) ifa match is found.

Start by installing and soldering theIC sockets for U1 through U4. Solder U5and U6 directly to the board, using thecomponent orientations shown in Fig.5. Mount the rest of the componentsto the board, paying particular atten-tion to C5, C13, D1, BR1, 01, and 02 toprevent installing them backwards.When soldering the crystal (XTAL1),leave a small space between the bot-tom of the crystal and the PC board.There is a chance that the metal caseof the crystal could short the two sol-der pads together if it is pushed flushagainst the board when soldering.

At the time of this writing, CND (Call-ing Number Delivery) was not avail-able on calls originating outside thelocal area code (at least not in theauthor's community). As mentioned,with JU1 in place, the microcontrolleris configured to ignore the areacodes on all incoming calls; only thelast seven digits of each number in

42 your list of telephone numbers will

have to be entered into memory. Inthe future, when the CND link hasbeen extended nationwide, you cansimply remove the jumper and re-en-ter your list of telephone numbersusing their area codes.

After all components are solderedto the board, inspect both sides forsolder bridges or cold solder joints,which appear as dull blobs of solder,and correct if necessary. Carefullyplug U1 through U4 into their sockets,making sure that they are orientedcorrectly. Make sure that all IC's areseated correctly with no pins bent ormisaligned.

Cut four pieces of insulated hook-up wire, about 6 -inches long, and twistthem into pairs, then solder one endof each pair to the board terminalslabeled S1 and S2. Cut two pieces ofdifferent -colored wire to the samelength, and twist and solder them tothe LED terminals on the board (takecare to note which color is connected

+5V

R1410K

R1510K

C.)LED1

'SEE TEXT

C8 10., T

+5V

to the cathode terminal and which isconnected to the anode).

The two telephone cords that pluginto the telephone and into the walljack can be made from a singlephone cord with a modular plug oneach end (such cords are availablefrom Radio Shack and elsewhere) bycutting it in half. Label one cord TO WALLJACK and solder its red and green wiresto terminals J1 and J2. Label the othercord TO TELEPHONE and solder its red and

green wires to the terminals labeledJ3 and J4. Note that the yellow andblack wires on both cords are notused. Use an AC wall adapter with anoutput of 9VDC, such as the RadioShack 273-1455; cut the coaxial plugoff the end of the output cord andsolder the " + " and " - " wires to theappropriate terminals on the board.

The size of the PC board and thelocation of the mounting holes aresuch that it will mount directly into aplastic enclosure available from Digi-

START

INITIALIZE PORTS

OPEN RELAY

GET 8 -BIT WORDFROM U2

YES

YES

NO

GET AND DISCARDLENGTH, DATE, AND TIME

GET 10 DIGITNUMBER FROM U2

READ NUMBERS FROMDIRECTORY 'A"

NO

CLOSE RELAY

GET AND DISCARDAREA CODE

GET 7 DIGITNUMBER FROM U2

READ NUMBERS FROMDIRECTORY "B'

Key (part no. SR131G-ND), but it is notcritical that you use that box. Any en-closure large enough to accommo-date the board and front -panelmounted switches will do.

Drill a hole in the top of the en-closure for the LED, and two holes for

CLOSE RELAY

WRITE NUMBER TODIRECTORY B

NO

TURN ON LED

GET NUMBERFROM PHONE

NO

GET NUMBERFROM PHONE

YES

WRITE NUMBER TODIRECTORY "A'

TURN OFF LED

CLOSE RELAY

NO

CLOSE RELAY

Fig. 3. This flowchart shows how Ills operating program works. A pre-programmedmicrocontroller is available from the source mentioned in the Parts List.

NO

the switches. Label the enclosure ac-cordingly, using dry -transfer lettering(available from art or office -supplystores), and mount the switches andLED to the enclosure and connectthem to the appropriate points on theboard using the wire pairs you pre-viously installed. Mount the PC boardin the enclosure and cut three slots inthe seam of the plastic case for thepower cord and the phone cords.Finish by running these wires outthrough the slots and assembling thetwo halves of the plastic case.

Operation. You must call your phonecompany and subscribe to the CallerID service for your Caller Block unit to 43

44

E N 0

It 00I`/Tc7-0-01 0

0 0

00 00000 0000000000 1111 0

001000000 00010 et 0 :71PL

3 1/8 INCHES

7 I-

0O 00

_J

H [4Fig. 4. The Caller Block is assembled on a double -sided PC board. Both sides of thehoard are shown here full size.

PARTS LIST FOR THE CALL BLOCKER

RESISTORS(All resistors are I:4 -watt. units)RI. R2. RI0-R15-10.000-ohmR3 -470.000 -ohmR4-18.(XX)-ohmR5-15 ,(KM-ohmR6 -270,000 -ohmR7, R8 -2000 -ohmR9 -75 -ohmRI6--- I50 -ohm

CAPACITORS(.2-0.2-0-, Mylar

C3, C4--470-ph ceramic disc16-WVDC. tantalum

C6-04. CIO. (14. C15 -0.1-11.F.!Mylar

('9-.0141.1-. lar('II, C12---15-ph ceramic disc

16-WVDC. electrolytic

SEMICONDUCTORSUl-PICI6C55-XT P.

microcontroller, integrated circuitI Microchip

U2-MCI45447,identification receiver, integratedcircuit (Motorola)

U3--MCI45436, DTMF receiver,integrated circuit (Motorola)

U4-93LC56, serial EEPROM,integrated circuit (Microchip)

U5-PS2505-1 optoisolator orequivalent, integrated circuit

U6-781,05 low power 5 -voltregulator. integrated circuit

Ql. Q2-2N440I general-purposeNPN silicon transistor

DI-IN4148 general-purpose silicondiode

LEDI-Red light -emitting diodeBR 1-1.0-amp. 200 -PI V. fullwave-

bridge rectifier


KI-5-VDC. DPDT relayMOVI-130 VRMS, metal -oxide

varistorXTALI-3.58-MHz. TV colorburst

crystalSI. S2-SPST toggle switchEnclosure. PC board. IC sockets,

wall adapter (9 -volt DC. see text).telephone cord with modularplugs, hook-up wire, solder.hardware. etc.

Note: The following items areavailable from WeederTechnologies. P.O. Box 421,Batavia, OH 45103: A double -sided etched and drilled PC board(WTCBL-13), $11.50: a kit of allboard -mounted componentsincluding a pre-programmedPICI6C55 (WTCBL-C), $34.50: apre-programmed PICI6C55 only(PIC-CBL), $18.00. All ordersmust include an additional $3.50for shipping and handling. U.S.and Canadian orders only, please.Ohio residents must add 6% salestax. Please call your localtelephone company to verify that"Caller ID" is available in yourarea before ordering.

3 1/8 INCHES

function, It may take several days tobe hooked up, depending on yourlocal telephone company, so it is agood idea to do that in advance.

Plug the phone cord labeled To wALLJACK into the telephone jack on thewall and plug the cord labeled TO TELEPHONE into your telephone. Plug the ACadapter into the wall and the CallerBlock should be ready to go.

Using a piece of paper, make twolists of telephone numbers, one foreach directory. As explained earlier,one directory can contain a large listof all your friends and family whomyou wish to allow to ring your phoneon a daily basis. The other list can bemore selective, listing only those mostimportant callers for times when youwant more privacy. Each directorycan hold up to 255 characters includ-ing the "#" sign, which is used to sepa-rate the telephone numbers. Thatworks out to 31 different 7 -digit tele-phone numbers or 23 different 10 -dig-it numbers (those with area codes).

When programming the CallerBlock, you may enter just a prefix suchas "752", and all numbers beginningwith "752" (752-0001 through752-9999) will ring your phone. Thatcomes in handy if you wish the peoplein your neighborhood to be includedin your list but you do not want to enterall their numbers separately. Also, ifthe unit is configured to use the areacode, entering a three digit prefix will

TOTELEPHONEWALL JACK

TOTELEPHONE

TO 9VADAPTER

oJ2

0J1

0J4

o J3

Fig. 5. Use this parts -placement diagram when assembling the PC board. The jumper(JUI ) should be installed if you do not want to include area codes with each telephonenumber stored in memory.

include all callers from that particulararea code.

Note: If you configured your CallerBlock to use area codes (omittingJU1), all numbers entered into memo-ry must include the area codewhether local or long distance. Thenumber "1" is not required at the be-ginning of long distance numbersand should not be used when enter-ing your numbers.

To enter your list of telephone num-bers into memory, first verify that thebypass switch (S1) is inactive (open),then switch S2 to the desired directoryand pick up the phone while holdingdown the "#" button on its keypad.The LED should light indicating thatyou are in the programming mode.Release the "#" button and begin en-tering your telephone numbers withyour phone's keypad. At the end ofeach telephone number press the"#" button, and after all numbershave been entered press the "*" but-ton to end programming. An examplewould be as follows (note that

hyphens have been added for claritybut obviously would not be entered):

555-1212#555-0110#231-0675#753-9607#752#444-2276#786#225-35#831-097#*

Note, that a prefix can be anylength less then a full number. If youmake a mistake, press the "*" buttonto end programming then hang upthe phone and begin again. If the "#"button is accidentally pressed twice ina row while programming, it is inter-preted as being a prefix consisting ofno numbers, thereby allowing anytelephone number to be acceptedand ring the phone. After you havefinished entering numbers into thecurrent directory and ending bypressing the "*" button, simply hangup the phone, switch S2 to the seconddirectory, and repeat the process withyour second list of numbers.

If you run out of memory while en-tering your telephone numbers, theprogramming mode is automatically

LED1

S2

terminated. Count the characters inyour list, including the "#" sign be-tween the numbers, and verify that itdoes not exceed 255 characters.

If you have a problem with certaincallers whose numbers' you've en-tered into memory but can still notring your telephone, call your phonecompany and ask if their exchange isincluded in your Caller ID network. Al-though the people calling you do nothave to subscribe to Caller ID as youdo, their exchange must be in theCaller ID network in order to ring yourphone.

Closing the BYPASS switch (S1) allowsyou to bypass the circuit and allow allcalls through as normal. When pickingup the telephone to make a call, thecircuit is also automatically bypassed.Note, that power must be applied tothe Caller Block or the telephone willbe disconnected from the phone line,even when the unit is switched to by-pass or when trying to make a call If

you wish to use an answering ma-chine, place it in the line ahead of theCaller Block. 45

to

to

rn

ll

46

BY SCOTT HENDERSHOT

pC -based analog -to -digital (A/D) con-version is a hot topic

these days. So there are nu-merous A/D adapter boardsavailable for PC's. However,most of them seem to begeared toward digitizingwaveforms in the megahertzfrequency range and, as aresult, cost megabucks. How-ever, for many types of sig-nals, high-speed, high -costconversion is not necessary.For example, unless you aredoing some special, re-search you probably do notneed to take temperaturemeasurements at 50,000samples per second even ifhigh resolutioti is a must.

So what data -acquisitiontechnique will yield a costsavings by permitting slow,high -resolution analog -to -digital conversion? Voltage -to -frequency (or V/F) con-version, which is very usefulfor digitizing steady-state orslowly changing DC signals.Voltage -to -frequency con-version is seldom discussedin texts on analog -to -digitalconversion, probably be-cause of its generally slowconversion speed. However,it offers some important advantagesover other techniques. For example,its inherent integration of the input sig-nal makes it more immune to noise.Also, high -resolution is more easily ob-tainable with this form of conversion.

You can take advantage of all thatby building the project presented inthis article. It is a 0- to 12 -volt analog -input adapter that uses the AnalogDevices AD654 voltage -to -frequencyconverter IC. That IC has several fea-tures that make it very attractive to theexperimenter including extremely lowcost, low support -component count,and the ability to be powered by asingle 5 -volt supply. The circuit uses aPC's game port as the interface, has aresolution of 1 part in 12,000, and canbe built for about 15 dollars.

As you read through this article youwill probably see opportunities to

Add a DVM

DVMDigital Voltmeter Soft, are

ffi

o your PCTurn your PC into a digital voltmeter for automated testing, torecord readings over time, and more.

customize this project for your ownneeds. Fortunately, there are manyparameters that can be changed(which we'll explore in moderate de-tail) once you understand how eachaffects the overall system. The intenthere is not to fully illustrate the AD654,but to present the basics of a com-plete ND converter project and allowthe experimenter to use it as a startingpoint for other designs.

The Circuit. Looking at the sche-matic diagram in Fig. 1, you will noticethat there are only two active compo-nents: U1 and U2. Taking U1 first, it is anAnalog Devices AD654, which is theactual analog -to -digital converter.More accurately, it is a voltage -to -fre-quency converter; it accepts an ana-log input voltage (the differencebetween its - V, and +Vin inputs)

and generates a squarewave. Thechip was chosen for its ease of imple-mentation. For example, it can bepowered from a single supply such asthe 5 volts available from a PC's gameport. The full-scale input range can beset by a single resistor, and it has alinearity of 0.1% or better.

Integrated circuit U2 is an LM324single -supply general-purpose op -amp with an input impedance of250-megohms set up as a voltage fol-lower. It provides input protection andbias -current compensation for theAD654. Other single -supply op -ampswill also work, however the LM324 isreadily available while others mightbe difficult to find.

As mentioned, the output of U1, andtherefore the circuit, can be con-nected to any digital input on the PC.For example, one of the parallel -port

W.

VIN

0 0

R7

1052

R1

10MEGD2

11t4458

R21MEG

e- r-+

R410K

J2

0

+5V

C30.1

2

four

4

GND

R310MEG 01

1N458

4

(12-a

11

R510K

four +VsU1

AD654

GND +C2

VIN

LIA824 IC6R6 t1K

R8100?

C2.01

C4 -.1

R92.2K

C510

LED1 0

Fig. I. Effectively, the adapter can be broken down into a voltage -to -frequency adapterwith a signal-conditioner/protection-circuit at the front end.

handshaking inputs or one of the but-ton inputs on the game port. We de-cided to use the game port (via J2)because of its 5 -volt outputs. Bear inmind that the AD654 has an open -collector output capable of sinkingabout 10-mA. So the input of the com-puter port you use must be tied to 5volts through a pull-up resistor residingeither inside the computer or theadapter circuit. The inputs on thegame port are tied high within thecomputer, so we don't need to worryabout this. If you decide to use an-other input port (such as the parallelport), you will need to deliberately pullthe signal line high.

The AD654 can accept input volt-ages of up to 4 volts less than thepower supply. Since we will be usingthe PC's 5 -volt supply, our maximuminput voltage will be 1 volt. In order toextend this range to 12 volts, a voltagedivider is used ahead of both U1 andU2 to scale the input down to 1 volt.Looking at the schematic you will seethe divider is made up of R1, R2, andR3. Resistors R2 and R3 are in paralleland have a total resistance of 909,000ohms. In series with the 10-megohmresistor that provides a 12:1 attenua-tion of the input signal. Componentvalues are not critical and 5% typesare okay to use. Any accuracy prob-lems will be compensated for in soft-ware. Using the attenuator will reducethe input impedance to 10 megohms.This is about the same as most digitalvolt meters and should not be a prob-

lem for most applications.Once supplied with appropriate in-

put, the AD654 requires the additionof only two passive components inorder to operate. These are a timingcapacitor (C2) and a scaling resistor(R6). The scaling resistor sets the over-all input -voltage range and the ca-pacitor sets the full-scale frequency.

The scaling resistor converts the IC'sinput voltage to a current. That resistormust be selected to provide 1 mA ofinput current at the full-scale inputvoltage. For example, our full-scale in-put voltage will be 1 volt, so by Ohmslaw, the resistor needs to be 1k. Youmight be tempted to substitute an-other resistor to achieve some otherfull-scale voltage. That is fine as longas you observe the design param-eters that follow.

The timing capacitor should be se-lected by using the following formula:

C = V/(10Rt)

where f is the full-scale frequency (10kHz for our design), R is the scalingresistor, and V is the full-scale inputvoltage, which is 1 volt for our circuit.This sets the value of our timing ca-pacitor, C2, at 0.01-pf It is possible todesign the circuit to operate at ahigher frequency, but most PC's willnot be able to keep up with the signal.This frequency is suitable for PC/XT andlater machines.

The only other criteria for the ca-pacitor is a stable temperature coeffi-cient. The following types are recom-

PARTS LIST FOR THEANALOG -INPUT ADAPTER

SEMICONDUCTORSUl-AD654 voltage -to -frequency

converter, integrated circuitU2-LM324 operational amplifier,

integrated circuitDI, D2-IN458. or 2N5089 diodeLED I-light-emitting diode

CAPACITORSCl-not usedC2-0.01-p,F polypropyleneC3. C4, C6 -0.1-11F monolithic

bypassC5 -1011.F. 6-WVDC, Tantalum

RESISTORS(All fixed resistors are Vs -watt, 5%

units unless otherwise indicated.)RI, R3-10-megohmR2-1-megohmR4. R5 -10.000 -ohmR6 -1000 -ohmR7. R8 -10 -ohm, 1/4 -wattR9 -2200 -ohm


11-not used12-Male DB-I5 connector

Printed -circuit board or perfboard,I4 -pin IC socket, 8 -pin IC socket,connector hood, binding posts,hardware, solder, wire, etc.

The following are available fromSCOTTECH (485 Old RidgeRoad. Webster, NY 14580; Tel.716-671-8783. FAX716-787-2788): Software, $5.00;printed -circuit board, $12.00; a kitincluding software, circuit board,Ul, C2. and the IC sockets,$25.00. Add $3.00 shipping withyour order, and NY State residentsmust add 81/2% sales tax.

The AD654 voltage to frequencyconverter is available from NewarkElectronics (N. Ravenswood Ave.,Chicago, IL 60640-4496; Tel.312-784-5100)

mended: polystyrene, NPO ceramic,and polypropylene. Other types arenot recommended due to their poortemperature stability.

With the output of the AD654 con-nected to a TTL-input port and tied to5 volts. it will generate a 0 -5 -voltsquare -wave. The frequency of thesquare -wave will be linearly propor-tional to the input voltage. The com-ponent values chosen will give us c10 -kHz square -wave at full scale. That 47

21/le INCHES

Fig. 2. It is recommended that you usethis foil pattern for making your ownadapter circuit hoard.

translates to 10 Hz per millivolt. Actu-ally, because of the voltage divider,our scaling will be 0.833 Hz per milli-volt. However, because of the way thesoftware measures the voltage, it is

not necessary to have a 1 -to -1 cor-relation of Hz to millivolts.

Diodes D2 and D1 provide inputprotection in case of polarity reversalor over -voltage. These should beSchottky or low -leakage type diodes.

Resistor R5 provides bias currentcompensation to the AD654. Togetherwith C6 they form a single -pole filterwith a time constant of 1 millisecond.That helps suppress noise at the inputto prevent false readings. Rememberwe are measuring voltage changesof only 100 microvolts.

A 10-µ,F tantalum capacitor (C5) atthe point where the supply voltage

coco enters the circuit quiets any power -

supply noise to produce a cleansquare -wave output. The additional

o resistors and 0.111F monolithic capac-itors shown in the schematic just de -coupleLU the IC's.

w

Software. Precise timing software isthe key ingredient to using this tech -

u_nique on a PC. Most high-level lan-guages are inadequate for microse-

48 cond timing so the software must be

Fig. 3. If using a PC hoard based on the author's foil pattern, stuff it as shown here.Note the polarity of the diodes, CS, and LEDI as you proceed.

written in machine language, Tospare you the agony of assembly -lan-guage programming, all of the nec-essary interface routines completewith manuals have been made avail-able by the author (see Parts List).There are demo programs that showyou how to call the data -acquisitionroutines from C and BASIC, and linka-ble object modules for both lan-guages. There is also a quick librarythat can be loaded into the Quick -BASIC environment.

A single call is all that is necessary toacquire a sample from the adapter.The functions return a floating pointvalue that is already adjusted for thecurrent calibration. The supplied func-tions have a conversion time of about17 ms. That will provide about 60 sam-ples per second. This sample rate waschosen to reduce the effects of 60 -cycle noise. Each sample will last ex-actly as long as one cycle of a 60 -Hzsinewave, therefore fully integratingany noise. The functions have beentested with Microsoft BASIC andQuickBASIC, Microsoft C and Quick C.I cannot guarantee their com-patibility with other compilers.

Along with the sample programs,there is a program called DVM,EXE.This program is a digital voltmeter ap-plication. When run, it expects to finda square -wave signal on the button -0input of the game port. If the signal isthere, the program reads the fre-quency and displays the calculatedvoltage. Complete instructions forusing DVM are provided with the soft-ware.

For convenience, all this software

+ 5VDC

BUTTON 0

POSITION 0

GROUND

NC

POSITION 1

BUTTON -1

+5VDC

+5VDC

BUTTON -2

POSITION -2

GROUND

POSITION -3

BUTTON -3

+5VDC

GAME PORT ADDRESS

201H BASE ADDRESSBIT 4 BUTTON 0BIT 5 BUTTON 1BIT 6 BUTTON 2BIT 7 BUTTON 3

Fig. 4. You can use the additional buttoninputs on your PC's joystick port to receivedata from multiple adapters.

has been posted on this magazine'sbulletin board. The telephonenumber is 516-293-2283 and the pro-tocol is no parity, 8 data bits, and 1stop bit. However, if you go this routeyou'll have to do without the manuals.[At the time this was being preparedfor publication a Windows version ofthe software was due to be released.Contact Scottech for further informa-tion-Editor]

Construction. The circuit should beconstructed using a printed -circuitboard. You can obtain one from theauthor (see Parts List for ordering infor-mation), etch your own using the pat-tern in Fig. 2, or design your own.

If you design your own PC boardconsider the following precautions.


How to Split a Power SupplyWhat can you do if you need a dual supply,

but only have a single -supply source? Build our splitter, of course!

BY MICHAEL A. COVINGTON

What do you do if you have asingle output power supplyand you need a dual one?

That's a problem that often arises withop -amp circuits. Many op -amp con-figurations require a split supply, butmost lab -type power supplies haveonly a single output. That means youcan't use them to power op -amp cir-cuits . or can you?

You can if you use the supply splittercircuit presented here. It neatly splitsits input voltage. -anywhere from 8 to30 volts in half to give you two equalvoltages of opposite polarity.

How it Works. The key idea is that ina dual -supply powered circuit,

INPUT

REGULATOR

01 LOAD1

a

'GROUND'

LOAD2

i' l Iransistor% QI and Q2 regulaterl e "ground mirage to hold it tnidrtavh 'Ilteen the t and terminal vvItages.

The splitter connects between the existing power supply and the circuit to be powered,in this case an op -amp circuit.

"ground" is midway between thepositive and negative supply volt-ages. Accordingly, all we have to do isderive a regulated voltage halfwaybetween V + and V - , and we'redone.

Unfortunately, ordinary voltage reg-ulators can't do the job. The reason isthat they can only source current, notsink it. However, the ground terminal ina dual supply may have to eithersource or sink current, depending onwhich half of the load is drawing morecurrent at the time.

Figure 1 shows the idea behind ourspecial two-way voltage regulatorthat does what's needed. If Load 1

and Load 2 are drawing exactly equalcurrents, then the voltage betweenthem is already halfway between V +and V - , so the regulator need not doanything. If the currents drawn by thetwo loads are different, then either Q1or Q2 will conduct extra current tomake up the difference. That way theground voltage stays exactly halfwaybetween the positive and negativerails.

Figure 2 shows the circuit. In it, R1 andR2 divide the input voltage in half Op -amp U1 reproduces that voltage at the"ground" output terminal by makingeither Q1 or Q2 conduct as much asnecessary. Capacitors C1 and C2 49

INPUT OUTPUT

R1

10Kt

012N3055

Cl100

0 cGND

D1

2.5A X U1 6

50PRV 2 741

4+02 C2

R2 s MJ295510K

Fig 2. In the actual circuit. a 741 (17 -amp (L'h is used as the regt huor that controlsthe action of Q1 and Q2 to maintain a voltage between the input ty Itages

The smaller components are mounted on per/hoard while the capacitors are mounteddirectly to the binding posts. They, in turn, are mounted In holes in case just like thetransistors.

hold the output voltage steady whenthe load changes suddenly.

PARTS LIST FOR THEPOWER -SUPPLY SPLITTER

SEMICONDUCTORS1.1- I \I' 4 I opciational amplifier,

inter:rated tirtuitQI _ rs, il)+S silicon NPN pov.er

11.1111

Q2 1124 5 wihcnn PNP rowerttUtiNtstin

1)1- 2 s amp, SO Pl reetiher diode


RI, R2 10,000 -ohm watt, S',resistor

I00 p.F. 20 Vv DC.eicttrui\tic eapatitor

Enk. tosure hirkfing po,N, Circuithoard, solder. wire, optional heatsinks liar QI and 1)2. eh.

50

Diode D1 is present to protect thecircuit's input from reversed polarity byblowing the fuse of the main powersupply. If your power supply has nofuse or you might one day use thecircuit with an un-fused supply, placeD1 in series with the circuit (say, in thepositive input line and pointing to-ward R1) instead. That will afford thesame protection with only slightly re-duced regulation and voltage output.

Purists may object that the turn -onvoltages of Q1 and Q2 are 1.2 voltsapart, producing an operational"dead zone" that the output of U1must cross in order to catch up with anunbalanced load. It is possible to biasQ1 and Q2 to eliminate the deadzone, but that proved unnecessary inpractice because U1 can cross thedead zone very rapidly. After all, C1and C2 guarantee that changes inthe load balance won't be in-

stantaneous. In extensive testing, thiscircuit gave excellent regulationpowering not only op -amp circuits,but also "messy" loads such as buzzersand flashing light bulbs,

Construction. Note that althoughthis unit has separate input and out-put terminals for V + and V , the in-puts and outputs are internallyconnected together. So, unless D1was placed in series, you can buildthe splitter with three terminals in-stead of five if you wish,

Because there are so few compo-nents, a printed -circuit board is notnecessary. Instead, the smaller com-ponents (D1, R1, R2, and U1) can bemounted on a small piece of perf-board. Capacitors C1 and C2, on theother hand, should be placed closeto the output terminals. If you usebinding posts, put them exactly 3/4 -

inch apart so that dual banana plugscan mate with them.

With regard to using a replacementfor U1, the best op -amp to use is actu-ally the cheap 741 specified. Higher -performance op -amps such as theTL081 do not improve performance.

Using it. As mentioned, you can usethe splitter with fixed- or adjustable-

voltage power supplies ranging from8 to 30 volts. Make sure your powersupply is isolated. That is, neither V +nor V should be connected to thepower -supply's cabinet ground orpower -line ground.

How much current can the splitterhandle? That's a tough question. Re-member that as long as Load 1 andLoad 2 are equal, the splitter doesn'tdo any work at all. For most op -ampcircuits, that's typically the case.

As a rule of thumb, this splitter canhandle at least 250 mA of differencebetween the two halves of the loadwhile operating with a 30 -volt input. Todo that, either Q1 or Q2 has to dissi-pate almost 4 watts and will get quitewarm. If, in practical use, you find thetransistors getting so hot that you can'ttouch them comfortably, add a heatsink. With the specified transistors andan adequate heat sink, this splittercan handle several amps.

Remember that Q1 and Q2 are"hot" in another sense, too: the caseof Q1 is connected to V + , and Q2's toV So don't let anything accidentallycome into contact with them.

Everyone is interested in theweather. Some of us even haveour own temperature and hu-

midity gauges to tell us what's hap-pening day by day. But perhaps theweather forecasting instrument thatholds the greatest mystery for mostpeople is the barometer-a devicethat is used measure absolute airpressure. Weather forecasters rely onthe prevailing atmospheric pressure(specifically the direction of thechange in pressure) to predictthe next weather front. For thatreason, it is important that smallchanges in the barometric read-ing be detected.

Unfortunately, some homeanalog barometers may not besensitive enough to detect min-ute changes in pressure. But theDigital Barometer described inthis article is designed to addressthe deficiencies of such units. TheDigital Barometer --an easy -to -build, reliable, portable weather -forecasting instrument that willprovide years of accurate baro-metric readings-uses a 41/2 -digitLCD readout to display readingsfrom 28.00 to 31.99 inches of mer-cury.

Pressure Fundamentals.Pressure can be expressed inmany ways. Barometers are oftencalibrated in inches of mercury --14.7 PSI is equivalent to 29.92inches of mercury; that is the typ-ical barometric reading ob-tained at zero altitude or sealevel under average weatherconditions.

Most people are accustomedto the concept of gauge pres-sure, often specified as pounds -per -square -inch gauge (PSIG). Thatspecification refers to differentialpressure with sea -level atmosphericpressure (14.7 PSI or 29.92 inches ofmercury) as the reference. A barome-ter, however, displays absolute pres-sure (PSIA), which is the differencebetween the current pressure andzero pressure (a perfect vacuum). Abarometer, therefore, is a differentialpressure gauge that is referenced tozero pressure.

Pressure Sensor. Before one canunderstand the operation of the Dig-ital Barometer, it is important to know

the theory of operation of an abso-lute pressure sensor. The pressure sen-sor used in our barometer (developedby Motorola Semiconductor Products,Inc.) contains a monolithic siliconpiezoresistor that generates an outputvoltage that varies with applied stress.

The pressure sensor is a four -termi-nal device that contains two cham-bers that are separated by a silicondiaphragm and piezoresistive ele-ment. The resistive element is de-

,14,ki Elkin=

44A

4 PORT/1,13U! BAROMETERti 4 4 C

PortableBarometerBe your own weather forecaster

with an easy -to -build circuit that,by sensing small changes in the

barometric pressure, lets youpredict the next weather, front.

BY ANTHONY J. CARISTI

signed to respond to absoluteatmospheric pressure (pressure mea-sured with respect to a perfect vac-uum- zero pounds -per -square -inch -absolute or 0 PSIA).

One chamber of the sensor is ex-posed to atmospheric pressure bymeans of an external port. The otherchamber (evacuated to as perfect avacuum as possible by modern man-ufacturing techniques) is sealed. Inthat way, the diaphragm of the sensoris under constant stress from the dif-ference between atmospheric pres-sure on one side, and essentially aperfect vacuum on the other. The me-

chanical stress placed on the di-aphragm (and piezoresistive ele-ment) by atmospheric pressurecauses the sensor to generate an out-put voltage that is proportional to theapplied pressure as seen by the openport of the sensor.

About the Circuit. Refer to Fig. 1.

Power to the circuit is provided by a 9 -volt transistor -radio battery (B1). Thecircuit draws about 6 mA of current.

Since battery voltage falls off withuse, a fixed 5 -volt regulator, U1, isused to maintain a constant volt-age to the circuit, thereby ensur-ing retention of barometer ac-curacy as the battery is de-pleted.

The piezoresistor within SENS1 isconnected between pins land 3of the sensor assembly, and isdriven by the regulated 5 -voltsupply. The taps on thepiezoresistor (which are internallyconnected transversely acrossthe element and are brought outto pins 2 and 4) sense the dif-ferential voltage developedacross the pressure -sensitive ele-ment in SENS1 to provide an out-put voltage that varies linearlywith absolute pressure.

Under normal conditions (inwhich atmospheric pressureplaces stress on the piezoelectricresistor), the sensor's output volt-age is a finite, but very small invalue. At sea level that voltage istypically about 20 mV. Over arange of 29 to 31 inches of mer-cury, the output of the sensorchanges by only about 1.3 mV

A differential analog amplifier,comprised of U2 -a and U2 -c,boosts the output of the sensor by

an amount determined by the valuesof resistors R1 through R6. The outputvoltage at pin 8 of U2 -c represents theamplified output voltage of the sen-sor, offset by a DC bias of 1.5 volts that'sgenerated by a 100k Thevenin equiv-alent, comprised of R1 and R3.

Op -amp U2 -b (which is configuredas a voltage follower) is fed from thewiper of potentiometer R8. That com-bination forms an altitude correction/calibration adjustment and is used tocompensate for circuit tolerances aswell as for any error caused by thealtitude of the barometer. It also per-mits the DC bias generated in the

52

analog amplifier by R1 and R3 to be The differential output of the ana-nulled out. log amplifier (at pins 8 and 7 of U2 -c

S1

B1

+9V

U1

AN78L05

._cTio

R82K

G C21

R1

332KV V V

s

R976.8K

R7

150K

R3143K

3

VS

SENSMPX2100AP

GND

R4100K

4.044

R215.8K

41044

R5

R6100K1046

TO R100

1/4 LM324N

TO U3PIN 30

1 7V

1/4 LM324NFig. I. At the heart of the Digital Barometer is the MPX2100AP 15-PSIA pressuresensor. SENSI, which outputs a differential voltage that is fed to an amplifierarrangement built around 1.12. an LM32-1 quad op -amp.

and U2 -b, respectively) is fed to thedifferential analog input terminals ofU3 (an ND converter that contains allthe necessary circuitry to drive a 31/2 -

digit LCD readout), shown in Fig. 2.Analog -input sensitivity for a full-scaledisplay of 1999 is determined by thereference voltage applied betweenpins 35 and 36 of U3, and is equal totwice the reference input. In this proj-ect, only the three full digits of the NDconverter are used. The half digit, 1, isnot required. But a leading "2" or "3" isrequired; let's see how it is generated.

Since the circuit uses a readily avail-able 31/2 -digit A/D converter and is re-quired to respond only to a limitedrange of pressure, an unusual readoutsystem was used to produce a displaycontaining four digits. The ND con-verter generates the three least -sig-nificant digits (LSD's) of the display. Asimple logic circuit provides the mostsignificant digit (either a "2" or a "3").The reference voltage (0.1 -volt) ap-plied between pins 35 and 36 of U3 is

39 38

a

.1

9

IIcb II Id

DISP1LCD004

I 11111 1111 BP

35 34 7 5 36 6 37 30 29 11 10 9 31 32 12 25

1112

24 15 14 13 26 27 21 20 19 18 17 22 23

33

28

2

3

4

8

16

13

1/4 4030BE

+5V Ai

U2 0PIN 8

U2PIN 7

GND-'

1/4 4030BE

23 16 24 15 18 17 22 12 11 10 9 14 13 25 5 4 3 2 8 6 7C 0 b C

U3ICL7106CPL

abc e

BACK PLANEDRIVE

131 30 32 36

6

1/4 40308E

1/4 4030BE

E.-C301

R101MEG

35

40-440%-R12 C4

4.12K .47

R11100K

40

R13100K

39 38

R14100K

C5100pF

29 28

S R1547K

27

C6 C7.47 .22

21

26

Fig. 2. The output of the op -amp arrangement in Fig. 1 (at pins 8 and 7 of (12-c andU2 -b. respectively) is fed to the differential analog -input terminals of AID converter113, which processes the input signal and displays that information as a barometric -pressure reading.

4"

L owlII

2 3/16 INCHES

Fig. 3. The Digital Barometer was builton two single -sided, printed -circuitboards -one of which (the one shownhere) is referred to as analog board. Youcan use this full size template (and theone in Fig. 4) to etch your own boardsor purchase a set from the supplier listedin the Parts List.

1 15/16 NCHES

Fig. 4. The template for the otherboard -called the display board (whichcontains the AID converter, digital logic,and LCD readout) -is shown here fullscale.

1 2 3 4

R3 R2 R5

-R6-

U2

-R4-

-R1-

TODISPLAYBOARD

Fig. 5. When assembling the analog board (guided by this parts -placement diagram),it is recommended that sockets be used for the DIP IC's.

C1

C2I 1/11.%)

R7

R9

R8

TO U3PIN 31

TO U3PIN 30

determined by voltage divider com-prised of R11 -R13.

The decimal point of the LCD read-out is hard wired for a display resolu-tion of 0.01 inches of mercury. Thatcauses the three least -significant dig-its of the display to read 8.00 for adifferential input voltage of 0.08 volts,9.00 for 0.09 volts, 0.00 for 0.10 volts,and 1.00 for 0.11 volts.

The gain of the analog amplifier ischosen so that a variation of 1.00 inchof barometric reading as detected bythe sensor is translated into a changeof 0.01 volts at pin 8 of U2 -c. Thatcauses the display's three least -signifi-cant -digits to read 8.00 at 28.00inches, 9.00 at 29.00 inches, 0.00 at30.00 inches, and 1.00 at 31.00 inches.

Generation of the most -significantdigit of the display (2 or 3) is accom-plished via U4 (a 4030 quad two -inputexclusive -OR gate). The XOR gate pro-duces a logic 1 output only when thesignals applied to its two inputs areopposite to each other. The key togenerating either a 2 or 3 is deter-mined by examination of the "g" seg-ment of the second most -significant -digit, since the barometer need dis-play only 28 to 31 inches of mercury. Ifthat digit is either an 8 or a 9, its "g"segment is energized and the most -significant digit must then be a 2. If thesecond most -significant digit is either0 or 1, the "g" segment is dormant and

B1

S1

II II+

the most -significant -digit must thenbe a 3.

Integrated circuit U4 -c simulta-neously examines the "g" segment ofthe second most -significant digit andthe back -plane waveform generatedby U3. When the "g" segment is active(8 or 9), the output at of U4 -c at pin 10is at a logic 1. Otherwise it is zero fordigits 0 and 1. Integrated circuit U4 -b isused to conditionally invert the back -plane waveform, so that its output isidentical to back -plane when the dig-it is 0 and 1, and is an inverted back -plane when the digit is 8 or 9.

Integrated circuit U4 -b pin 4 feedssegment "e" of the most -significant -digit, which must be displayed as a "2"when the next digit is 8 or 9. Similarly,U4 -a is used as a conditional inverterso that the "c" segment of the most -significant -digit is always driven op-posite to the "e" segment as requiredfor a "3."

Finally, U4 -d is used as an inverter sothat the remaining segments, com-mon to display a digit of 2 and 3, arealways energized. The logic circuit,comprised of U4, always generatesthe correct most -significant -digit ofthe barometric reading, but cannotdisplay any digit other than 2 or 3.

mm

-4

0zcn

owco

zco

Construction. The Digital Barometer 8was built on two single -sided, printed -circuit boards; one is referred to as 53

r

TO ANALOG BOARD

+5V GND

U3

-C7- C-C6--

3 C4 DISP1**

'SEE TABLE 1

**SEE TEXT

Fig. 6. After assembling the display board, install all of the jumper connections listedin Table 1 on the component sided of the board. The LCD module is then mounted tothe copper side of the board.

R14

I

615

R12

R13

U4

-R10 ---

TO U2 TO U2PIN 8 PIN 7

analog board (which contains theanalog circuit and regulated powersupply) and the other is called thedisplay board (which contains the A/Dconverter, digital logic, and LCD read-out). Splitting the circuit into two sec-tions ()lows stacked printed -circuitconstruction so that the entire projectcan be assembled into a small en-closure, complete with the battery.The readout was mounted on copperside of the digital board to allow it toprotrude through the cover of thehousing, for ease of viewing.

If you do not wish to etch your ownboards, they are available from thesource given in the Parts List. Tem-plates for the two printed -circuitboards are shown in Fig. 3 (the analogboard) and Fig. 4 (the display board)full scale. The parts -placement di -

Lo, agrams of the analog and displayboards are shown in Fig. 5 and 6, re-

mspectively. Also shown in those di-agrams are the hard -wire connec-

>go; Lions between the boards and to the

external components.It Is recommended that sockets be

6 used for the DIP IC's. Because of theS limited space in the recommended'II enclosure, only low -profile sockets

ItIt should be used. The LCD readout canbe soldered directly into the digital

F2 board. If desired, a socket for the7, readout can be fabricated by cutting

u_a 40 -pin DIP socket in half lengthwise.

54 Do not install the readout or IC's into

TABLE 1-JUMPER CONNECTIONS

From To

U3 pin 2 DISP1 pin 18U3 pin 3 DISP1 pin 19U3 pin 4 DISP1 pin 20U3 pin 5 DISP1 pin 21U3 pin 6 DISP1 pin 22U3 pin 7 DISP1 pin 23U3 pin 8 DISP1 pin 17U3 pin 9 DISP1 pin 14U3 pin 10 DISP1 pin 15U3 pin 13 DISP1 pin 26U3 pin 14 DISP1 pin 13U3 pin 15 DISP1 pin 10U3 pin 16 DISP1 pin 29U3 pin 17 DISP1 pin 31U3 pin 18 DISP1 pin 9U3 pin 22 U4 pin 8U4 pin 8 DISP1 pin 32U3 pin 23 DISP1 pin 30U3 pin 24 DISP1 pin 11U3 pin 25 DISP1 pin 27U4 pin 2 DISP1 pin 5U4 pin 3 DISP1 pin 7DISP1 pin 12 DISP1 pin 6U4 pin 11 DISP1 pin 12

the boards until you are instructed todo so later on during the checkoutprocedure.

Pay strict attention to the orienta-tion of all polarized components; ifany of the polarized components(electrolytic capacitors and IC's) areinadvertently installed backwards, thecircuit won't work and there is the pos-sibility of damage to one or more ofthe components.

The accuracy of the barometer re-lies on the tolerance and stability of

PARTS LIST FOR THEDIGITAL BAROMETER

SEMICONDUCTORSUl-AN781...05 5 -volt, 100-mA.

voltage regulator, integrated circuitU2-LM324N quad op -amp,

integrated circuitU3-1CL7106CPL 31/2 -digit AiD

converter, integrated circuitU4-CD4030BE quad exclusive-oR

gate, integrated circuitDISPI-LCD004 41/2 -digit or similar

LCD readout ( Digi-key)SENSI-MPX2I(X)AP 15 -PSI A

pressure sensor (Motorola)

RESISTORS(All fixed resistors are Vi -watt, I%

metal -film units, unless otherwisenoted.)

R I -332,000 -ohmR2-I5.800-ohmR3 -143.000 -ohmR4 -R6, RI I, RI3 -100.000-ohmR7 -150,000 -ohmR8 -2,000 -ohm PC mount, cermet

potentiometerR9 -76,8(X) -ohmR 10- -megohm 1/4 -watt. 5% carbonR12 -4120 -ohmR14 -100,000 -ohm, 1/4 -watt. 5%,

carbonR15 -47,000 -ohm, 1/1 -watt. 5%,

carbon

CAPACITORSCI -10-µE 15-WVDC, axial -lead

electrolyticC2 -0.1-µF, ceramic -discC3 -0.0I -µE ceramic -discC4. C6--0.47-1.LE ceramic -discC5-I(X)-pF. ceramic -discC7- 0.22-µF, ceramic -disc


B1 -9 -volt transistor -radio batterySI-SPST miniature toggle or slide

switchPrinted -circuit materials, enclosure

(Radio Shack 270-222 or similar).IC sockets, battery holder andconnector, wire, solder, hardware.etc.

Note: The billowing items areavailable from A. Caristi (69 WhitePond Road, Waldwick, NJ 07463):Pressure sensor iSENS1). $42.50:a set of two printed -circuit boards,$19.75; 78L05 5 -volt regulator(U I) 2.00; LM324 op -amp ( L12 ).$2.00; 1CL7106CPL AID converter(U3), $16.50; 4030 quad xoR gate(U4), $2.00: set of eleven 1%metal -film resistors, $4.95. Pleaseadd $3.00 postage/handling to allorders. New Jersey residents pleaseadd appropriate sales tax.

the amplifier and ND converter re-sistor values. For that reason, it is nec-essary to use only 1% metal -filmresistors where specified in the PartsList. Ordinary carbon resistors are notstable enough for the circuit, andshould not be used in place of themetal -film types.

Analog Board. The pressure sensor isa reasonably sturdy device, but caremust be taken when forming theleads at right angles so that the bodylies flat on the board. Use two long -nose pliers when bending the leads-one to prevent stress on the leadwhere it enters the plastic body andthe other to bend the terminals to theproper position.

Before forming the leads, locate pin1 of SENS1, which is identified by asmall indentation cut into the flat ter-minal of pin 1. Once you have identi-fied pin 1, you'll be able to form theleads in the correct direction so thatthe sensor can be properly installed inthe circuit. The use of mounting hard-ware for the sensor is optional; if used,place the head of the screws on thebottom side of the board to allow it torest on the bottom of the enclosure.No pneumatic connection to thepressure port of the sensor is requiredfor the barometer except during anoptional calibration procedure (thatis discussed later).

After completing the analogboard, examine it very carefully foropens, shorts, and cold solder joints. Itis much easier to correct a problem atthis stage rather than later on shouldyou discover that the barometer doesnot work.

Digital Board. To keep the size of thebarometer as small as possible, theLCD readout was mounted on thecopper side of the board. That allowsyou to mount the display board to thecover of the enclosure with the dis-play protruding through a 23/46 by 15/16 -inch rectangular opening. Drill fourholes in both the display board andcover to accommodate the mount-ing hardware. Be careful not to drillthrough any of the copper conduc-tors.

The display board requires anumber of jumper wires to completethe circuit; a listing of those connec-tions are given in Table 1. Use #24 or#26 insulated, stranded wire for the

HOSETEE

CLAMP

APPLYVACUUM

HERE

PRESSURESENSOR

HEIGHT OFWATER COLUMN

Fig. 7. For those who wish to obtain thegreatest possible accuracy from hecircuit, this set up can be used tocalibrate the Digital Barometer using aprocedure founded in some basic laws ofphysics.

jumper connections. All jumper wiresmust be placed on the componentside of the display board to allowroom for the readout module on theopposite side.

After all other components havebeen installed, install the LCD's socket(as described earlier), if one is to beused, on the copper side of the dis-play board. Do not install the LCD atthis point; the circuit must be deter-mined to be bug -free first, with noopens, shorts, cold solder joints, orother construction errors.

Final Assembly. There are 4 con-nections that must be made betweenthe analog and display boards; +5volts, ground, and the two differentialoutputs from U2 -c and U2 -b. Use dif-ferent color insulated stranded wirefor those connections and be sure toallow sufficient lead length to permitthe boards to be placed as requiredin the enclosure.

In the author's prototype, two cor-ners of the analog board were cut offat 45° angles to permit the board tobe positioned near one end of thespecified enclosure, allowing suffi-cient room for B1 next to the board. It isnot necessary to fasten the analogboard to the recommended en-closure; it will automatically be held in

place when the cover is screweddown.

The wiring between the two circuitboards must be positioned so that itdoes not lie across the top of U3 whenthe cover of the enclosure is put inplace; otherwise, there may be insuffi-cient clearance when attempting toseal the enclosure that is specified inthe Parts List.

Connect a battery clip to the cir-cuit, When wiring the clip to the circuit,be sure to observe proper polarity asindicated in the parts -placement di-agram. Switch S1 can be installed onthe side of the enclosure. Because ofthe limited space within the en-closure, use a miniature toggle orslide switch.

Checkout. For the initial checkout ofthe circuit, do not install the IC's (U2,U3, or U4) or the display (DISP1) on theboards. The power supply will bechecked first, using any DVM or VOM.Connect a fresh battery to the circuit.Connect the negative lead of thevoltmeter to the circuit ground (nega-tive battery terminal) and turn on S1.Measure the output of the regulated5 -volt supply at pin 3 of SENS1. Youshould get a reading of from 4.8 to 5.2volts. If that checks out, proceed withthe remainder of the test. If you do notobtain the proper read-ing,troubleshoot the circuit and cor-rect the problem before proceeding.

If the power supply is not deliveringthe correct voltage, check the termi-nal voltage of the battery whenpowering the circuit: The minimumpermissible voltage is 7 volts. Checkthe orientation of U1. With the batterydisconnected, measure the resis-tance between the 5 -volt bus andground to be sure that there is no shortcircuit in the wiring.

When you are satisfied that thepower supply is operating properly,disconnect the battery. Very carefullyinsert U2-U4 and DISP1 into their re-spective sockets, with the correct ori-entation. When installing those com-ponents, be sure that none of the ICpins are inadvertently bent under-neath the body of the IC.

With all of the components in-stalled, apply power to the circuit,and measure the voltage at pin 8 ofU2, with respect to ground. You shouldget a reading of about 1.8 volts. If yourreading does not coincide with that 55

voltage, carefully check to make surethat R1 through R6 are of the correctvalues. If in doubt, use an ohmmeterto measure the resistance of any re-sistor after disconnecting one endfrom the board. Try a new LM324N.Check the sensor for correct orienta-tion in the circuit. Set R8 to mid -posi-tion and measure the voltage at pin 7of U2. A reading of about 1.7 volts isnormal. If you do not get the correctreading, check R7-R9 for the correctvalues.

The next step involves checking theoperating range of R8, and if neces-sary changing R7 and/or R9 to com-pensate for normal tolerance varia-tions in the sensor and other circuitcomponents.

Connect the negative lead of thevoltmeter to pin 7 of U2, and thepositive lead to pin 8 of U2. Set thevoltmeter to a sensitive range (such as200 -mV full scale). Apply power to thecircuit and record the reading as R8 isadjusted to each end of its range. Ide-ally, the maximum and minimumreadings should be centered around100 mV (0.1 volt), with an adjustmentrange of at least ± 10 mV. It may benecessary to tailor the values of R7and/or R8 in order to obtain a volt-age -adjustment range that covers 90to 110 mV as R8 is varied from oneextreme to the other.

Once R8 has been centered about100 mV very slowly adjust R8 over itsrange and note the reading of thedisplay. You should be able to obtainbarometric readings of at least 29.00to 31.00 inches of mercury. Re-member, the circuit is not capable ofdisplaying numbers such as 27 and32, so ignore such readings, whichcan occur at either end of the adjust-ment range.

If you do not obtain a legitimatebarometric display, or if the display istotally blank, the problem probablylies with the display board. A totallyblank display indicates that the back -

plane -drive signal generated at U3pin 21 (a 5 -volt peak -to -peak square -wave of about 50 Hz) is absent. Checkall of the components associated withU3. Check the orientation of DISP1 tobe sure it is installed correctly.

Check the wiring to be sure thatthere are no solder bridges across theclosely spaced conductors. If possibletry a new ND converter (U3) and XOR

56 gate (U4). If the display is energized,

but one or more of the digits has seg-ments that are either incorrectly ener-gized or extinguished, the problem ismost likely with the connections be-tween U3, U4, and DISP1.

Carefully review Table 1 to deter-mine whether all of the jumpers arecorrectly wired. Check the solder con-nections to DISP1 for opens and shorts.When the barometer is operatingproperly, set R8 to the current baro-metric reading, which can be ob-tained by calling a local airport ortuning into a radio or TV weather re-port.

Precision Circuit Calibration. TheDigital Barometer can provide relia-ble and accurate readings since thetolerance of the sensor and circuitcomponents are reasonably good.For those who wish to obtain thegreatest possible accuracy from thecircuit, a simple calibration pro-cedure using some basic laws ofphysics can be used to trim the ampli-fier gain for the particular sensor usedin the circuit.

Calibration involves using a water -

column manometer, which can beassembled from easily obtainablematerials. The manometer is used tosimulate a pressure difference of 1inch of mercury so that the circuit ischecked at two different pressurereadings, 30.00 and 29.00 inches. Re-sistor R2 can then be trimmed to pro-vide a precise circuit gain for achange in pressure of 1 inch of mercu-ry.

Figure 7 illustrates the manometersetup. The materials require include aclean bottle, clear plastic tubing, ahose "tee" to make a 3 way connec-tion, a hose clamp, a short length ofwood, and a scale. (A small amount offood coloring can be used to makethe water level easier to see.) The ac-curacy of this procedure is deter-mined by how precisely you canmeasure and set the height of thewater column, which can be markedon the length of wood.

The bottle and plastic hose can besecured to the wood to keep the tub-ing above the top of the bottle asstraight as possible. Keep the assem-bly vertical during the procedure.With the pressure sensor connectedto the manometer as shown in Fig. 7,apply power to the circuit. Open theclamp and be sure that there is no

water or bubbles in the tubing abovethe water level in the bottle. Set po-tentiometer R8 so that the displayreads 30.00 inches.

Now gently apply suction to theopen end of the tubing so that thewater level rises in the tube. Carefullyraise the level of the water 1319/32inches (34.5 centimeters) above thelevel in the bottle. Close the clamp sothat the water in the tubing remains atthe desired level. Note the reading ofthe barometer, which should be 29.00inches.

If the reading is greater than 29.00,the amplifier's gain is set too low, andthe value of R2 needs to be reduced.If the reading is below 29.00, R2 needsto be increased. The required changein R2 will be extremely small, possiblyless than 100 ohms. Ifs best to use asmall carbon resistor connected in se-ries to raise the value, or a large car-bon resistor connected in parallel toreduce the value.

Once the optimum value for R2 hasbeen determined and placed in thecircuit, the barometer is calibrated.Adjust R8 for the prevailing barometricpressure.

Using the Barometer. Once R8 hasbeen adjusted, it needs no further at-tention unless the altitude of the ba-rometer is changed by moving it toanother location. If so, reset R8 by ob-taining a current barometric readingfrom an airport or weather report.When operating the barometer, youmay see some fluctuation in the least -significant digit of the display; it shouldhover between two readings. That'snormal for the ND converter used inthe circuit.

Also note that the circuit is far moresensitive than the common analogbarometer, and will give a continu-ously updated pressure reading. Thus,you may see small changes in read-ings as the ambient air pressure var-ies. That can be very noticeable on awindy day. To use your barometer forweather forecasting, note the direc-tion of change in readings token anhour or more apart. Increasing read-ings usually indicate fair weatherahead; falling barometric pressure in-dicate stormy or unstable weather ison its way.

Be sure to turn off the instrumentwhen not in use. That ensures longbattery life.

It's a high -voltage apparatus that has earned a placein history. Now irs your tarn to get in on the fun!

Aiw imshurst machine? So that's whatit's called!' my neighbor said, hiseyes as wide as half dollars. I

often heard that comment.I took the Wimshurst machine off the shelf in

my den and placed it on the desk before him. Iturned the hdndcrdnk, the black plastic platesspun, and sparks ,umped between the metalglobes. "It will create 75,000 volts," I com-mented. He leaned away from the machine. Asthe plates spun, the machine hummed andsparks snapped be-ween the globes. The smell

'THEof ozone soon filled the room.

WIMSHURSTMACHINE

The photo on ttis page is 'eprinted, wish per-mission, from boo.< #3573 "Homemade Light-ning," by R.A. Ford. Coy/right 1990 by TABBooks, a ciyision of McGraw-Hill, Blue RidgeSummit, PA 17294: Tel 800-233-1-28 or717-794-2191.

I'm sure that you, too, have seen aWimshurst machine although you maynot have known it's name. You probablyknow more about its cousin, the Van deGraaff generator. Both are electrostaticgenerators, but that is where the sim-ilarity ends. The Van de Graaff gener-ator creates static charges by friction,while the Wimshurst device does it byinduction. In the early days of elec-trostatics, the principle of induction wasalso known as "influence." In fact, themachine is more correctly called a"Wimshurst influence machine."

The Wimshurst machine played animportant role in the early years ofelectrostatics. It provided high -voltagesnecessary for experiments in X-ray. Butbefore I tell you how the Wimshurst ma-chine works, let's take a quick look atthe science of electrostatics.

Electrostatics. Electrostatics was firstnoticed sometime in 600 BC when theGreek philosopher Thales discoveredthat amber attracted light objectswhen rubbed. The phenomenon notonly demonstrated a fundamentalconcept of electrostatics, but also gaveus the word "elektron," meaning amberin Greek.

When Italian physicist AlessandroVolta invented the 'Voltaic pile" (or bat-tery) in 1800, the science of elec-trostatics changed forever. Volta's newinvention provided scientists with a sta-ble, dependable source of movingcharges (i.e., DC). This invention was aturning point in electricity because nowscientists could study electrodynamics,whereas before they were limited tostudying electrostatics.

Triboelectric Effect. It's been a whilesince the days of Tholes, but we all knowa few modern ways to make electricityby rubbing. Shuffling our shoes acrossthe carpet on a dry day causes a sparkbetween our finger and a metal door-knob. Likewise, rubbing a glass rod withflannel and then pulling them apartcauses the flannel to hang unnaturallytowards the rod. The rubbing actioncauses the glass to develop an abun-dance of positive charges and the flan-nel an abundance of negative ones.Once pulled apart, the difference incharge of the two materials causes theattraction of the flannel to the glass.

The same happens when you rub pa-per against a plaster wall or woodendoor. It sticks to the vertical surface be-cause the rubbing creates opposite

58 charges on the paper and the wall. Try

it with a balloon and you'll see the sameeffect. In each of these cases, rubbingcreates segregated electric chargesand static electricity is the result.

In high school, you probably saw aVan de Graaff generator. It madesparks fly, fluorescent tubes glow, andyour hair stand on end when youtouched its dome. The Van de Graaffgenerator works in a way that is similarto rubbing glass with flannel, except

+ + +

CHARGE COMB

METAL DOME

UPPERPULLEY

DISCHARGE

COMB

[MOVING BELT

INSULATING

COLUMN

LOWER PULLEY

BASE

+CHARGESUPPLY

Fig. 1. A Van De Graaff generator placesa high -voltage charge on its metal domeby friction, whereas a Wimshurst machinecharges via induction.

WARNING!!! This article deals with and in-volves subject matter and the use of materialsand substances that may be hazardous tohealth and life. Do not attempt to implement oruse the information contained herein unlessyou are experienced and skilled with respect tosuch subject matter, materials and sub-stances. Neither the publisher nor the authormake any representations as to the accuracy ofthe information contained herein and disclaimany liability for damages or injuries, whethercaused by inaccuracies of the information,misinterpretations of the directions, misap-plication of the information or otherwise.

that the rubbing is made continuous byusing a moving belt inside the gener-ator (see Fig. 1). As an electric motorturns the belt, metal combs in the gen-erator's dome and base strip chargesfrom the belt. As a result, the dome andbase develop opposite charges. Small,classroom -sized Van de Graaff gener-ators produce 200,000 volts. Largerones, like those used for sub -atomicparticle research, create several -mil-lion volts.

Glass rubbed with flannel, and pa-per, or balloons sticking to a wall, andthe Van de Graaff generator are allexamples of creating static electricityusing friction. That is also known astriboelectric charging. As we men-tioned, there is another way to createstatic electricity and that is by induc-tion.

Induction. John Canton, in 1753, wasthe first to put forward the concept ofinduction of charge. He demonstratedthat when a charged body is broughtclose to a neutral body, the neutralbody develops a charge of equalmagnitude but opposite polarity. Oneof the earliest devices to demonstrateinduction was the "electrophorus." Theelectrophorus is the simplest elec-trostatic generator.

Later in 1787, Abraham Bennet, theinventor of the gold -leaf electroscope,developed the first simple machine toinduce electrostatic charges. The de-vice was called a "doubler" because ofits ability to progressively accumulatestatic charges. His doubler did not usefriction, but used Canton's inductionconcept to generate separate positiveand negative charges.

New varieties of doublers, or "influ-ence machines" as they were sooncalled, were developed by Nicholsonin 1788, Belli in 1831, and Lord Kelvin in1860. Also in 1860, Varley built the firstsuccessful high -voltage influence ma-chine. Other induction devices weresubsequently developed by AugustToepler and Wilhelm Holtz. But it wasn'tuntil 1878 that British engineer JamesWimshurst invented the first depend-able device to inductively generatestatic electricity. The Wimshurst influ-ence machine was born.

The Machine Itself. Before we ex-plain how a Wimshurst machine works,its a good idea to describe its structure.The Wimshurst machine has three ma-jor parts: rotating parallel plates, neu-tralizing rods, and collecting combs

(see photos). Today's bench -sized dem-onstration units typically have 12 -inchdiameter plates. During the heyday ofelectrostatics, larger Wimshurst ma-chines (used for research or poweringearly X-ray machines) had multiplepairs of plates several feet in diameter.

Each Wimshurst machine develops amaximum electrostatic potentialbased on the number of plates used,their diameter, and the spacing be-tween them, Interestingly enough, in-creasing the rotating speed of theplates does not increase the maximumdischarge voltage. Only increasing thenumber of pairs of plates increases thedischarge voltage.

The plates can be any sturdy, non-conducting material, such as glass orplastic (see Fig. 2). The plates aremounted in pairs, separated by aquarter -inch gap, on a horizontal shaft.The closer the plates are mounted toeach other, the better the machine willoperate. The plates are turned by beltsand pulleys from a common crank-shaft, but they rotate in opposite direc-tions. A difference in pulley diameterscauses the plates to spin several timesfaster than the handcrank.

Metal -foil strips called "sectors" are

DISCHARGE BALLS

SWITCH(LEVERTO CONNECT LEYDENJAR TO DISCHARGE(LIFT UP TO TOUCH

BAR "A')

LEYDEN JAR1 OF 2)

BASE

Resources.

Wimshurst electrostatic generators areavailable from Edmund Scientific Company(Cat. No. B70,070), 101E. Gloucester Pike,Barrington, NJ 0 8 0 0 7-1 3 8 0; Tel.609-573-6250, and from The Chem Shop,1151 South Redwood Road, Salt Lake City,UT 84104; Tel. 801-973-7966.

Also, contact Dynamic Systems, Inc., 48Sunlight Drive, Leicester, NC 28748; Tel.704-683-1280, Fax 704-683-3511.

See the following books:The Wimshurst Machine: How to Make

and Use ft, by Alfred W. Marshall; LindsayPublications, Inc., PO. Box 12, Bradley, IL60915-0012.

Electrostatics: Eiploring, Controlling andUsing Static Electricity. by A.D. Moore; An-chor Books, 1968.

Handbook of Electrostatic DischargeControls, By Bernard S. Matisoff; VanNostrand Reinhold Company, 1986.

Electrostatics: Principles, Problems andApplications., By Jean Cross; Adam Hilger,1987.

evenly spaced along the outer surfaceof each plate. Those help extract ex-cess charges from the non-conductiveplates.

The charges that accumulate on thesectors are removed by pairs of collect -

COLLECTING COMB(FROND

The Wimshurst machine is composed of many parts. However, not all Wimshurst machineshave Leyden jars although this one does.

FRONT PLATE

FRONTNEUTRALIZING ROD

CENTER HUB

DRIVE BELT (FRONT)

PULLEY (FRONT)

HANDCRANK

ing combs made of tinsel threads. Eachpair of combs is mounted on a U-shaped bracket, with one brush touch-ing the front plate and the other brushtouching the rear plate. The two U-shaped brackets are mounted op-posite one another. They carry the ac-cumulated charges to the machine'sdischarge balls.

There are two "neutralizing rods" thatspan the diameter of each plate andalso have metal tinsel combs on eachend. The front and rear rods are per-pendicular to each other and are posi-tioned at an angle of 45° to 60° from themachine's base.

In addition to the three basic parts ofa Wimshurst machine, they typicallyhave two built-in Leyden jars, which arevery simple glass and foil capacitors.Each Leyden jar can be electricallyconnected to a collecting comb bymeans of a hinged rod. If the Leydenjars are not connected to the collectingcombs, then a continuous arc jumpsbetween the discharge balls when thehandcrank is turned. If the connectingrods are lifted to touch the collectingcombs, then a sharp (and intense) snapof electricity jumps between the dis-charge balls every few seconds.

It is important to notice that all themetal parts of a Wimshurst machineare built with rounded edges. A funda-mental rule of electrostatics is thatcharges find it much easier to "jump"from a pointed surface than from arounded one. Any sharp points on themachine would allow the charges todissipate quickly.

How it Works. Remember that theWimshurst machine is an induction de-vice. It doesn't depend on friction tomake an electrostatic charge. As you'llsee, quadrants of negative andpositive charge are created across theplates by induction between the frontand rear plates. As the plates rotate,these positive and negative chargesare syphoned off through the metalsectors by the collecting combs.

As you may recall, the principle ofinduction requires that an object beinitially charged before you can use itto induce a charge in something else.That holds true for the plates on theWimshurst machine. Even before youcrank the handle the plates have somestatic charges on their surface. Thecharge is created from the incidentalrubbing and handling of the machine,and the machine actually amplifies thisinitial imbalance of charge. 59

Looking at Fig. 2 let's say that quad-rant CD of the rear plate had a slightnegative charge before the crank wasturned. That would induce a smallpositive charge on the front plate in thesame sector. As you turn the crank, elec-trons on the front plate sector at point Care repelled by the electrons built upon the rear plate. That pushes them upthe neutralizing rod to A, which is amore desirable place to be becausethe rear plate has a positive chargethere. That leaves electron -starvedquardrant CD with a net positivecharge, and electron -rich quardrantAB becomes more negatively charged.

Note that the rear plate-rotating inthe opposite direction-works in a re-ciprocal fashion: Electrons movethrough the rear neutralizing rod from Bto D. So on the rear plate, quadrant ABbecomes more positive and quadrantCD becomes more negative. That per-mits the front neutralizing rod to scoopup more repelled electrons and so on.

In the region of the collecting combs,the front and rear plates have the samecharge. The charges on the plates re-pel each other in those areas. That per-mits the collecting combs to scoop upthe excess charges and send them tothe discharge balls. The charges willcontinue to accumulate until surfaceleakage or a spark between the dis-charge balls dissipates them.

You can see the charge leakage byplacing the device in the dark, settingthe discharge balls a good distanceapart, and looking at its corona dis-charge radiating from the edges of theplates. You'll also see tiny, purple arcsaround all combs.

Setting the gap between the dis-charge balls an inch or two apart,causes a continuous shower of sparksto jump between the spheres. Con-

, necting the Leyden jars to the dis-charge spheres allows you to separate

2 the balls by a larger distance to pro-duce big crackling sparks.

Demonstrations. Here are some

odemonstrations that you can try with aWimshurst machine. For instance, with

t; the Leyden jars disconnected, move6 the discharge balls far enough apart socr1- that there is no spark when you turn the

handcrank. Light a candle and hold itclose to one ball and then the other. At

g the positive ball, the flame will be at-°' tracted toward it, and at the negativeLL one, the flame will be repelled.

For another experiment, hold a60 piece of cardboard between the dis-

NEUTRALIZING ROD(FRONT PLATE)

COLLECTING COMBS(METALLIC)

QUADRANT AB

METAL FOIL SECTORS

FRONT PLATEROTATION

NEUTRALIZING ROD(REAR PLATE)

NON -CONDUCTINGPLATE

C

OPPOSITE CHARGES INDUCEDON PLATE AND METAL SECTORSBY METAL ROD OF COLLECTING

COMBS

Fig. 2. This is the charge distribution on the front plate. Front and rear neutralizing rodsscoop the charges up and move them to more desirable quadrants. The collecting combscan then wisk them away.

Here you can plainly see the Leyden jarsand discharge spheres. The jars areconnected to the spheres by lifting up thelevers attached to them.

charge balls (again with the Leydenjars disconnected). Allow the shower ofsparks to jump through the cardboard.Inspect the cardboard and notice thatthe hole caused by the spark is bulged

on both sides. That shows that sparksact like AC current, they oscillate be-tween the discharge balls.

For a different effect, start by cuttingsome kind of shape out of a piece ofaluminum foil. A good example is a let-ter, such as T or L. Paste the aluminumdesign to a piece of cardboard. Usinga knife or razor blade, cut the design inseveral places to create discontinuitiesin the foil (don't let the cuts exceed Y32inch wide). Connect each end of theshape to one of the discharge balls.Run the machine and watch the arcs asthe charges jump the cuts in the foil.

Safety. You must respect the potentialof any electrostatic generator. Undernormal use, they are safe, but youshould not let that lull you into a falsesense of security. You must never at-tempt to condense, or store, thecharge they produce without fullknowledge of its dangers. Simplysaid, avoid connecting any capaci-tor to any electrostatic generator.

.U LEfi

01

3111111111

LTAGEVoltage doublers are an easy and inexpensive way to

40-14-experiment with high voltage.

01K

IN )I-----)

IF YOU'VE BEEN LOOKING FOR A WAYto generate high voltage, you'veundoubtedly run across the volt-age doubler. Voltage doublingusing diode -capacitor combina-tions is a common practice. How-ever, whole banks of doublers,called cascades, can also be usedfor producing extremely high DCvoltages from moderate to highAC voltages. Such high DC volt-ages may be needed for TV sets,lasers, air purifiers, industrialsmoke -stack dust removers,negative -ion generators, and, ofcourse, for experimenting, onwhich we'll concentrate here.

Half -wave doublerFigure 1 shows a half -wave volt-

age doubler; we'll assume that Cland C2 are initially discharged.During the first half -cycle shownin a, the upper input terminal ispositive and the bottom negative,so D1 conducts and Cl charges toabout 170 volts peak. Diode D2can't conduct, since it's back -bi-ased, so C2 discharges throughRL. In the second half -cycle (b),the analysis is similar, exceptthat D2 conducts and C2charges.

The circuit is really a transfor-merless voltage amplifier. WhileTi can provide isolation, as wellas increase the AC voltage ini-tially going into the doubler, theamplification due to the doublingaction would occur without it.When the polarity reverses, boththe input voltage and the chargeacross Cl are in series like twobatteries, producing about 340volts peak. One problem, though,is that a half -wave doubler can'tbe used with a load that drawsmuch current.

Full -wave doublerLet's see how a full -wave voltage

doubler is related to and builtfrom both positive and negativehalf -wave rectifiers. Figure 2-ashows a half -wave rectifier with apositive output, Fig. 2-b showsthe same version with a negativeoutput, and Fig. 2-c shows thetwo combined into a full -wavevoltage rectifier.

The full -wave voltage doublershown in Fig. 3 has been redrawnfor greater clarity; it has betterregulation than a half -wave ver-sion, and is easier to filter. Thecircuit produces nearly doublethe peak AC voltage of 170 volts,or about 340 volts peak acrossRL. For the first half -cycle (a), D2is cut off and D1 conducts, sothat Vc1 equals approximately170 volts DC. On the next half -cycle (b), the positive voltage isreplaced by a negative voltage, soD2 conducts and DI is cut off. RLgoes across Cl and C2 in series,effectively creating a doubledlevel of about 340 volts DC.

Warning!! This article deals with andinvolves subject matter and the use ofmaterials and substances that may behazardous to health and life. Do not at-tempt to implement or use the informationcontained herein, unless you are experi-enced and skilled with respect to suchsubject matter, materials, and sub-stances. Neither the publisher nor the au-thor make any representation as for thecompleteness or accuracy of the informa-tion contained herein, and disclaim anyliability for damages or injuries, whethercaused by or arising from the lack of com-pleteness, inaccuracies of the informa-tion, misrepresentations of the directions,misapplication of the information, or other-wise.

RALPH HUBSCHER

Unlike the half -wave voltagedoubler, the full -wave version hastwo capacitors across RL ratherthan one. Whereas Cl shown inFig. 1 is cut off and unsuppliedfor half of every cycle, Cl and C2in Fig. 3 are supplied on alternatehalf cycles. When the capacitorcorresponding to the diode that'scut off discharges, it can only doso through the capacitor beingsupplied, slightly decreasingboth its current and the max-imum voltage it reaches.

Measuring high -voltage DCVoltage measurements will be

possible only to about the secondor third stage of a cascaded volt-age doubler with most volt-meters. Beyond that, you'll needto use either a high-yoltage DCmeter or an external voltage di-vider for use with a standardhigh -impedance voltmeter (10megohms or more).

A good voltage divider that canbe used for the purpose of high -voltage measurements is theRCA SK3868/DIV-1, a high -volt-age DC divider; it's used in TV's toreduce the final anode voltage go-ing to the CRT to the level re-quired for the focus voltage. Itconsists of resistors R1 (200megohms) and R2 (40 megohms)in series, as shown in Fig. 4.There are three leads, one for thefree ends of each resistor, and theother at their juncture. If you putboth a 10-megohm meter (shownas ZM in Fig. 4) and a 2.7-megohm resistor (R3) in parallelwith the 40-megohm resistor(R2), you can achieve almost ex-actly 100:1 range multiplication,for a full-scale deflection of 20kilovolts DC. 61

120VAC

120VAC

C1

02

D2

__...1104

170V I 340VD1 PEAK '162

Rt

FIG. 1-HALF-WAVE VOLTAGE DOUBLER.During the first half -cycle (a), D1 con-ducts, D2 cuts off, C1 charges to 170 voltspeak, and C2 discharges through RL. Forthe second half -cycle (b), the Input polar-ity Is reversed, and both the Input and Clare in series, producing 340 volts peak.Now D1 cuts off while D2 conducts, andthe current divides between C2 and RL;the cycle then repeats.

a

120VAC

25VDC

-25VDC

FIG. 2-TWO HALF -WAVE RECTIFIERS,one with a positive output (a) and onenegative (b), combine to make a full -wavevoltage doubler (c).

Cascaded voltage doublersFigures 5-8 show four addi-

tional voltage doublers. The oneshown in Fig. 5 is the moststraightforward. If you build it,use 1N4007 diodes with peak in-verse voltage (PIV) ratings of 1kilovolt for D1-D6. and0.068-0.1 1.1.F capacitors withworking voltages of 400 volts DC.Figure 5 is electrically identical tothe one in Fig. 6, so keep that inmind if you should come acrosseither format. Figure 7 shows anextended version that's better

120VAC

T1

120VAC

.111.-

a

D1

170V C1

D2 170V C2

tti

V

R,

C2 DISCHARGESSLIGHTLY, IFPREVIOUSLY

CHARGED

Cl DISCHARGESSLIGHTLY, IFPREVIOUSLY

CHARGED

FIG. 3-FULL-WAVE VOLTAGE DOUBLER, redrawn for greater clarity. For the first half -cycle (a), D2 Is cut off and D1 conducts, producing about 170 volts DC across C1. On thenext half -cycle (b), D2 conducts and D1 is cut off. The output voltage is now across C1 andC2 in series, doubling the level to about 340 volts DC.

HV

(RED WIRE)

WHITEWIRE

HVDC TV CRT FOCUS DIVIDERRCA PART 0SK3868/01V-1

R1

200MEG

R240MEG

R3 BLACK2.7MEG WIRE

Zm

10MEG

DC

VOLTMETER

CD

FIG. 4-TO MEASURE HIGH VOLTAGESwith an ordinary 10-megohm meter, youcan use the RCA SK3868/DIV-1 high -volt-age divider. The circuit provides a 1:100voltage division, allowing 20 kilovolts tobe measured on a 200 -volt scale.

Cl

120VAC

C3 C5

FIG. 5-THIS CASCADED DOUBLER uses1N4007 diodes rated at 1 kilovolt PIV, andcapacitors from 0.068-01 p.F with a 400 -volt DC working voltage.

stabilized for moderate -currentapplications: it's called either aCockcroft -Walton or Greinachercascaded voltage doubler.

You can use a sewing needle asan emitter for the doubler shownin Fig. 8 to generate "coronawind." That will sound like a his -

(46Vs D6

4Vs

C6

D5

, C4

2Vs ffD2

D4

-L. C2

- Vs

D3

D1

T1g-

=-- -M

4 - 6Vs

C5

2 - 4Vs

C3

0 2Vs

C1

Vs

FIG. 6-THIS VOLTAGE DOUBLER is thesame as the one shown in Fig. 5, althoughit's drawn differently. You should be ableto recognize both versions.

sing noise. (We'll shown you howto demonstrate the "wind" lateron.) The circuit delivers 3.75kilovolts DC when powered from120 volts AC. or 7.5 kilovolts DCwhen powered from 240 volts AC.

The output of a cascaded volt-age doubler should be termi-nated with no less than 200megohms, and only then be al-lowed to extend beyond a protec-tive plastic case, for safety.Voltages as high as 5 megavoltsDC have been generated using

cascaded voltage doublers, es-pecially when operating in a pres-surized atmosphere. The biggestadvantage to using voltage dou-blers is that they use inexpensivelow -voltage parts. Otherwise, ifall the parts had to be of the high -voltage variety, you would have touse expensive and rather largecapacitors like the one shown inFig. 9.

If you have problems with thecircuit in Fig. 8 (or any otherhigh -voltage circuit), you mustdischarge every capacitor (we'lltell you how in a minute) beforeyou check for malfunctions.When examining the circuit forproblems, closely check the sol-der connections, and then the di-ode directions and continuity.The 1N4007's should have a re-sistance of 1.1K when forward -biased and be open when reverse -biased, while the capacitors

120VAC

FIG. 7-THE COCKCROFT-WALTON, orGreinacher cascaded voltage doubler,has improved performance for moderate -current applications.

should all have infinite resis-tance.

To properly discharge capaci-tors, build a discharging wandlike the one shown in Fig. 10. Usea 2 -foot wooden (or plastic)dowel, and connect a stiff wire tip

FIG. 9-HIGH-VOLTAGE CAPACITORS. A0.25 ILF, 7.5 -kilovolt capacitor is on theleft; a 100 pF, 15 -kilovolt capacitor is in themiddle; and 0.0005µF, 5 -kilovolt capacitoris on the right.

WATERTIP

PIPE(PIANO WIRE)

ITWIRE No

BRACKET

DOWEL

FIG. 10-A HIGH -VOLTAGE GROUNDINGwand is used to discharge capacitors. A 2 -foot wooden dowel is attached to a stiffwire tip. The metal tip must be connectedto an earth ground, such as a cold -waterpipe.

FIG. 11-HOMEMADE SWITCH for high -voltage DC. The line filter at left is a safetymeasure to keep high -voltage DC out ofthe house wiring.

FIG. 12-A TYPICAL CASCADED voltagedoubler. Note the wide spacings betweenthe diodes, the long connecting wires,and the smooth solder joints.

FIG. 13-THIS IS THE DOUBLER from Fig-ure 12, after being sealed in candle wax.For better protection, you can immerse itin pure paraffin oil.

(piano wire works well) to a coldwater pipe as earth ground with agood electrical connection. Dis-charge all capacitors twice, sincethey generally either hold charge,or tend to recharge from othercapacitors. Don't use an AC lineground or chassis ground in-stead of an earth grounded waterpipe, or you may blow a fuse ordamage parts.

Figure 11 shows a switch forhigh -voltage DC that you can usewith any of the cascaded voltage -doublers shown here; standardswitches may present a shockhazard. Also, use an electromag-netic interference (EMI) line filterlike the one seen at left in thephoto to keep high -voltage DCout of house wiring, and to pre-vent shock from static charge.The EMI filter is from CorcomCorp. (1600 Winchester Road,

if --K KT K ---F1( / AS EMITTERSEWING NEEDLE

RIC1 I C3 C5 I C7 C9 , C11 C13 C15 Cu7 C19 C21 C23 C25 1 ELECTRODE

1MEG

T1DI rrt rt' rrt (r4 rt. t

I I -200MEG FOR CORONA WIND3.75/7.5kVDC

120/240# * * * * $# *$T * * t26

VAC L-+ L-4 L-1 L-4C2 C4 I C6 C8 I C10 C12 C14 C16 I C18 C20 C22 C24 C26 I

1-/A1-4 * 1 )F--/--)[-/-*--/A"A t )1-/

FIG. 8-THIS 25 -STAGE VOLTAGE DOUBLER will generate "corona wind." It delivers 3.75kilovolts DC when powered from 120 volts AC, or 7.5 kilovolts DC when powered from 240volts AC.

DI -D26: 1N4007, IkV PIV

CI -C261.068- .1µF. 400VDC

FIG. 14-CORONA WIND DEMONSTRATION. The flame will deflect toward the groundplate. Note the black ground wire attached near the screw threads.

RIVET

ti

WIRE

/41SOLDER WIRE.E.ONTO SIDE

FIG. 15-THIS ION -MOTOR ROTOR, whenplaced on top of the sewing -needle emit-ter, will spin In a circle.

SPHERICALELECTRODE

SPHERICALELECTRODE

PENDULUMUSING LAC-

QUER -COATEDPLASTIC BALL

a

"(04'&:V.VVERYLIGHT

COTTONBALL

b

SPHERICALELECTRODE

SPHERICALELECTRODE

FIG. 16-TWO CHARGED DOOR KNOBScause a suspended, lacquer -coated plas-tic ball to behave like a pendulum (a). Alight cotton ball will bounce back andforth without being suspended on a string(b).

Libertyville, IL 60048, tel.312-680-7400), Model 2061 israted at 20 amps, 250 volts, and50-400 Hz. The high -voltage DCswitch in Fig. 11 also uses an old100 -amp fuse box, shown on theright; it may look like an antique,

64 but it will prevent any shocks.

When you build a cascadedvoltage doubler, you can encasethe circuit in pure paraffin oil orcandle wax to reduce the chancesof getting shocked. It will alsominimize corona loss, so thehigh -voltage DC arrives where it'sneeded. Figures 12 and 13 show atypical ladder -type voltage dou-bler before and after being sealedin wax.

ExperimentsThere are many experiments

that can produce observableeffects due to the high -voltage DCproduced by voltage doublers. With a high -voltage emitterpointed at a ground plate (used toattract ions), with a burning can-dle placed in between them (seeFig. 14), you'll see the candleflame deflect toward the metalplate. You can make a rotor for an ionmotor, using a light pivot madefrom a rivet with thin, stiff wire(like piano wire) attached, asshown in Fig. 15. The rotor mustbe balanced on top of the sewing -needle emitter (much as in acompass) used for the doublershown in Fig. 8. (We ran a similarconstruction project in Radio -Electronics, February, 1991.)When powered up, the rotor willspin and a hissing sound will beheard. Both ends of the wire arebent at opposite right angles, sothe emitted electrons propel thewire in a circle. You shouldsharpen both ends of the rotorwire to provide a sharp surface

good for corona generation andelectron emission. The sharp-ened ends will have a small radi-us of curvature (a tight curve orbend), giving rise to a highly dis-torted electric field at its surface.The high electric field is whattends to ionize air molecules inthe vicinity.

Another experiment you couldtry involves holding a fluorescenttube near the emitter. The tubewill glow, but be careful not totouch the terminals on the ends.or you'll get a shock. Lines of force of an elec-trostatic field can be demon-strated by placing the electrodes(the high -voltage DC output andground) in a tray covered withcastor oil containing somefarina. The farina will producethe pattern of the electric fieldlines: similar to iron filingsshaken lightly on a piece of paperin the presence of a bar magnet. If you place two round doorknobs on insulated stands madefrom plastic cups filled with can-dle wax, and then charge them.then a plastic ball suspendedfrom a string will be drawn to andtouch the positive electrode, andfall back to center when thespheres are discharged (see Fig.16-a). A plastic ball coated withconductive lacquer swings to-ward the positive electrode like apendulum: when the ball anddoorknob touch, the ball be-comes positively charged, so theyrepel one another. It then swingstoward the negative side, ab-sorbs electrons, becomes nega-tively charged. and is repelledback to the positive. The processrepeats indefinitely as long as thehigh -voltage DC is present, and itwill continue to operate for sometime after it's shut off. The chargeexchange is slow, and there'll bearcing at the positive electrode. A grounded metal ball alter-nates between both electrodes,like the conducting plastic ball.However, the arcs are smaller dueto its greater weight, and shouldbe observed at both ends, butmore on the positive side. A light cotton ball should bedrawn to the positive electrodeand hang there by itself, asshown in Fig. 16-b. It's then re-pelled 0.5 -inch toward the nega-tive electrode, and the processshould repeat indefinitely.

As one who supplies parts tothose who experiment withhigh voltage, I get a lot of let-

ters and phone calls from frustratedbuilders that go like: "Can you supplyan inexpensive XXX microfarad ca-pacitor at a working voltage of YYY?My only source wants $249 for one."Sometimes, a high price is justified;other times, a seller has the only ca-pacitors of a special value available,and will soak you for the maximumdollar.

It is feasible to build your own ca -

Build capacitors thatreally pack a punchfor high voltage fun!

BY ANTHONY CHARLTON MAKEYOUR OWN

HIGHVOLTAGE

APACITORSpacitors of any voltage and energystorage size for either AC or DC use.The process involves a step-by-steplogical approach that we'll presenthere. We'll explain how to plan andconstruct a capacitor, where to getmaterials, safety considerations, tipsand hints, and include a few simpleprojects.

A Capacitor's Description. A ca-pacitor consists of two or more platesof a conductive material separatedby an insulating substance called a

dielectric. A dielectric may be solid,gel, liquid, or gas. A capacitor's abilityto store energy is measured in eithermicrofarads (µ,F), nanofarads (nF), orpicofarads (pF). Micro means one mil-lionth, nano stands for one billionthand pico for one trillionth (farads arealso used, but in high voltage workthey are impractically large units).Several factors affect capacitance.The formula for determining capaci-tance is:

C = (0.224KA/c1)(n-1)

66

where C is the capacitance inpicofarads, K is a constant that de-pends on the insulator (or dielectric)between the plates (called the di-electric constant), A is the area of oneconductive plate in square inches, d isthe separation between adjacentplates in inches, and n is the numberof plates. As you may know, differentinsulators have different dielectricconstants. Table 1 shows the values ofK for some common materials andthe peak voltage they can withstandper 1/4000th inch (called a mil) of thick-ness. This rating is called the punctureor breakdown voltage.

Dielectrics. The better the insulat-ing property of the dielectric, thehigher its resistance, and the less di-electric leakage loss present. In lowcurrent, high voltage power supplies,minimizing all sources of loss is impor-tant to prevent undue power -supplyloading. For that reason, plastics areby far the best materials for large ca-pacitors. A serious project should in-volve one of the plastics.

Lexan, Polystyrene, and Plexiglas inparticular are easy to glue, and canbe cut with a table saw using a plas-tics blade, or a carborundum impreg-nated all-purpose cutting blade likeZippity-Do (which is cheaper). A sabresaw with a really coarse wood bladewill also work (other blade types clogor chip). Such plastics may be drilled

WARNING!! This article deals with andinvolves subject matter and the use ofmaterials and substances that may behazardous to health and life. Do not at-tempt to implement or use the informa-tion contained herein unless you areexperienced and skilled with respect tosuch subject matter, materials, and sub-stances. Neither the publisher nor theauthor make any representations as forthe completeness or the accuracy of theinformation contained herein and dis-claim any liability for damages or inju-ries, whether caused by or arising fromthe lack of completeness, inaccuracies ofthe information, misinterpretations of thedirections, misapplication of the informa-tion or otherwise.

with high quality steel drill bits or spe-cial plastic bits. They must be drilled at300 RPM or slower to prevent chip-ping and melting, and be sure toleave the protective film or paper onthe plastic when working with it,

Mylar, Polyethylene, Nylon, and es-pecially Teflon are difficult to work withas they are very slippery. The best wayto attach plates to any of those mate-rials is to use a glue specifically de-signed for the material. Polyvinylchloride (or just PVC) is moderatelyslippery. It can be glued with a PVCcement, or foil plates can be at-tached using silicone RN

Glass is, in principle, an even better

TABLE 1-DIELECTRIC CONSTANTS AND BREAKDOWN VOLTAGES

InsulatorDielectricConstant

Puncture Voltageper 0.001 Inch Notes

Air 1.0 30 1

Window glass 7.8 200Polyethylene 2.3 450Paper (bond) 3.0 200Polycarbonate (Lexan) 2.96 400Teflon 2.1 1000Polystyrene 2.6 500Epoxy circuit board 5.2 700 2, 3Pyrex 4.8 335Plexiglas 2.8 450PVC (rigid type) 2.95 725Silicone RN 3.6 550Polyethylene terphthalate (Mylar) 3.0 7500Nylon 3.2 407 4Mineral Oil, Squibb 2.7 200 2, 5Shellac 3.3 200

NOTES: All measurements at 1 MHz unless otherwise noted.1Tested with dry air.2Tested at 300 HZ using a Healthkit IM -2320 Multimeter and homemade capacitor.3Estimate, based no experiences.4Lowest value of 3 types.sEstimate. Probably higher. A 0.040" gap withstood over 10,000 volts DC before breakdown in onetest.

dielectric. It also has the advantageof being easy to glue to with SiliconeRN or Krazy Glue, and it is readilyavailable and cheap. However, it is

fragile, and may contain impuritiesthat allow conductive paths for de-structive arcs. Contradictorily, for yourfirst capacitor or Iwo, we suggest thatyou try a type made with glass to gainexperience, since they go togethereasily and are cheap.

Many industrial capacitors are oilfilled. Oil has an extremely high resis-tance, so it does not measurably in-crease leakage. Silicone transformeroil is the best liquid insulator, but israther hard to obtain. Mineral oil, onthe other hand, is readily availablefrom most pharmacies. Although ithas a low dielectric constant, it canbe used in a variety of simple ways tomake very good high voltage capac-itors.

For example, a dandy variable DCcapacitor can be made by immers-ing a junked AM -radio tuning capaci-tor of the movable -plate type inmineral oil so its shaft and connectionleads come out of the container's top.If you wish to try this idea, make abso-lutely certain the "cold" plates of thecapacitor (the moving plates) are atground potential. Use a good, large,non-metal knob for adjustment. A 100 -to 365-pF variable capacitor with a 1-kVDC breakdown voltage (i.e., a platespacing of 1 mm) becomes a 270- to985-pF unit with 7500-VDC break-down rating. Try pricing a 7500 -voltvariable capacitor sometime, andyou'll see the advantage to this ap-proach!

You can use mineral oil in designs ofyour own, too. Immersion of a home-made capacitor in mineral oil willgreatly improve its voltage rating andlifetime.

Paper is an excellent dielectricwhen saturated with mineral oil. Try20 -lb. bond computer paper whichhas a 4 mil thickness. Prepare this inex-pensive capacitor by interleavinglayers of dry paper with aluminum foil,and then immerse the capacitor in oiluntil the paper gets saturated.

One disadvantage to using oil in

home-made capacitors is that thetape or glue used to bond the assem-bly must be oil -resistant. Silicone RN isthe best glue for these purposes.

Design Considerations. There are

DO NOT TOUCH BALLWHEN JAR IS CHARGED!

METAL BALL

PLEXIGLAS PLUG - MAKEFROM Ye SHEET

GALLON MAYONNAISE JAR

ALUMINUM FOIL ONOUTSIDE IS THE

NEGATIVE TERMINAL

FOIL ON INSIDEIS THE POSITIVE

PLATE

WIRE CONTACTS BOTTOM

Fig. I. The classic Leyden jar is the oldest storage capacitor we know of. They areeasy to make and the materials only cost around $2.

several things to consider when de-signing and constructing your owncapacitor. Let's point out each onebefore moving to the construction de-tails. The first and most important thingto concern yourself with is safety De-spite the romance of high voltage, it isfoolish to needlessly risk your life. Sinceyou will probably be working with le-thal voltages, observance of all safetypractices for high voltage (or I -V) is

absolutely essential. For someguidelines, see the bared text entitled"High Voltage Safety."

The next aspect to consider is ca-pacity, If you have a specific capaci-tance in mind, you can design acapacitor using the information pro-vided elsewhere in this article. Try oneof the designs described later. Or per-haps you prefer experimenting in-stead. Either way, when building forthe first time, we suggest making small

designs first to get used to techniquesand quirks before you invest lots oftime and money.

You must also take into considera-tion the voltage that will be applied tothe capacitor. That will affect yourchoice of a dielectric and thus its re-quired thickness. Should you use aninadequate dielectric or thickness,sparks or arcs can result. A spark is atemporary breakdown that a lot ofcapacitors will survive, but an arc isserious: it is a path burned into thedielectric or other component. Arcscarbonize materials, producing ahighly conductive channel that oftenrenders an apparatus useless andvery likely dangerous. Except In spe-cial cases where the insulator is a"self -healing" type (like air, oil, andsome plastics), a single arc will ruin thecapacitor.

To compensate for the impurities

High Voltage Safety.

High voltage is considered any value over500 volts AC or DC. When you attach acapacitor to high voltage, you are multiply-ing its hazard manyfold. Therefore, experi-menters must take extra precautions toavoid painful shocks and possible elec-trocution. Here are a few guidelines to fol-low when working with high voltage:

Label your project in several locationswith: "Danger. High Voltage" where appro-priate Such a warning label is providedhere for you to copy (see Fig. A). Keepchildren, pets, and curiosity seekers awayfrom the apparatus. Cover all bare leads.wires, connection terminals, and possiblepoints of contact with high voltage putty ora cover fabricated from thick clear plastic.

Work in a dry location. Working in adamp basement or workshop courts disas-ter. Wear rubber -soled boots or sneakers.Stand on a thick rubber mat

*ANGEF)HIGH

VOLTAGEFig. A. Curiosity can hurt more thanjust felines. so use this warning label onaN }your high voltage projects to protectthe unwary from harm.

Never put your body in a position tobecome a conductor. Locate your appara-tus away from appliances, metal doors andwindows frames, heating ducts. vents, ra-diators, sinks. or water pipes. All theseitems can become a deadly ground if yourbody comes between them and high volt-age.

Always pull the plug when working ora high voltage circuit unless you must testit. When testing a live circuit, use utmostcaution. Keep one hand in your pocketUse clip -on test leads that are rated twicethe voltage of the live circuit. Use a highvoltage probe whenever possible ---its in-sulating handle will help protect you.

Use NE -2 neon lamps to indicate liveor stored high voltage. Bleed off the chargeon capacitors with a power resistor beforeperforming adjustments.

Adequate ventilation must be pro-vided for circuits that produce largeamounts of ozone such as Jacob's laddersor Tesla coils.

mr11r"

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that often appear in materials thatare not highly refined for capacitor 8

use, we must add a safety margin tothe thickness of the dielectric. In the 67

case of DC, a good rule of thumb is a50% margin. For example, say youneed a 500 -volt DC capacitor usingpolystyrene. Consulting Table 1, notepolystyrene's breakdown is 500 voltsper mil, thus 1 mil is required. Adding50% gives you 1.5 mils, which is ade-quate for pure DC. You can always usea thicker dielectric if it's expedient,providing that you adjust the numberof plates or their size to accommo-date the wider plate separation. Itshould be mentioned that when mak-ing a paper capacitor, you should usea healthy safety margin since paper isnot always uniform in thickness.

In comparison to AC, DC puts rela-tively little stress on a capacitor. Bycontrast, AC reverses the dielectrics'polarity every cycle. So the dielectricin an AC capacitor must have twicethe thickness required in an equiv-alent DC capacitor. Further, whenconsidering dielectrics in AC applica-tions, you must deal with the peak volt-age-not rms (Root Mean Square)voltage-that they will be exposed to.If you wish to convert an rms voltageto its equivalent peak sinewave value,multiply it by 1.414.

So, to roughly calculate the propervoltage rating needed for an AC ca-pacitor, you first double its requiredrms voltage rating then multiply by1.414. To further simplify this calcula-tion, all one needs to do is multiply theAC (rms) voltage in question by 2.828.Now divide the voltage by the punc-ture -voltage rating to get a prelimin-ary thickness value. Finally, you mustadd a safety margin of 50% to 100%.The actual percentage depends onthe characteristics of the applied ACvoltage. For a pure sinewave AC, wesuggest a 50% safety margin whereas

8 high frequency, non -sinusoidal ap-plications such as Tesla coils require afull 100% extra thickness.

If one is available, equip an os-cilloscope with a high voltage probe

co to visually observe exactly what thecircuit is doing so you can determine

ci3 the proper safety margin. An os-zo cilloscope will also enable you to de-

tect destructive voltage spikes andsuperimposed AC (also called AC rip-ple) so you can design a capacitor tohandle those harmful excursions.

Of course physical size, weight, and

u_al fragility are also important charac-

teristics of capacitor design. If you68 have size limitations, Mylar is the best

12" 10"

2" x 6" ALUMINUM -FOIL TABSECURED WITH CRAZY GLUE.

COPPER -CLAD PC BOARD.

COPPER MUST BE BRIGHT,SHINY CLEAN AND FREEOF CHEMICAL RESIDUE!

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Fig. 2. For a single -section capacitor, use one double -sided PC board. For multiplesections, use several single -sided boards clamped together or bolted together withnylon screws.

dielectric material to use since it has avery high puncture voltage per mil,and thus makes a very compact ca-pacitor. Plastics are light, so most ca-pacitors will weigh less than tenpounds. The toughest plastic is Lexan,which is difficult to crack even with ahammer and is often used for vandal -proof windows. Glass is the worst ma-terial for a lightweight, durable ca-pacitor, and can even crack under itsown weight when lifted. Take all thisinto account when selecting your ma-terials.

Of course, the overall cost in laborand materials should also be consid-ered before constructing a capacitor.Calculate beforehand the cost ofyour materials. Paper and poly-ethylene are the cheapest. Glass isthe next higher price. Labor time isabout the same with Plexiglas, Lexan,and glass sheet capacitors. Exoticplastics such as Teflon are not neededunless your application demands ex-treme chemical and thermal deterio-ration resistance. Polyethelene hasexcellent chemical resistance, but

breaks down gradually upon ex-posure to ozone gas (always presentaround high voltage) becoming brit-tle and less resistant to arc puncture.

That brings us to another importantconsideration: the capacitor's usefullife. To enhance a capacitor's lifekeep the working voltage at or belowthe rated specification in both DCand AC applications. We discoveredthat charging at no more than 70% ofa capacitor's working voltage resultedin an amazing 10 -fold increase in life-time for one type of commercial ca-pacitor. Also, for DC capacitors, watchout for voltage reversals. If your systemhas a lot of inductance, reverse volt-age swings are always produced. In-crease the safety margin if a lot ofinductance is in the circuit. Further-more, the temperature should bekept below 120°F As mentioned ear-lier, watch out for superimposed AC,voltage spikes, and ringing. Thesetypes of AC waves can drasticallyshorten lifetime. Tesla coils have noto-rious ringing. To repeat: if feasible, usean oscilloscope to visually analyze

your circuit. Often a power resistor in-serted in the current path to the ca-pacitor quenches ringing. With thiscriteria under our belts, let's look atsome problems your design and con-struction methods should prevent.

Signs of Trouble. Your assemblytechniques should seek to minimizethe likelihood of a few possible prob-lems. Luckily, all of them can be pre-vented at least in part by using ampleamounts of insulating material such asNo -arc or Corona Dope and/or highvoltage putty on all exposed areas. Aplastic case to enclose the apparatusis also recommended (more on thatlater).

Still and all, you should know whatproblems the insulation is preventing.The first problem insulation relieves isthe possibility of electrical shock.

Insulation also minimizes the pro-duction of ozone-a gas createdwhen high voltage causes three oxy-gen atoms to join together. Ozone hasa tart, sweet "electrical" smell, and is100 times as poisonous as carbonmonoxide. Beware: it quickly causesheadache, nausea, vomiting, and re-spiritory irritation. In addition to insulat-ing all the exposed HV areas, youshould also operate your equipmentwith good ventilation if it producesany ozone.

Closely linked to ozone generationis corona leakage. It is produced by acharge being leeched off a highlycharged object by the air. That typ-ically produces ozone. However,sometimes a device (such as a VandeGraff generator) is constructedspecifically to display corona dis-charge, and insulating it would defeatthat purpose. In such cases, goodventilation is the only practical meansof hazard prevention.

Ozone can also be created by arc-ing, which can occur anywhere. How-ever, ozone production is not thegreatest hazard arcing presents. At 50kV, a spark can arc between an unin-sulated contact and your body if youcome within 2 inches of the contact.Arcing commonly takes two forms: di-rectly through a capacitor's dielectric(as mentioned earlier), or across theedges of a capacitor's plates to anadjacent plate. A snapping sound in-dicates the presence of arcing, sokeep your ears open.

Arcing from the edges of a capaci-

tor plate, or anywhere the shape of aconductor changes abruptly (such asthe tip of a nail) is called point dis-charge. It can be readily observed ina dark room at very high voltages.Small, bright blue pinpoint(s) are seenleaking electrons into the air, accom-panied by a hissing sound and copi-ous ozone production.

Once again, insulation and properventilation are the proper solutions toall these problems, and there aresome specialized techniques to insu-late your capacitors and otherwiseimprove the safety of your high volt-age projects. Let's get to those now.

Construction Requirements. A keyingredient in a good assembly is aproper case. Your capacitor's housingmust protect it against moisture, dirt,and accidental discharge. Plasticcases for dry capacitors are easy tomake with acrylic sheets glued at allcorners with Silicone RTV. Oil -proofcases can be made for immersedmodels, but you will need to rough -upthe plastic at the sealing edges withsandpaper and use both a bondingand second fillet glue coating for aliquid -proof seal. Metal cases can bemade from PC boards cut on a shearor large paper cutter and soldered atthe edges. Copper roof flashing(available at hardware stores) workswell too. However when using metal,always beware of contamination bysolder rosin, solder bits, and othercrud, which can short out plates orotherwise reduce efficiency.

Whether a capacitor is enclosed orexposed, discharge paths must bewide enough to avoid arcs to thecase, adjacent plates, terminals, con-nections, or components. That is es-pecially important in situations whereconductors must be left uninsulated.Note that the space from each plateto the edge of the dielectric must bewide enough to stop any spark from"crawling" over the edge of one plateto another.

Power leads must be capable ofwithstanding the full voltage of thecharge plus at least a 50% safetymargin. TV anode wire, which comesrated up to 40-kVDC, makes greatleads. Vinyl tubing or aquarium airhose may be slipped over leads toincrease their voltage rating.

Make sure the plates are securelymounted or they will tend to shift, ormake a noisy rattle when used withAC. Glue or compress the assembly tohold it secure. With regard to mount-ing, keep in mind that glues that dryby evaporation of a volatile chemicalmight not set properly if "buried" in-side an assembly away from air, andcould thus become a fire hazard.

Rolled -up capacitors may be heldsecurely by wrapping the interleavinglayers of foil and insulator tight aroundan insulating mandrel and then tap-ing with a clear PVC tape. Where nec-essary, coat the ends with Silicone R1V.That will eliminate end -arcing flash-over and corona loss. Alternatively, al-though it is somewhat brittle, paraffin(with a puncture voltage of 250 volts/

TOP INSULATING SHEET

THICKNESS EXAGGERATED TO SHOW AREA OF COPPER

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ALUMINUM FOIL CONNECTION TABS

Fig. 3. With this design, you may stack as many plates as you wish, provided there arean equal number of plates attached to each lead.

70

BOTTOM PLATEFOIL TAB

INSULATING DIELECTRIC

NOTE EDGE SPACING

FINISHED CAPACITOR

BOTTOM PLATE FOIL TAB

MANDREL

J- MAW

ALUMINUM FOIL

ALUMINUM FOIL

INSULATING DIELECTRIC

TOP PLATEFOIL TAB

Fig. 4. A rolled -up capacitor, like that shown here, can provide the greatestcapacitance in the smallest space. Note that the dimensions in the side view have beengreatly exaggerated for the sake of clarity.

mil) is an excellent insulator for theends of rolled -up capacitors and theedges of flat -plate type capacitors. Ifyou want to use melted paraffin wax,heat the wax only in a double -boilerpan, since if it gets too hot it can catchfire. Be sure to apply several coats,allowing the wax to harden betweeneach coat. Liquid electrical tape alsomakes a great end seal, however it issomewhat hard to find. Try mail-orderdistributors for that product.

High voltage terminals for your proj-ects can be made from plastic rodsdrilled through to accept connectionwires. You may add a nut and bolt ontop for convenience. However,beyond about 3,000 VDC this methodsuffers from point discharge. Metal

balls make good terminals. Cleanthem up with a wire brush or steelwool to eliminate rough spots. The au-thor uses fishing floats covered witheither aluminum foil or nickel printpaint for up to 10 kVDC. Split the bob-bin first with a razor blade, remove theline holder and spring, and glue it to-gether again with epoxy.

Furthermore, as you work, keep allmaterials as clean as possible. Notonly will your work have a better ap-pearance, but arcs and burn-throughs due to contaminants will beprevented. High voltage easily tracksalong dust, surface contamination,and even finger oil (which containssalt). Also, we shall refer to a "section"as consisting of two conductive plates

with an insulating dielectric betweenthem.

By now we hope you have a goodunderstanding of the principles andtechniques involved in making yourown capacitors. Without forgettingsafety, let's talk about how to buildsome simple capacitors, any of whichcan be modified for your application.

A Leyden Jar Capacitor. LeydenJars are one of the first types of ca-pacitors made, having been inven-ted nearly two and a half centuriesago. Their development was first re-corded in 1745 by Ewald von Kliest. In1746, Peter van Musschenbroeck ofLeyden, Holland experimented fur-ther with the invention. We can buildour own modernized units with a gal-lon -size wide -mouthed mayonnaisejar. The project only costs about $2,and is good to at least 10 kVDC at 2.5nE Units we've tested at 15 kVDC didnot fail; at that voltage, the capacitorsstored just under 1/3 joule each.

First select a jar without bubbles,cracks, or blemishes and that has amouth large enough to comfortablyslip your hand through. Next, carefullyclean it out. You'll use aluminum foilinside and out as the conductiveplates (see Fig. 1 ) . Cut a foil disk 1 -inchbigger than the bottom of the jar.Now coat the dull side of the foil andinside jar bottom with a thin, evenlayer of rubber cement. Let both dryfor 10 minutes, and press together.Smooth with firm hand pressure. Avoidexcess wrinkles. Do the rest of the in-side except the top inch of the bottleusing three or four pieces of foil. (It iseasiest to do the plate in pieces in-stead of all at once, since rubber ce-ment "grabs" and it is difficult to re-position the foil once contact hasbeen made.) Now do the outside foilplate in pieces, leaving the top inchbare. Check the foils with a continuitytester to determine if the pieces are ingood electrical contact. Areas of foilnot in contact can be bridged withstrips of foil or nickel -print paint.

For the top cover, cut two disks ofclear plastic, one slightly smaller thanthe rim, the other 1/4 -inch larger thanthe rim. Glue the two pieces togetherto form a plug. Drill a 1/4 -inch holethrough the plug's center. Cut and in-sert a length of 1/4 -inch (outer diame-ter) metal rod or tubing through thishole. Attach a ball to its top, and sol-

der a wire or small -link chain to itsbottom. The wire must make goodelectrical contact with the foil. Let theassembly dry for a day with the coveroff, to allow vapors from the rubbercement to dissipate, then cement thecover on with silicone or Krazy Glue.

PC -Board Capacitor. Some nifty lowinductance capacitors can be madefrom pieces of copper -clad epoxy cir-cuit board (see Fig. 2). For a simpleIwo -plate capacitor, you can use onedouble -sided sheet. For multiple sec-tions, use single -sided board.

To prepare each board, start byetching away a 1 -inch strip fromaround all its edges. That process canbe simplified by first masking off thestrip, spraying the bare copper withan etch -resistant paint, removing themasking tape, and then etching.

Clean the board after etching, andrinse with de -ionized or distilled water.Thoroughly air-dry the sections, or usea blow dryer. Attach strips of alumi-num foil to each plate.

If you are building a multiple -sec-tion capacitor, connect the aluminumfoil strips together as shown in Fig. 3and secure them using glue or nylonbolts at each corner. Spray thefinished assembly with several coatsof an insulating product, or paraffin.

If you use the dimensions shown inFig. 2 and a 0.060 -inch gap betweenplates, you can achieve a capaci-tance of 1.94 nF (1940 pF) per section.When deciding on the gap width touse, keep in mind that the greater thespace between successive plates thelower the chance of arcing. For exam-ple, a 1 -inch spacing gives you a 30%larger gap than a 20 -kV spark canjump. Insulation will further improvethat margin.

The Stacked Sheet Design. Thistype is virtually identical to our PCboard capacitor, but it can be de-signed to handle considerably morevoltage. You simply substitute sheetplastic or glass dielectrics, and gluealuminum foil in place of the copperfor each section (refer to the PCboard capacitor drawing in Fig. 3 asneeded). All in all, it's an easier designto build, as it does not involve theeffort of etching copper, and you cancontinue to add sections to your origi-nal prototype to increase its capacityas future demands require.

Sources

All types of plastics: United States Plas-tics Corporation, 1390 Neubrecht Lane,Lima, OH, 45801; Tel. 800-537-9724.Company changes for catalog and re-quires a minimum -amount order. Write orcall for details.

High voltage rectifiers and meters, MCMElectronics, 858 E. Congress Park DrCenterville. OH 45459-4072, Tel513-434-0031. Free catalog.

Sheet and formed metals, plastics, andprecision tools: Small Parts, Inc,, PO Box381966, Miami, FL 33238-9980: Tel.305-751-0856. Free catalog.

Information, technical assistance, highvoltage parts, and kits: Allegro ElectronicSystems, 3 Mine Mountain Road, Corn-wall Bridge. CT 06754: Tel. 203-672-0123(9 AM -12 noon EST weekdays). Free cat-alog.

When building a large capacitor ofthis type, we suggest that you usenylon bolts at the corners to hold it alltogether. The bolt holes should bepre -drilled before assembly, and allchips cleared away. Make sure theplate -to -edge spacing is adequatefor the voltage you will subject thecapacitor to. Add extra spacing if youintend to use bolts at the edges.

Glue foil carefully to the top of thefirst plate using a small amount ofspray adhesive, Krazy Glue or R1V sil-icone. Press it smooth and let it dry. Aphotographic finishing roller is handyfor flattening foil. Repeat the pro-cedure for the second sheet, orient-ing the foil connection tab in theopposite direction. Keep the platesand dielectrics aligned as assemblyproceeds. Repeat this procedure foras many sections as you want. Alwayskeep the final number of plus andminus plates equal.

Put an insulating sheet above andbelow the last plate and secure theassembly with nylon bolts. Do not overtighten or the center of the assemblywill "bow." Finally, clean the ends witha very small amount of isopropyl (rub-bing) alcohol and wipe dry. Smear acoating of silicone R1V over all theedges.

Roll -Up Design. The kind of capaci-tor depicted in Fig. 4 can providelarge capacitance in a small size.They are a little trickier to make thanstacked -section type capacitors, soyou might want to try a few small pro-

totypes first. The design uses a layeredapproach (as shown), and we sug-gest using only one section as it is diffi-cult to align and wrap multiplesections. By contrast, a single sectionseveral feet long is not too unwieldy.

Aluminum foil works great in thesecapacitors. You'll find the oven/broilertype, which is heavy-duty foil, far easi-er to work with than the plain variety.Polyethylene and Mylar are the mostcommon dielectrics, but you can ex-periment with other materials.

Looking at the figure, note the ori-entation and shape of the foil plates(A) and (C). They can be easily se-cured to the dielectric (B) using dou-ble -sided Scotch tape. Note also theedge spacing. An outer covering ofdielectric (D) will prevent the finishedcapacitor from having a "hot" case,which might be a hazard. With thosepoints in mind, lay the foil out on asmooth sheet of paper, which in turnshould be laid out on a smooth, hardsurface to prevent wrinkling. Carefullyassemble the four layers as shown inthe drawing. Strive to make them flatand smooth.

Wrap the capacitor "sandwich"around a non-conductive mandrel orspool.- - ideally made of plastic orglass rod (be careful not to break aglass rod). Try to make the roll straightand free of lumps and wrinkles. Whenit's all rolled up, secure it with plenty oftape. The author uses clear package -sealing tape for this. Now secure thepositive foil tab (assuming it's going tobe for DC) to the mandrel using tape.Finally, coat the exposed ends with aninsulating product like silicone RN,

The remaining foil connection tabmay be reinforced by rolling it arounda small metal dowel. A nail, or a cut-off piece of 1/8 -inch uncoated brazingrod is suggested. Apply glue to holdthe assembly together.

Foil tabs can be strengthened byadding "ribs" of adhesive from a hotglue gun. Similarly, the tabs can bemade tear -resistant by applying hotglue where they enter the capacitor.

Note most problems with this designcome from particle contaminantsthat stretch a dielectric thin in spotswhere they are trapped by the tightlyrolled dielectric. Another trouble is in-adequate edge spacing, causingarcing across the ends. Careful plan-ning and assembly will eliminate bothheadaches. 71

In these times of rising costs anddiminishing real income, everydriver knows how very costly vehi-

cle maintenance can be, even for rel-atively minor repairs. For mostproblems, a driver has no alternativebut to take the vehicle to a servicetechnician. Sooner or later all vehiclesrequire repairs to the battery andcharging system, and if the problem isnot diagnosed properly, you couldspend a fortune replacing perfectlygood parts. But there is one area-thevehicle's battery and starting andcharging systems-that can bemonitored and diagnosed for possi-ble problems using a simple elec-tronic circuit.

Although modern automotive bat-teries and their charging systems canbe somewhat complex, diagnosingthem is relatively simple. And diagnos-ing electrical -system problems is justwhat the Automotive Electrical Diag-nostic System described in this articleis designed to do.

Some Background. The AutomotiveElectrical Diagnostic System monitorsthe battery's terminal voltage to de-termine the condition of the vehicle'selectrical system under various oper-ating conditions --including standingidle prior to starting, cranking, idling,and running with various electricalaccessories in operation. It keeps tabson your vehicle's battery, alternator,regulator, and cranking voltagethrough a set of 5 LED's.

Under normal conditions, a greenY LED lights and remains lit as long as?3 the battery voltage is normal prior to2 starting the engine. The other 4 LED's

light only when a malfunction occurs.( The circuit is powered from the vehi-

cle's electrical system, so It does not

o require a separate power source. Itcan be permanently installed in the

cou vehicle (to continuously monitor elec-

trical system status) or assembled as acc

l3 portable unit and connected to anyw-i vehicle with a 12 -volt electrical sys-4 tern.

All vehicles that use a 12.6 -volt lead -acid battery as the power source will

at. fall within certain voltage constraintsas the vehicle is operated. For exam -

72 ple, a battery in good condition will

An Electrical -SystemAnalyzer

for your car

ELECTRICAL SYSTEM ANALYZER

\\\

WEAR

OR

WEAH BATTERY

- NOT CHARGING

I=I%NT

OVE RCHARGING

-4

Build an electrical systemmonitor that can alert youto any one of severalproblems so that you canavoid the inconvenienceand expense of a road-

service call.

BY ANTHONY J. CAR ISTI

have a terminal voltage of about 12.6volts when the engine has not been inoperation for some time. Duringcranking, battery voltage should notfall below 9 volts.

When the engine is operating, thevoltage regulator should hold the ter-minal voltage between 13.5 and 15.0volts. When all accessories are on, thealternator should be able to maintainat least 13.5 volts across the battery atabout 1500 or 2000 engine RPM. Table1 illustrates normal battery -terminalvoltages under various operatingconditions and specifies possible

faults when any of the voltages levelsfalls outside of the acceptable limits.

Circuit Operation. At the heart ofthe Automotive Electrical DiagnosticSystem (see Fig. 1) is the MaximMAX8 214ACPE five -stage voltagecomparator, U1, which also contains abuilt-in 1.25 -volt precision reference.The chip also has an on -board logiccircuit that allows the outputs of two ofthe comparators to be inverted bytying the mode -select input (ms, at pin15) high (more on that later).

In the Automotive Electrical Diag-nostic System, the non -inverting inputsof all of the comparators are tied toU2's internal 1.25 -volt reference, whilethe ms input is tied high, causing theoutputs of comparators U1 -a and U1 -bto be inverted. When any comparatoroutput is driven low, its associated LEDlights.

Resistors R1. R6 form a voltage -di-vider network that provides specificvoltage levels at five junctions, eachof which feeds the positive input ofone of U1's five comparators; thuseach comparator receives a knownpercentage of the input (battery)voltage. Since each of the five com-parator circuits are similar, an expla-nation of one such circuit will cover all.Take U1 -e (the Nor CHARGING MCNiTCR), for

example.When the engine is at rest, the bat-

tery voltage is 12.8 volts or more, so thevoltage at the R3/R4 junction exceedsthe 1.25 -volt reference, causing theoutput of U1 -e to go high, holdingLED3 off and indicating that thecharging system is delivering currentinto the battery at engine idle. Even ifthe battery voltage rises above 12.8volts (which is normal during engineoperation), the output of U1 -e at pin10 remains high, keeping LED3 off. Butshould the charging system fail, bat-tery's terminal voltage will dip below12.8 volts, causing LED3 to light, alert-ing the driver to the condition,

Comparators U1 -c and U1 -d oper-ate in a similar manner, lighting theirLED's if the battery voltage falls below9.0 and 13.4 volts, respectively, indi-cating wEiki<BATTEW (insufficient cranking

voltage) or ,NSUFFIC EN' CHAPG NG(defective alternator operation).

TABLE 1-AUTOMOTIVE ELECTR CAL FAULTS

Condition Normal Voltage Possible Fault

Vehicle at rest 12.6 volts <12.4 volts: bad cell orseverely undercharged battery

Cranking >9 volts <9 volts: Weak battery

Idling >12.8 volts <12.8 volts: Not charging;bad alternator or wiring

Runningminimum load

>13.4 volts <13.4 volts: defective alternatoror voltage regulator

Runningminimum load

<15.2 volts >15.2 volts: Overcharging;defective regulator

Runningmaximum load

>13.4 volts <13.4 volts: alternatordefective or belt slipping

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Fig. I. The Automotive Electrical Diagnostic System is built around a MaximMAX82I4ACPE five -stage voltage comparator, which contains a built-in 1.25 -voltprecision reference, and on -board logic that allows the outputs of two of thecomparators to be inverted.

Comparators U1 -a and U1 -b haveto sense voltage levels higher than12.4 and 15.2 volts, respectively to lighttheir LED's. To accomplish that, themode -select input of U1 is connectedto Vcc, thereby inverting the outputs ofthose two comparators, and causingtheir respective LED's to light when thebattery voltage rises above 12.8 voltsprior to starting (a normal condition),and above 15.2 volts (OVERCHARGING).

In order to prevent ambiguity be-tween a NOT CHARGING and INSUFFICIENT

CHARGING situation, which would causetwo normally off LED's to simulta-neously light, R7 and D2 are placedinto the circuit. A low output from U1 -e(NOT CHARGING) pulls down the refer-ence input of U1 -d rtINSUFFICIENT CHARGING)

to prevent LED4 from lighting. Thus,when the vehicle charging system isnot working at all, LED3 (NOT CHARGING)

will light and LED4 (INSUFFICIENT CHARGING)

will not.Integrated circuit U2, a fixed 8 -volt

voltage regulator, maintains a con-stant supply voltage for U1. Diode D1has been placed in the circuit to pre-vent component damage in case thelead polarity is accidentally reversedwhen connecting the AutomotiveElectrical Diagnostic System to the ve-hicle's electrical system.

Construction. Although circuit wir-ing is not critical, and could havebeen done on perfboard, the author'sprototype was assembled on a smallprinted -circuit board measuringabout 2 by 21/2 inches. A template ofthat printed -circuit pattern is shown inFig. 2. A printed -circuit board is avail-able from the source specified in theParts List.

A parts -placement diagram for theprinted -circuit board is shown in Fig. 3.Regardless of which method of con-struction you choose, it is strongly rec-ommended that you use a socket forU1. That makes testing and trou-bleshooting the circuit easier should itever be necessary.

When installing the parts, be verycareful to properly orient the polar-ized components. Just one compo-nent inadvertently installed back-wards will prevent the circuit fromoperating and may cause damageto itself and/or other components. So,as always, double-check the positionof the parts before soldering themin place.

When connecting the LED's to thecircuit board, use flexible insulated

r-

44°

ft2 3/16 INCHES

Fig. 2. The project MIS assembled on asmall printed -di -twit hoard measuringabout 2 hr 2 1/2 inches. A printed -circuitboard is available from the sourcespecified in the Parts List.

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73

PARTS LIST FOR THEAUTOMOTIVE ELECTRICAL DIAGNOSTIC SYSTEM

SEMICONDUCTORSUl-MAX8214ACTE voltage -

comparator, integrated circuit(Maxim)

U2---AN781,08 8 -volt. IOU-mAvoltage regulator, integrated circuit

DI-1N4(X)4 I -amp 4(X) -Ply siliconrectifier diode

D2 --1N4148 general-purpose silicondiode

LEM. LED3-LED5-Red light -emitting diode

LED2-Green light -emitting diode

RESISTORS

(All resistors are 1/4 -watt. V.4, metal -

film units. unless otherwise noted.)RI--100.000-ohmR2 -4530 -ohmR3 -357 -ohmR4-487 ohmR5 -1300 -ohmR6 -9530 -ohm

R7 -220,000 -ohm, 1/4 -watt,carbon

RK-R12---470-ohm, 1/4 -watt, 5e;;carbon

CAPACITORSC1 -100-µF, 25-Vv'VDC, radial -lead

electrolyticC2. C3 -0.1-µF. ceramic -disc


Printed -circuit materials, IC socket,enclosure, test leads, alligatorclips, solder, hardware. etc.

Note: The t011owing parts areavailable from A. Caristi, 69 WhitePond Road. Waldwick, N.J. 07463:printed -circuit hoard. 511.95; a setof 6 metal -film resistors, 54.50:Ul, 12.50; U2. S2.50. Please addS3.00 postage/handling. NewJersey residents please addappropriate sales tax.

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Fig. 3. Assemble your printed -circuit board acording to this parts -placement diagram.It is strongly recommended that a socket be provided for Ul.

wire; solid wire is not recommendedsince it has a tendency to break. It isalso important to use 1% metal -filmresistors where specified. The ac-curacy of the circuit depends on the1% tolerance of those resistors. Thetolerance of ordinary carbon resistorsis simply not sufficiently accurate orstable for that part of the circuit.

The circuit board can be housed inalmost any a small plastic or metalenclosure. If you house the circuit in ametal enclosure, be sure that no part

74 of the wiring touches any metal sur-

face, and use a rubber grommetwhere the input leads of the instru-ment pass through the enclosure. TheLED's should be mounted to the frontpanel of the enclosure and eachshould be labeled to indicate itsmeaning. Use a green LED for LED2(BATTERY OK) and a red unit for all others.

If you wish to permanently install theproject into a vehicle, connect thenegative input wire to any metal partof the chassis. It is best to connect thepositive input lead directly to one ofthe wires of the ignition switch, which is

powered only when the ignition isturned on. That avoids unnecessarybattery drain when the vehicle is notin use.

Note: To maintain the AutomotiveElectrical Diagnostic System's ac-curacy, it is not recommended thatthe positive lead of the circuit be con-nected to any part of the vehicle'selectrical system in which a significantvoltage drop from the battery positiveterminal occurs, especially with ac-cessories turned on. To measure thevoltage drop in the vehicle wiring,connect a DC voltmeter between thepositive battery terminal and thepoint at which the Automotive Elec-trical Diagnostic System is to be con-nected. A voltage drop of more than0.05 volts with accessories operatingindicates that some other point in thewiring should be chosen for the circuit.

For a stand-alone unit connectlengths of 18- or 20 -gauge flexible ortest -lead wire (color -coded red forpositive and black for negative) to thecircuit for the input leads. Those wiresshould be terminated in alligator clipslarge enough to grip the terminals ofthe vehicle's battery. Such clips areavailable from most auto -parts orelectronics -supply outlets.

When the assembly phase is com-plete, examine the project very care-fully for cold solder joints (which mayappear as dull or rough blobs of sol-der). Also, examine the board for shortcircuits, especially between adjacentIC pins. Correct any errors that youfind.

Checkout. It is important to benchtest the project before installing orusing it on a vehicle. To test the projectyou'll need an well -filtered variable,DC power supply with an adjustmentrange of at least 8 to 16 volts, and anaccurate DC voltmeter. Connect thevoltmeter and project input leads tothe output of the power supply, whileobserving the proper polarity of bothsets of wires.

Start with the supply set to its mini-mum voltage and slowly raise the out-put until it reaches 8 volts. At that point,both LEDI and LED3 (mAx BATTERY andNOT CI-LARGNG) should light. The project'sdisplay is indicating that the batteryvoltage is below 9 volts, and also thatthere is no charging current. Note fur-ther that LED2 (BATTERY OK) I$ ex -


Build aTelephone Scrambler

Keep your privileged conversations completely private with this audio -scrambling circuit.

How private are your privatetelephone conversations? Ifyou are like most people, you

take for granted that conversationsheld thebetween you and the person on theother end. It is a very uneasy feeling tofind out later that someone has beenlistening in.

Well worry no more, for theTelephone Scrambler described inthis article is designed to dispose ofthat problem. The TelephoneScrambler, which connects betweenyour telephone and wall jack anddoes not require modifications to yourtelephone, is designed to scrambleyour voice before sending it over thetelephone line. Anyone picking up anextension or monitoring your con-versation with a telephone -line trans-mitter will hear nothing more thangarbled sound. Only the party at theother end of the telephone line, usinganother Telephone Scrambler, will beable to understand what you are say-ing.

Theory. There are many differentways to scramble audio signals thatcontain speech so as to render thespeech unintelligible. The difficult partof making speech unintelligible lies indoing so in a manner that allows it tobe restored to its original form. TheTelephone Scrambler accomplishes

BY TERRY J. WEEDER

that task by shifting all high frequen-cies to low frequencies, and all lowfrequencies to high frequencies.

The exact amount of frequency shiftthe distance in Hz

that the original signal is from the cen-ter frequency of 1790 Hz. For example,the parts of your speech pattern thatlie at 2290 Hz (500 Hz above the cen-ter frequency) will be shifted to 1290Hz (500 Hz below the center frequen-cy), and vice versa, the parts of yourspeech that lies, at say 2790 Hz (1000Hz above the center frequency) willbe shifted to 790 Hz (1000 Hz belowthe center frequency), and vice versa.The result is a garbled audio signalsounding similar to that of listening toan SSB-modulated carrier with an AMreceiver.

The process of restoring the audioto its original form can be accom-plished in the same way. Simply per-form the task of frequency shifting onthe scrambled audio for a secondtime, and all the products in thespeech pattern will be shifted back totheir original frequency. (The processis analogous to double -inverting asignal in a digital system.)

About the Circuit. Figure 1 shows afunctional block diagram of the Tele-phone Scrambler. The circuit is com-prised of two hybrid networks, two low-pass filters, two balanced modulators,

an oscillator, a divide -by -1000 coun-ter, and two buffer stages. The circuit'soperation is really quite simple. Theoscillator provides a 3.58 -MHz signalthatto provide a 3.58 kHz signal, which isthen fed through the buffer to one ofthe modulators. While that's going on,speech is applied to the hybrid net-work, which transfers the signal to thebalanced modulator, where the twosignals are mixed (to producescrambled audio), and then output tothe telephone line through a low-passfilter.

A schematic diagram of the actualcircuit is shown in Fig. 2. The circuit ispowered from a 13 -volt AC wall trans-former, the output of which is convert-ed to DC by a fullwave bridge rectifier(BR1) and regulated by a pair of Zenerdiodes (D1 and D2) to form a dual ( )

6.2 -volt supply. Power for the relay (K1)is tapped from the output of BR1 (priorto regulation) so as not to put an un-necessary strain on the regulated 6.2 -volt source.

Initially, relay K1 is de -energized, sothe circuit is bypassed through K1'snormally closed contacts, allowingthe telephone to operate in the nor-mal manner. A PS2505-1NEC op-toisolator/coupler (U6) is used tomonitor current flow through the tele-phone and provides base bias cur-rent for Q1 only when the telephone is 75

0

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TELEPHONE

Fig. 1. The Telephone Scrambler scrambles all outgoing dialog from the telephone,while it descrambles all incoming dialog from the telephone line.

off hook. With base bias present,pressing S2 (SCRAMBLE) turns on SCR1and current flows through Q1, current -limiting resistor R44, and the coil ofrelay K1, activating it. Current alsoflows through LED1, indicating that thecircuit is in the scrambled mode.

Diode D3 is included in the circuit to

protect Q1 from reverse voltagespikes, resulting from the de-energiza-tion of K1. Capacitor C15 providestemporary bias current for Q1 duringthe break in telephone current whenthe relay armature is in the process ofmoving between sets of contacts.

When K1 (a DPDT relay) Is energized,

one set of its contacts places the pri-mary winding of transformer T1 acrossthe telephone line, completing thecurrent loop that alerts the central -office switching equipment to an off -hook condition, while coupling audiosignals to and from the telephoneline. At the same time, -6 volts is ap-plied to the telephone through K1'ssecond set of contacts and trans-former T2's primary winding, providingoperating power to the telephone(like in the normal off -hook condition),and allowing audio to be coupled toand from the telephone.

The circuit includes two hybrid net-works. The first consists of the second-ary winding of T1, coupled with U3 -dand R22 through R24. The secondhybrid network is comprised of thesecondary winding of T2, coupledwith U3 -c and R19 through R21. Thosenetworks allow signals from the sec-ondary windings of the transformersto be applied to the inputs of op -amps U3 -c and U3 -d. The outputs ofop -amps U3 -a and U3 -b are in turnapplied to the secondary windings of

SEMICONDUCTORSU 1-4069 hex inverter, integrated

circuitU2-4040 I2 -stage ripple -carry

binary counter/divider, integratedcircuit

U3-LM348N quad op -amp,integrated circuit

U4, U5-LM 1496N balanced-modulator/demodulator, integratedcircuit

U6-PS2505-1NEC optoisolator/coupler. integrated circuit

SCRI-S401E-ND I -amp. 400-PIV.non -sensitive gate. siliconcontrolled rectifier

QI-2N4400 general-purpose NPNsilicon transistor

BRI-I-amp. 200 -Ply, fullwave-bridge rectifier

DI, D2 -1N4735. 6.2 -volt Zenerdiode

D3 -1N4148 general-purpose siliconswitching diode

LED1, LED2-Light-emitting diodeMOVI-130-VRMS, metal -oxide

varistor

RESISTORS(All fixed resistors are 1/4 -watt. 5%

units.)RI--10-megohm

PARTS LIST FOR THETELEPHONE SCRAMBLER

R2 -1200 -ohmR3, R40 -4700 -ohmR4, R39 -47,000 -ohmR5, R38 -20,000 -ohmR6, R7, RI9, R20, R23 R24, R36,

R37 -10,000 -ohmR8, RI4, R30, R35 -3900 -ohmR9. RIO, R28, R29. R42 -51 -ohmRII-R13. R15, RI7, R18, R25 -R27.

R3I, R32. R34--1000-ohmR16. R33 -6800 -ohmR2I, R22 -560 -ohmR41 -I30 -ohmR43, R47 -220 -ohmR44 -750 -ohmR45, R46 -20 -ohm

CAPACITORSCI, C4. C16, C17, C20 -0.1-µF,

MylarC2. C3-36-pF, ceramic -discC5, C14-220-pE ceramic -discC6. C13 -0.02-11E, MylarC7, C12 -0.033-0..F, MylarC8, C11 -1-1.1,F, I6-WVDC. tantalumC9, C10 -22-µF. 16-WVDC,

electrolyticC15 -2.2-µF. I6-WVDC, tantalumC18, C19 -100-µF, 35-WVDC,

electrolyticC21 -1000-11E, 35-WVDC,

electrolytic


XTALI-3.58-MHz, TV color -burstcrystal

T1, T2 -600 -ohm to 600 -ohm audiotransformer

KI-12-volt DC, DPDT relaySI. S2-SPST momentary

pushbutton switchPrinted -circuit materials, enclosure.

13 -volt AC. 800-mA walltransformer (Radio Shack no.273-1610 or similar), telephonecord, power cord jack, wire,solder, hardware. etc.

Note: The following items areavailable from WeederTechnologies. P.O. Box 421,Batavia. Ohio 45103: An etchedand drilled printed -circuit board(WITS -B). $14.00; a kit of allboard -mounted components(WTTS-C). $29.00; a pre -assembled circuit board ( \VTTS-A), $69.00. All orders mustinclude $3.50 for shipping andhandling. U.S. and Canadianorders only please. Ohio residentsmust add 6c4 sales tax. Aboveprices are tirr single units only. 2Telephone Scramblers arc required.

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Fig. 3. This template for the Telephone Scrambler's printed -circuit board is presentedfor those who prefer to roll their own. However, if you are so inclined, vou canpurchase a prefabricated board from the supplier listed in the Parts List.

T1 and T2. The outputs of U3 -a and U3 -b are also applied to the inputs of U3 -c and U3 -d, but because both invert-ing and non -inverting inputs of U3 -cand U3 -d are tied together, the signalsnullify each other (the signals causeno output change to either U3 -c orU3 -d).

At the heart of the circuit are thetwo LM1496N balanced modulators(U4 and U5). The audio signal appliedto pin 1 of both U4 and U5 is mixed withthe squarewave signal that is intro-duced to each IC at pin 8. The outputsof U4 and U5, taken from pin 6 of eachunit, consists of the sum and dif-ference of the two input signals. Thesum of the two signals is filtered out bylow-pass filters made up of U3 -a, R4through R7, and C5 through C7 for U4,and U3 -b, R36 through R39, and C12through C14 for U5. That leaves onlythe difference frequency.

The squarewave is generated byU1 -a (1/oth of a 4069 inverter) and a3.58 -MHz color -burst crystal. The 3.58 -MHz squarewave is used as the clockinput for U2 (a 4040 binary counter)whose output at pin 14 is the clockfrequency divided by 1000, which isequal to 3580 Hz. That 3580 Hz signal isapplied through buffers, consisting ofU1 -c through U1 -f, to pin 8 of U4 andU5, and determines the frequencyshift of the input signals at pin 1.

Circuit Construction. The Tele-phone Scrambler was assembled in aprinted -circuit board, measuringabout 47/8 by 37/8 inches, Atemplate ofthat printed -circuit artwork is shown inFig. 3. You can etch your own boardfrom the artwork, or purchase theetched and drilled board and a kit ofboard -mounted components fromthe supplier listed in the Parts List.

After obtaining the board and allthe parts in the Parts List, use the parts -placement diagram in Fig. 3 to guideyou in assembling the circuit. Start bymounting U1 -U6, T1, T2, and K1. Mount-ing those larger components first willmake it easier to orient your boardwith the parts -placement diagramand find the correct locations for theother components.

Next, mount the resistors, capaci-tors, and diodes. Pay particular atten-tion to the diodes and polarizedcapacitors to make sure that they arenot installed backwards. Finish by in-stalling the remaining componentsand the four jumper connections. Ver-ify that all IC's have their notchespointing in the right direction (ac-cording to Fig. 3). Also check to makesure that Q1, SCR1, and BR1 are notinstalled backwards.

Your enclosure must be largeenough to house the board and anypart of the switches and jacks thatextend into the enclosure. Prepare theenclosure by drilling four holes in thefront panel of the appropriate size toaccommodate the two switches (S1and S2) and two LED's (LED1 and LED2).Then drill a hole in the rear panel to fitthe power jack. (If a Radio Shack273-1610 wall transformer is used, it willmate with a Radio Shack 274-1563power jack. The inside diameter of awall transformers's connector variesfrom model to model so it would be agood idea to verify that wall trans-former you select mates with the jackyou select before mounting it to theenclosure.)

Mount the LED's, switches, andpower jack. Then position the boardand mark the locations for drilling. Drillthe four board -mounting holes andinstall standoffs. Drill out two moreholes in the back panel just, largeenough to run a piece of telephonecord through. File the holes smooth toprotect the insulation on the wires. Asingle telephone cord with modularjacks on each end can be cut in half,feeding the cut ends through theholes in the back panel. The authorput a 2 -inch piece of heat -shrink tub-ing over the cords where they passthrough the holes in the back panel toprotect them and a tie wrap on eachcord just inside the enclosure to pro-vide strain relief.

Label one of the cords TO WALL JACK(Continued on page 104)

To many PopularElectronics read-ers, loudspeaker

design may seem morethan a bit mysterious, whatwith all the talk of "Thiele/Small parameters," reso-nance, and various, morearcane terms. Indeed, formany, building a speakersimply means buying some"raw drivers" and puttingthem in any old cabinetthey might have around,and hoping for the best.Too often, such haphazardwork is a big disappoint-ment.

In contrast, when a hardysoul goes hunting for somemore substantial designtheory, all they might findare some very specializedphysics texts. All in all, it is adiscouraging situation, tosay the least.

However, all is not lost.Thanks to the work of twoinsightful engineers-A. N.Thiele and R. H. Small-thenumber of parametersyou'll need to chose be-tween to produce a goodloudspeaker design hasbeen reduced from hun-dreds to just three. Further,the three programs pre-sented in this article will letyour PC do the numbercrunching for you. Specifi-cally they'll tell you the volume anddimensions of the enclosure, and de-sign a crossover network for you.

To simplify this approach further,most driver manufacturers provideyou with some of the necessary pa-rameters to help you decide on a driv-er. Many even publish them right intheir catalogs and brochures. That willallow you to do some useful trial -and -error work with your computer, insteadof wasting many hours of hard workand dollars worth of wood and driv-ers.

The Cabinet. First we'll look at cab-inet design. At this point you might bewondering if a cabinet is at all neces-sary. There are two reasons why youneed a cabinet (more properlycalled an "enclosure"). The first is toprevent the sound radiated from the

Designyour own

LoudspeakersThree simple programs that will help youdesign your own speakers the right way.

BY WILLIAM R. HOFFMAN

rear of the driver (which is in oppositephase to that from the front) frommeeting and canceling the driver'sforward output. Second, an enclosureserves to flatten and extend the driv-er's bass output. (Anyone who hasever propped up a driver without acabinet and played some musicthrough it knows what happens to thebass.)

Specifically, we'll discuss the designof a sealed acoustic -suspension cab-inet. Of course, there are other typesof enclosures-bass-reflex, ducted -port, passive -radiator, horn, andmore. All of them could serve thesame purpose, but are much harderto design and build. So we will onlydeal with a simple, acoustic -suspen-sion system here.

The Basics. There are a couple of

preliminary concepts ofspeaker design that wemust first introduce. Let'sstart by defining the threeparameters that Thiele andSmall have given us. Theyare commonly abbrevi-ated f,,Qts (also referred toas Qt or the "Q"), and Vas. Asmentioned earlier, thoseparameters are commonlysupplied by a driver's man-ufacturer.

Let's start with f0, or thefrequency of oscillation.That refers to the naturalbass -resonance frequencyof a loudspeaker driverwithout a cabinet. It will beused to determine how lowin frequency our as-sembled system will effi-ciently go, or fb.

Second, Qt is the degreeof peaking that occurs inthe driver's response, againwithout a cabinet, at thebass -resonance frequen-cy. That is related to the Qwithin a cabinet, whichwe'll call Q- cab' The param-eter cab can be decidedmore or less by the design-er.

Last, Vas is the volume ofair contained within aclosed space that wouldhave the same com-pliance or "springiness" asthe suspension of the driv-

er's moving part (the cone).With the three manufacturer -sup-

plied parameters (fa, Qts, and Vas), acarefully chosen cabinet Q (Qo),and the help of our first design pro-gram, we can exactly specify the vol-ume of a sealed acoustic -suspensioncabinet. The enclosure will work withthe chosen driver to give the best bassresponse possible.

Now that you know the parameters,there are a two things we need tomention about using them to design aloudspeaker. First, there's more thanone possible enclosure for any givendriver. The difference between thedesigns is a trade-off between howlow we want the frequency responseto extend (fc,,,,) and how sharply wewant the response to fall off belowthat bass cut-off point determined by

(Qcab) 79

+20Second, we must understand that

once we have selected our driver, wecan only set one of those two param-eters (bass frequency response orbass fall -off). Although it can be eitherone. Once we decide the value ofone parameter, the remaining pa-rameter will be fixed by the laws ofphysics.

Realistic Bass Cutoff. It is importantto recognize that a given size driverhas a fairly well defined lower fre-quency limit. That is because themovement of the cone of a drivermust quadruple (yes, quadruple) forevery halving of frequency to main-tain the same acoustic output. Therecomes a point at which the driver willsimply run out of cone excursion (thedistance the cone can move) and itssound output will fall off sharply.

There are many advertisementsthat claim miraculous bass reproduc-tion from tiny speakers, but Table 1gives us some practical values toshoot for in our design work. Attempt-ing to push a driver much below thoserecommended frequency limits willusually result in weak or distorted out-put, or even driver damage.

The table also indirectly gives us aguide to our lower limit for the value offc,b, your speaker's bass -output ca-pability. That's because the driver in acabinet can't produce bass responselower than it can in free air-a naturalconsequence of the loading effect ofthe volume of air trapped in thesealed cabinet behind the driver. It

therefore sets a lower practical limiton one of our "designer -determined"parameters.

TABLE 1-LOWEST FREQUENCIES

Driver Size (inches) Low Limit (H2)

4 90 - 955 85 - 906 65 - 708 40 - 45

10 30 - 3512 25 - 3015 20 - 25

The Q. Now, we must consider the"Q" of the system (Qcab)' the otherparameter we can set ourselves.There are some things we need toknow about Q. To begin with, the high-er the Q, the stronger the bass outputwill be. You might be tempted to

80 make the Q as high as possible, but

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Fig. I. the Q is too high. there will he an undesirable peak in the frequenesresponse. Thus the speaker will sound to hoorm.

LISTING 19 PRINT:PRINT:PRINT:PRINT:PRINT10 PRINT " LOW FREQUENCY ACOUSTIC SUSPENSION LOUDSPEAKER"11 PRINT:PRINT12 PRINT "13 PRINT:PRINT:PRINT14 PRINT "15 PRINT:PRINT:PRINT17 PRINT "18 PRINT:PRINT:PRINT20 INPUT "22 PRINT:PRINT:PRINT:PRINT:25 ON P GOTO 30, 99030 INPUT "35 PRINT40 PRINT "45 INPUT "46 PRINT50 PRINT "55 INPUT "60 PRINT65 PRINT "70 INPUT "75 PRINT:PRINT:100 LET Z=(E/D)^2*(1.05)-1110 LET Y=B*E/D120 LET X=C/Z130 PRINT"135 PRINT140 PRINT"145 PRINT150 PRINT"155 PRINT"160 PRINT165 PRINT"170 PRINT"172 PRINT175 PRINT"180 INPUT"182 PRINT185 ON F GOTO 30, 990187 PRINT990 PRINT"999 END

DESIGN PROGRAM"

By William R Hoffman"

Copyrite 1993"

Proceed? (1=yes 2=no) and Enter ",P

Free air resonance of driver (Hz) =";B

Air volume compliance equivalent ordriver (Vas) in cu. ft =",C

Free air total Q of driver at"bass resonance (Qts) =",D

What is desired completed system Q "at bass resonance (Qcab) -",E

For an in cabinet bass driver Q of'E

The cabinet volume is ="X"cu. ft."

And the driver bass resonance frequencywill be ="Y"Hz"

If you wish to print this information,"use <prn scr> key at this time."

Would you like to try another driver',"( I =yes 2=no) and Enter ",F

Good listening!"

remember, the higher the Q the fasterthe speaker output will fall off belowf,. Also, just as in an electrical circuit,too high a Q can cause ringing andoscillation, which means highly dis-torted (typically muddy or boomy)bass.

So then, what would be a good val-ue? Figure 1 shows some Q curvespossible with our completed speakersystem. Notice, that for values muchover 1, a large peak in the output oc-curs, growing with increasing Q. Thepeak is what you must avoid if youwant good sound from the com-pleted system.

For some guidelines, look at Table 2where a range of Q values are sug-gested for various bass -driver sizes.The values are based on many fac-tors, including the size of the driveritself. Notice that very small driversshould have rather high Q values,while larger ones have lower Q's. Thereasons for those suggestions are verypractical. A tiny 4 -5-inch driver has sosmall a cone, and consequently suchweak bass output, that the slight peakdue to a high Q value actually makesthe sound fuller and more satisfying. Avery large driver, on the other hand,can produce so much bass outputthat a lower Q is required to controland smooth -out its response. So now,armed with some knowledge aboutfca and Qcob, we are ready to beginsome practical design work.

TABLE 2-POSSIBLE O's

Driver Size (inches) 0 Value Range

4 -5 0.9 - 1.26 0.8 - 1 08 0.7 - 0.910 0.6 - 0.9

12 - 15 0.5 - 0.8

Our First Program. To begin with,you need to enter the speaker -designprogram (see Listing 1) in a directoryalong with GWBASIC or BASICA, atleast one of which you must have torun the program. We called our pro-gram LFSPKR-1 BAS. It's job is to calcu-late the volume of the enclosure andthe bass resonance frequency of thesystem.

Be sure to carefully enter it as it is

shown, including all punctuation andspaces. They are very important to theprogram. All the blank spaces foundin the "input" and "print" statements

bring the displayed lines in the pro-gram to a nicely centered place onthe working screen, so be sure to in-clude them. If anyone needs help, orhas never entered or run a BASIC pro-gram, just find a friend or ask some-one about how it is done: the processis very easy. Really!

Running LFSPKR-1. Once the pro-gram has been typed in and enteredas LFSPKR-1.BAS, from the DOS promptwe can start it by typing GWBASICLFSPKR-1.BAS if you are using GWBASIC,or BASICA LFSPKR-1.BAS if you are using

BASICA. When the opening screenappears, the title, "Low FrequencyAcoustic Suspension Loudspeaker De-sign Program" appears. The programthen asks you, if you wish to proceed.After answering "1" (or yes), you will beasked for the values of f,, V05, and Q.in that order. Then, you will be prompt-ed for a value for Qb, the com-pleted system Q at bass resonance.That value must always be higher than

the bass resonance, Qts.Since we are only making trial -and -

error calculations, we can search fora bass driver/ tcab/ -Qcob combinationthat gives you the system a bass cut-off frequency/Q combination that youwant based on the guidelines alreadygiven. Once you've entered the driv-er -based parameters, choose avalue (Q,), from Table 2. Once thatis entered, the program will automat-ically calculate the interior volume ofa cabinet that will yield the Q- cab wespecified.

Typically it's good to line an en-closure with at least a single 1--2-inchthick layer of material. Typically Fi-berglas, which usually comes in a rollor in pillow form, is cut into bails thatcover the area of each of the interiorsurfaces, and fastened to the wallsusing a staple gun. Applying it thatway means you can probably expectto gain about 5% of the effective en-closure volume, something you canusually ignore.

LISTING 210 PRINT:PRINT:PRINT:PRINT:PRINTIS PRINT "16 PRINT:PRINT17 PRINT "18 PRINT:PRINT:PRINT19 PRINT "20 PRINT:PRINT:PRINT21 PRINT "22 PRINT:PRINT:PRINT23 PRINT "24 PRINT "25 PRINT "26 PRINT:PRINT:PRINT:PRINT27 INPUT "28 PRINT:PRINT:PRINT:PRINT30 ON P GOTO 40, 99035 PRINT:PRINT:PRINT:PRINT40 PRINT "41 INPUT "43 PRINT:PRINT:PRINT:45 ON A GOTO 5050 LET X=A^(1/3)*(12)55 LET Y=X*.6260 LET Z=X*1.6270 PRINT "71 PRINT "72 PRINT "75 PRINT:PRINT:PRINT80 PRINT "85 PRINT "90 PRINT "95 PRINT:PRINT100 PRINT"110 INPUT"120 ON B GOTO 35, 990990 PRINT:PRINT:995 PRINT"999 END

SPEAKER ENCLOSURE DIMENSIONS"

DESIGN PROGRAM"

By William R. Hoffman"

Copyrite 1993"

This is a program designed to calculate optimum"dimensions for a loudspeaker enclosure when only"the desired volume is known."

Proceed? (I=yes 2=no) and Enter";P

The desired enclosure internal"volume is (cu. ft.) =" ;A

rCD

mThe following are the INSIDE dimensions of a box" ;(73

of A"cubic feet that are optimized to provide the"least number of standing waves (or resonances)."

HEIGHT = "ZWIDTH = "XDEPTH = "Y

Do you wish to try another volume?"(1=yes 2=no) and Enter";B

Good listening!"

z

0ww

Cn-4

0

81

82

Now the Questions. Looking at ourcalculated values, we can ask our-selves: is the frequency of cab lowenough to ensure good bass re-sponse, but still within the limits sug-gested by Table 1? If it is somewherenear the limit shown in the table, thenwe will be getting all that we probablycan from it. Then we can ask: is the sizeof the suggested enclosure too big orsmaller than we want?

If the values are unsatisfactory, thenyou can simply try again. At theprompt, respond (1 = yes), and thenenter another value of Qcab for theparticular driver you are working with,or instead try the specifications of an-other driver that you might want touse. Because this is a computer pro-gram, you can easily try as manylimes as you want until you find theright set of specifications for yourneeds. If you wish, you can even havea printer (almost any printer will work)print out the entire screen with all theprompts and information contained.Simply use the PRNT sca key on the key-board.

What Volume? Now, the next logicalstep is to use the calculated volumeand another small program to get theactual internal cabinet dimensions(height, width, and depth.) as indicat-ed in. Fig. 2. But before we introducethe next program, let's discuss a fewpoints first.

For one, the enclosure volume cal-culated in our first program(LFSPKR-1.BAS) was the empty cabinetvolume. That is, no allowance wasmade for the space that would beoccupied by the driver (or drivers),any extra bracing you might want touse, or the crossover network.

Alternatively, you can change thevolume with acoustic damping mate-rial (Fiberglas, wool, or foam). Addingit will make the cabinet seem largerthan it really is. (Yes, I said larger!)When the acoustic waves from therear of the driver pass through thesesoft, absorptive materials, they are ac-tually slowed down by a smallamount-typically by about 20%-and are also partially absorbed. Theresult is that the enclosure seems"acoustically bigger" than it really is.Fora typical material like Fiberglas, upto 40% larger. So, if you were to lightly"fluff" fill a box of 1 cubic foot in vol-ume, it would acoustically look like

about 1.4 cubic feet to the driver.

The Second Program. Our secondprogram is a simple one. It asks for thevolume of enclosed empty air spacewe are looking for. In return, it gives usa set of inside (not outside) dimen-sions that offer the least opportunityfor resonances or standing waves todevelop within the enclosure. Also, itgives us a nice rectangular shape thatis pleasing to the eye.

To use the program, ENCDES-1.BAS,first enter it as shown in Listing 2. Likethe first program, be sure and enter itexactly as shown. Once this is done,

141--DEPTH(V)

WIDTH(X)

HEIGHT(Z)

Fig. 2. The second program uses thethree variables shown here to indicatethe cabinet dimensions.

you may access it along with yourcopy of BASIC by entering eitherGWBASIC ENCDES-1.BAS (if you areusing GWBASIC), or BASICA EN-CDES-1.BAS (if you are using BASICA) atthe drive prompt.

Once the opening screen appears,respond to the prompt with a "1," andENTER. Following that, another promptwill ask for the enclosure volume. Enterthe value in cubic feet, but rememberthat the box volume must include anallowance for the driver(s), acousticmaterial, bracing, and crossover, ifany. Once that is done, the programwill calculate and display a set of val-ues for the three inside dimensions, ininches. With these, then, we are readyto start building our new speaker sys-torn.

Now we need to talk aboutcrossover networks and the last of thethree programs to be presented. Theprogram calculates the exact valuesof inductors and capacitors that youwill need to complete your loud-speaker system.

TO AMPLIFIER

SPKR1TWEETER

SPKR2WOOFER

Fig. 3. A crossover network divides thefrequency spectrum up so only thenecessary sonic components arereproduced by each driver.

What is a Crossover? Most speakershave more than one driver. The rea-son for having multiple drivers is sim-ple: a big driver will reproduce bassfrequencies very well, but is very pooron the highs because its cone is toobig and heavy to move very quickly.While on the other hand, a little drivercan make its light little cone movevery rapidly and reproduce the highsvery well, but it cannot move enoughair to reproduce deep bass in anyquantity. Obviously, then, we musthave at least two different types ofdrivers (one large and one small) toreally be able to make a good, wide -

range speaker with low distortion.There are also speaker systems

made with even more drivers. Thereare some with three or even four,each one covering only a very narrowand specific range of frequencies (forwhich it is optimized.) In such a systemeach driver is physically smaller thanthe last as we move up from the bass,through the midrange, and on to thehighs. However, here we'll limit ourscope to a two -driver, or two-way sys-tem.

To prevent large drivers (called"woofer's") from distorting while tryingto reproduce highs, and small drivers(the smallest of which are called"tweeters") from blowing out fromlows, a crossover network is used. A

crossover separates the sonic fre-quency spectrum into the ranges

needed by each driver in a multi -wayloudspeaker. More specifically, it is acombination of inductors and capac-itors that form an electronic filter network.

The Crossover Components. Let'slook at an example circuit to see justwhat a crossover does. The simplecrossover circuit in Fig. 3 is applied toa speaker system that has a wooferand tweeter. Notice that there is aninductor wired in series with thewoofer, and a capacitor wired in se-ries with the tweeter. Those Iwo ele-ments divide the sound spectrumbetween the drivers.

The inductor when connected as inFig. 3, forms a low-pass fitter. That is, it

allows the low frequencies to passthrough it, but will block all high fre-quencies. The result is that only thebass is passed, which is fine since thewoofer can only mechanically re-produce bass any way, and thereforeit avoids wasting power from the am-plifier uselessly. The action of the low-pass filter on frequency is showngraphically in Fig. 4A

The capacitor, when it is connectedto a tweeter as shown back in Fig. 3,performs a function that is the compli-ment to the inductor It forms a high-pass filter. It will only allow high fre-quencies to pass, keeping the strongbass power out of the small high -fre-quency driver. That prevents possibledistortion and damage. The effect thehigh-pass fitter has is shown in Fig. 4B.

In addition, another capacitor isconnected directly across the termi-nals of the woofer to perform anothernecessary function: compensation.The voice coil of the woofer is just that,a coil, so it has a substantial induc-tance just like the inductor we areusing for the low-pass filter. If we didn'tuse C2, the voice -coil inductancewould defeat the crossover inductor,and no crossover action would takeplace. The compensation capacitornullifies the effects of the woofer'svoice -coil inductance and allows theinductor to create the low-pass func-tion we require.

The Crossover Program. Listing 3shows the BASIC program, calledXDES 1 BAS, which we will use to calcu-late the values of inductance and ca-pacitance we need for our crossovernetwork As with the previous pro -

RESPONSESLOPE = 6dB/OCT.

FREQUENCY

LOW PASS

07RESPONSE

SLOPE = 6dB/OCT.

FREQUENCY

HIGH PASS

Fig. 4. The action of the woofer and tweeter outputs of the two-way crossover areshown here in A and B respectively.

LISTING 35 PRINT:PRINT:PRINT:PRINT:PRINT10 PRINT "11 PRINT:PRINT12 PRINT "14 PRINT:PRINT:PRINT15 PRINT "16 PRINT:PRINT:PRINT17 PRINT "18 PRINT:PRINT:PRINT20 INPUT "22 PRINT:PRINT:PRINT25 ON P GOTO 30, 99027 PRINT:PRINT:PRINT30 INPUT "35 PRINT:PRINT40 PRINT "45 INPUT "46 PRINT:PRINT50 PRINT "55 INPUT "60 LET X=0B)/(6.28"A))*100065 LET Y=((1)/(C6.28*A))10^666 LET Z=((1)/(B*6.28A))`10^670 PRINT:PRINT:PRINT75 PRINT "80 PRINT:PRINT85 PRINT "86 PRINT:PRINT87 PRINT "90 PRINT:PRINT:PRINT95 PRINT "96 INPUT "97 ON Q GOTO 27, 990990 PRINT:PRINT995 PRINT"999 END

grams, carefully enter it in with yourcopy of GWBASIC or BASICA, beingsure to get all spaces and punctua-tions correct. Once that is done. wecan access the program like theothers by typing GWBASIC XDES-1.BASor BASICA XDES-1.BAS.

Once the opening screen appears,answer the prompt, and you will beprompted for the frequency (in Hz)that you want the crossover functionto take place at. Then you will beasked for the impedance of the bassdriver at the crossover frequency you

DESIGNING A LOUDSPEAKER "

CROSSOVER NETWORK"

By William R. Hoffman"

Copyrite 1993"

Proceed" (1=yes 2=no) and Enter";P

Crossover frequency is (Hz) =";A

Impedance of low frequency driver at"the crossover frequency is (ohms) =",B

Impedance of high frequency driver at"the crossover frequency is (ohms) =";C

L "X"inHy"

Cl = "Y"uF"

C2= "Z"uF"

Would you like to try another set orvalues ? (I --yes 2=no) and Enter",Q

Good listening!"

have just selected. You can usuallyfind that value from an impedancecurve supplied by the driver man-ufacturer. If not, use an impedancebridge that you or someone you knowmight have to measure the driver's im-pedance. If neither of these sourcesof information are available, then usethe nominal impedance of the driver(usually 8 ohms). This should get youfairly close, since our filter slope isquite shallow (as Fig. 4 shows, it's only 6dB/octave.)

(Continued on page 104) 83

Many of people like to keeptrack of the prevailingweather conditions using in-

door and outdoor thermometers, ba-rometers, and humidity gauges. High -

quality instruments provide accuratereadings, but the common, consum-er -type humidity gauges-calledhygrometers --leave much to be de-sired. Even relatively expensive de-vices that rely on the absorption ofmoisture by a human hair or someother organic substance cannot re-spond accurately to very low or veryhigh relative -humidity levels.

Modern solid-state technology hasbrought advances in relative -humidi-ty measurements by providing varioustypes of capacitive and resistive sen-sors that respond to varying amountsof moisture in the air. Many of those,however, are non-linear and may notgive reliable readings at humidity lev-els below 10% or more than 90%.

It's All Relative. Relative humidity(RH), a measure of the quantity ofmoisture contained in the air, is impor-tant to our well being and can be asignificant factor in forecastingweather conditions. The term "rela-tive" refers to the percentage of watervapor contained in a given volume ofair as compared to the maximumamount of moisture that can be ac-commodated in that volume of air atthe same temperature. A condition of100% RH indicates that the air is satu-rated with moisture, and that any fur-ther increase in moisture content willresult in precipitation (rain, snow, orcondensation).

Personal comfort is closely relatedto humidity. In the summer, mild tem-peratures accompanied by high rela-tive humidity are uncomfortablebecause the body's natural coolingsystem (which works by evaporationof perspiration) is slowed. Conversely,the reduction in relative humidity dur-ing the winter when indoor air is heat-ed causes one to feel cold unless thetemperature is raised to 72 degrees ormore.

Adverse reaction to insufficient orexcessive relative humidity is not lim-ited to the human body. You mayhave noticed that wooden drawersand doors tend to expand and stickduring the summer, and furnitureoften shrinks and cracks during the

84 winter. Even electronics, such as VCR's

ttt

O

foluittuuks

IMAM% 11111PIDITY0AUG

Relative-Humidi

GaugeKeep an eye on thehumidity with thiseasy -to -build digitalinstrument.

BY ANTHONY J. CARISTI

and copying machines, can fall preyto moisture unless the relative humidi-ty is kept within certain limits. In ex-treme cases, where high humidity iscoupled with high temperatures,mildew can form everywhere, caus-ing significant damage.

There are many ways to measurerelative humidity. One way is by de-tecting the rate of water evaporationusing an instrument, known as a psy-chrometer, that uses two ther-mometers. One thermometer has acontinuously moistened cloth cover-ing its bulb, which senses a reductionof temperature through moistureevaporation. That reading is com-pared to the dry-bulb thermometer,which responds to ambient tempera-ture. A chart is then consulted to de-termine the relative humidity thatcorresponds to the Iwo temperaturereadings. But that method, which canbe very accurate, is not convenient touse.

Another method of measuring hu-midity is the one used in many low-cost, consumer -type, relative -humidi-ty gauges that have been around formany years. Such instruments havebeen designed to respond to the per-centage of relative humidity throughthe action of a strand of hair; the hairchanges length in accordance withthe amount of water vapor that it ab-sorbs from the air. It is not surprisingthat such instruments are not veryaccurate and any reading of relativehumidity may not be meaningful, es-pecially at very low or very high hu-midity levels. There are also severalelectronic methods by which relativehumidity can be gauged; one ofwhich uses passive components thatchange capacitance or resistance inaccordance with the air's moisturecontent.

However, the Digital Relative -Hu-midity Indicator described in this arti-cle takes advantage of a newdevelopment in sensor technology toproduce a professional -quality instru-ment that is simple to build, accurate,and easy to use. The sensor-theIH3602L precision humidity sensor,one of several developed by Hy -CalEngineering (9650 Telstar Ave., ElMonte, CA 91731; Tel. 818-444-4000)-is linear over a range of 0 to 100%RH, and with proper calibration canbe used to produce an instrumentthat is accurate to within ± 2%.

Our circuit can measure and dis-play relative humidity with signifi-cantly better accuracy and resolutionthan can be obtained from common,consumer devices, thereby enablingyou to determine whether a humidi-fier or dehumidifier needs to beturned on or off. It can also be used toindicate an improvement in humidityonce you have taken corrective ac-tion, with the added advantage ofbeing able to give an indication ofwhether your humidifier, dehumidifier,or air conditioner is doing its job.

How it Works. Figure 1 shows a sche-matic diagram of the Digital Relative -Humidity Gauge. The heart of the hu-midity -measurement system is SENS1(the Hy -Cal IH3602L, an 6 -pin CMOSdevice with only 3 -active terminals),whose DC output voltage varies lin-early with RH levels from 0 to 100%.

The sensor is fed from a regulated 5-

volt source provided by U1, an

S1

POWER0

R5100K 36

25

U1

AN78L0511

R6 12G C2 R3 2.21K

131 100K

3 R7

SERI'5K 3 16

23IH3602L

4R8

100K 17R1

2.21MEG 3122

C3 I U2

18

15.01 1CL7106CPL

R2 I 0 24

100K R4. 32

2K14

1.3 V 27 9

D1 C4 J, R9 10

1N4004 .22 47K2 21

38C547

D229v

1N4004 3339

C6 ±.47T 34 40

26

Fig. I. At the heart of the Digital Relative Humidity Gauge is the Hy -Cal 1H3602L,

SENSI (a 6 -pin CMOS device with active 3 -terminals).

r:)0000000Inala 0

1

elgrq181,M0:c\y:

J3 INCHES

Fig. 2. The Digital Humidity Gauge vim assembled on two single -sided, printed -

circuit boards; a template for one of them-the sensor board-is shown here full size.

AN78L05 100-mA fixed regulator. Theoutput of the sensor (measured withrespect to its negative power inputterminal) is ratiometric and varies lin-early from 0.8 volts at 0% RH to 3.65volts at 100% RH when powered by 5 -volts. The sensor output is fed throughresistors R1 and R2, which form a volt-age -divider network that reduces theoutput voltage of the sensor to arange of about 35 to 158 mV. Thechange in voltage (28.5 mV/% RH) isused to drive U2 (an ICM7106CPL AID

converter that also contains an on-board oscillator, output latches, 7 -segment decoder/drivers, and aback -plane signal generator).

Potentiometer R4 is used to set pin30 of U2 at 35 millivolts, while the refer-ence voltage of the chip is set toequal the total change in voltage atpin 31, 123 mV (158 mV - 35 mV). Po-tentiometer R7 permits full-scale cal-ibration of the instrument for variationsin humidity sensors by allowing adjust-ment of the reference voltage of U2.

1412 11 10 13

15

16 e

17

3

4

5

7 8 9

R10100KWV

C7

*SEE TEXT

DISP1LCD001

1,2,6

BACKPLANE

Although U2 is capable of driving afull 31/2 -digit LCD readout, in this ap-plication only two sets of U2'sers are used (the most -significant andleast -significant digits are not used). Afull scale reading, 100% RH, occurswhen the analog input voltage isequal to the reference voltage. Thatwould normally give a display of100.0; but, since the most- and least -significant -digits are not used, 100%RH is represented by a Iwo -digit dis-play of 00.

Components D1, D2, and R4 set theanalog -ground terminal of U2 at pin32 to slightly above ground (at 1.3volts). Pin 31 of U2 (the positive analogdifferential -input terminal) is con-nected to the output voltage of thesensor through a voltage divider com-posed of R1 and R2. Diodes D1 and D2are also used to generate a fixed volt-age of about 1.3 volts, which is neces-sary to bias the analog input stages ofthe A/D converter. Pin 30 (the negativeanalog differential -input) of U2 is con-nected to the wiper of R4, allowingcalibration at 0% RH. Pins 35 and 36 ofU2 are the reference -input terminalsand are used to set the full-scalerange of U2. A full-scale output occurswhen the input voltage applied be-tween pins 31 and 30 is equal to twicethe reference voltage. 85

PARTS LIST FOR THEDIGITAL RELATIVE -HUMIDITY GAUGE

SEMICONDUCTORSU 1-AN781_,05 5 -volt. 1(X)-mA

voltage regulator, integrated circuitU2--ICL7106CPL 31/2 -digit A/D

converter, integrated circuitDISPI-LCD001 2 -digit liquid -

crystal display (Digi-Key)SENS1-1H36021. P/N sensor (Hy: -

Cal)

RESISTORS(All fixed resistors are 'n-watt, 514

units, unless otherwise specified.)RI--2.21-tnegohm, Vi -watt,

metal -filmR2. R3, R5. R8-100,(XX)-ohm, 1/4 -

watt. 1(4 metal film124 -2,000 -ohm. cermet, PC -mount

potentiometerR6 -2.210 -ohm. 1/4 -watt, 1% metal

filmR7 -5,000 -ohm, cermet PC mount

potentiometerR9 -47,0(X) -ohmR10 -1(1),0(81 -ohm

CAPACITORS25-WVDC, radial -lead

electrolyticC2- 0.1-µF, ceramic -discC3 -0.01-µF. ceramic -discC4 -0.22-µF. 50-WVDC,

metallized -filmC6 -0.47-µF, 50-WVDC.

metallized -filmC7-100-pF. 50-WVDC, ceramic -

disc


BI -9 -volt transistor -radio batterySI-SPST momentary -contact switchPerlboard materials, enclosure, IC

sockets, battery holder andconnector, wire. solder, hardware.etc.

Note: The following parts areavailable from A. Caristi, 69 WhitePond Road, Waldwick, NJ 07463:a set of two printed -circuit boardstOr $18.95; UI for $2.50; U2 for$17.75; LCD readout for S11.00;kit of 5 metal -film resistors for$3.00: sensor $65.00. Please add$4.(X) postage/handling to allorders. New Jersey residents,please add appropriate sales tax.

rnrn7, Construction. The Digital Relative-a_t Humidity Gauge was assembled on

two single -sided, printed -circuit86 boards, referred to as the senor and

the display boards. That type of con-struction allows the boards to bestacked if desired to permit the circuitto be housed in a very small en-closure. Figure 2 is a full-size templateof the sensor board, while Fig. 3 is afull-size template of the display board.

r

1111-1Y1'

[11-11/4 INCHES -01

Fig. 3. Use this full-size printed -circuitpattern to etch the display board foryour Digital Humidity Gauge.

bpi

92 I

91

fl I BP

oriented. It is mandatory that thosecomponents be placed in the circuitas shown. Doing otherwise will result inan inoperative circuit and possiblecomponent damage.

When assembling the sensorboard, do not install SENS1 (the humid-ity sensor); that part will be installedlater, when the circuit is calibrated. Besure to use a socket for U2. That per-mits the project to be serviced shouldit ever become necessary, and is wellworth the slight additional cost. Asocket for the display is optional; onecan be easily fabricated by cuttingan 18 -pin IC socket lengthwise in halfto produce two 9 -pin sections.

When handling the display module,be extremely careful not to exert ex-cessive force; that unit is fragile. Wheninserting the IC and display into theboards be sure to orient them prop-erly (as indicated in their respectiveparts -placement diagrams).

Note that most of the resistors spec-ified in the parts list are 1% metal -filmtypes. Those resistors are used in the

TO DISP1

di al d2 f2 b2

Ie1 cl e2 c2

'SEE TEXT

U2

C3

jzi I A C6

R9 I

D2fI R4 DD1

SENSl

R1

-C7 --R10--R5-- R6-

-R3- -R8-B1

11,Fig. 4. When assembling tl e senor board be sure that all of the polarized componentsare correctly oriented and installed in their proper locations.

R2 1C2

ICI

If you choose not to etch your ownboard, a set of boards can be ob-tained from the source given in theParts List.

Figures 4 and 5 are the parts -place-ment diagrams for the sensor and dis-play boards, respectively. Whenassembling the boards, be sure thatall of the polarized components (thedisplay module, IC's, diodes, andelectrolytic capacitors) are properly

S1

POWER

circuit to ensure temperature stability,as well as ensure that its calibrationdoes not change with temperaturevariations or from component aging.Ordinary carbon resistors don't havethe necessary stability, and should notbe used as a substitute for metal -filmresistors.

The sensor and display boards areconnected to each other via a set of15 wires; labeled "a" through "g," and

TO U2

el al e2 a2o 0 0 0

f2

11 1b2 d2 bl dl

c2 cl BP

TO U2

Fig. 5. The display board (as shown bythis parts -placement diagram) holdsonly one component, the LCD readout.Although, from appearances it may seemdifficult to assemble this board thewrong way, looks can he very deceiving,so make absolutely sure that the readoutis properly oriented on the board.

design is to stack the two boards ontop of each other and mount the as-sembly to the front panel of the en-closure, with a rectangular openinginto the front panel to allow the dis-play to be viewed.

Connect a 9 -volt battery con-nector to the circuit, and mount a 9 -volt battery holder inside the en-closure to secure the battery in placeso that it does not rattle around.

There is just one operating control,S1. It is recommended that a momen-tary "spring -return" switch (either tog-gle, slide, or pushbutton) be used forS1 so that there is no possibility of acci-dentally leaving the unit turned onafter taking a humidity reading.

When you have finished assem-bling and wiring the project (with theexception of installing the humiditysensor), examine the circuit very care-fully for shorts, especially between ad-

NC

VOUT

+9V

+5V

+VIN -VIN

Fig. 7. The Hy -Cal 1H3602L sensor,SENSI (a 6 -pin CMOS device with onlyUl

7M-06

R11*10K

TEST

3 -active terminals), is at the heart of thehumidity measurement system. The1H3602L provides a DC output voltagethat varies linearly with relative

1GDl POTENTIOMETER humidity levels from 0 to 100%.14

TO R1

'SEE TEXT

Fig. 6. The Digital Humidity Gauge canbe calibrated using two procedures. Thefirst is a two -point method, outlined inthis diagram. It requires a potentiometerand digital voltmeter to set R4 and R7.(See the text for more details.)

BP (for back plane) in the parts -place-ment diagrams. For those connec-tions, use #24 AWG or similar insulatedstranded wire. Do not use solid wire; itwill break during handling of theboards. Be sure to allow sufficient wirelength in accordance with the finalposition of the assembly in the se-lected enclosure. The most compact

jacent IC terminals, and cold solderjoints (which may appear as rough ordull blobs of solder), and correct anydeficiencies; it is far easier to fix aproblem at this stage than during thecheckout procedure when you turnthe project on, only to find that thecircuit does not work.

Two -Point Calibration. Either of twoprocedures can be used to calibratethe circuit: the two -point method (de-scribed here) and the one or two -point precision method (to be dis-cussed a little later on). In the two -point method, see Fig. 6, a potentiom-eter and a digital voltmeter are usedto set R4 and R7. It is recommended

that the two -point calibration be per-formed first to verify the proper opera-tion of the circuit. Afterward, thesecond, more accurate, calibrationprocedure can be performed if desir-ed (more on that later).

In Fig. 6, a 10k potentiometer (R11) istemporarily substituted for the humidi-ty sensor. That unit is connected inparallel with C2, and the voltage at itswiper is then fed through R1 to thefollowing circuitry as if it were the out-put of SENS1. By setting the voltage atthe wiper of R4 to 0.08 volts and thewiper R7 to 3.65 volts, the digital dis-play will show the correct reading.

Connect a fresh 9 -volt battery (or awell -filtered 9- to 15 -volt DC walladapter) to the circuit. Turn S1 on andmeasure the output of U1 across C2.The reading should be 4.75 to 5.25volts. If the voltage falls within thatrange, record the actual value on apiece of paper. If you do not obtainthe correct voltage, disconnectpower and troubleshoot the circuit tolocate and correct the fault.

Check the orientation of C1, U1, U2,D1, and D2. Examine the circuit boardcarefully for any possible shorts, es-pecially between closely spacedcopper conductors. Check the out-put voltage and polarity of the powersource to be sure it is delivering atleast 8 volts to the regulator. If every-thing looks good, try a new regulator.When the fault has been corrected,record the actual output voltage ofthe regulator and proceed with thecheckout.

With power applied to the circuit,the display should show a two -digitnumber that varies as R11 is adjusted. Ifeither or both of the digits have incor-rectly illuminated or extinguished seg-ments, the problem lies with one ormore of the 14 segment connectionsbetween U2 and the display.

Refer to the schematic diagram todetermine which wire connectionsbetween U2 and DISP1 are at fault. Usean ohmmeter to verify any suspectconnections which can be eithershorted or open circuits, or miswiring.If the display is totally blank, the prob-lem most likely lies with the back -plane signal that runs between U2 pin21 and DISP1 pins 1, 2, and 6. Refer toFigs. 4 and 5 to verify that U2 and thedisplay module are properly orientedand correctly wired together.

If an oscilloscope is available, pin 87

21 of U2 can be checked for the pres-ence of the back -plane signal. A 60 -

Hz squarewave of about 6 volts peak -

to -peak is a normal indication. If thatsignal absent and the component ori-entation and wire connections arecorrect, try a new U2.

At this point, with the display read-ing properly, the calibration can beperformed. First calculate the nomi-nal output voltage of the sensor, for0% and 100% RH, by using the follow-ing two expressions:

= (0.8)M/5V, = (3.65)M/5

where VL is the nominal output of thesensor at 0% RH, V, is the nominaloutput of the sensor at 100% RH, and Vis the regulated supply voltage asmeasured and recorded earlier.

Set R7 to mid -position. Connect thenegative lead of the digital voltmeterto the anode of D1, and the positive

TIGHT FITTINGCORK

MAIN PC BOARD

SENSOR

UNDISSOLVEDSALT

SATURATEDSALT

SOLUTION

Fig. 8. The second calibration methoduses the first procedure, coupled withthe one outline in this illustration. (Seethe text for full details.)

lead to the wiper of R11. Adjust R11 sothat the DVM reading is equal to theVL value calculated above. Adjust R4so that the display reads 00. Note: TheND converter is capable of display-ing negative numbers even thoughno polarity sign is used in this circuit. Asa result, the display will vary from bothsides of 00 as R4 is adjusted.

Set R11 so that the DVM reads V, ascalculated earlier. Adjust R7 so thatthe display reads 00 (representing100% RH). Perform those steps severaltimes to ensure that neither R4 nor R7need further adjustment.

TABLE 1-RH OF VARIOUS SATURATEDSALT SOLUTIONS

Salts Percent RH

Lithium Bromide 6.4Lithium Chloride 11.3Potassium Acetate 22.7Magnesium Chloride 32.8Potassium Carbonate 43.2Magnesium Nitrate 53.2Potassium Iodide 69.1Sodium Chloride 75.3Ammonium Sulphate 81.1Potassium Chloride 84.3Potassium Nitrate 93.8

end of the scale and the other nearthe high end; select the appropriatesolutions using the information in Ta-ble 1. However, an easier one -solutionmethod can be used if desired. Toperform the one -solution calibration,adjust R4 first for 0% RH (as discussedearlier), and then use a saturated so-lution of sodium chloride (commontable salt and water) and adjust R7 fora display of 75% RH.

To calibrate the circuit, the humiditysensor must be temporarily con-nected to the sensor board via a

TABLE 2 -CORRECTION FACTOR FOR VARIATIONS IN AMBIENT TEMPERATURE

AmbientTemperatureIn Degrees F

Uncorrected Relative Humidity Reading

20% 30% 40% 50% 60% 70% 100%

25 -1.2 - 1.7 - 2.3 2.8 3.4 4.0 5.735 -1.0 -1.4 -1.8 --2.3 2.8 -3.2 4.645 -0.8 -1.0 -1.4 1.7 2.1 2.4 , 3.555 -- 0.5 -0.7 - 0.9 -1.2 - 1.4 1.6 2.365 -0.3 -0.4 - 0.5 -0.6 0.7 0.8 1.275 0 0 0 0 0 0 085 0.2 0.3 0.5 0.6 0.7 0 9 1.295 0,4 0.7 1.0 1.2 1.5 1.7 2.5

105 0.7 1.1 1.5 1.9 2.2 2.6 3.7

That completes the initial calibra-tion of the instrument. If you do notelect to perform the more precise cir-cuit calibration described below, re-move the test potentiometer andplace the sensor into the circuit (asshown in the parts -placement di-agram, Fig. 4).

One or Two -Point Precision Cal-ibration. The known property of asaturated salt solution having a spec-ified RH level in the enclosed airspace above the liquid level providesthe means for a simple but accuratecalibration of the instrument. In thismethod, a sealed bottle containing amixture of a specified salt and waterproduces a known level of relative hu-midity in the air above the liquid at agiven temperature.

To calibrate the unit, the sensor isplaced in the air space of a sealedcontainer that holds the saturated saltsolution. When equilibrium is reached(when the liquid and water vapor inthe bottle are at the same tempera-ture), the appropriate potentiometer(R4 or R7) adjustments are made asdescribed below.

The most accurate method is a Iwo-

point calibration performed by se-lecting two solutions, one near the low

three flexible wires. To aid in the hookup of the sensor, a pinout diagram ofthe sensor is shown in Fig. 7. Note (asmentioned earlier) although the unit isa 6 -pin device, it has only 3 activeterminals.

Refer to Fig 8. The solutions are pre-pared by mixing a sufficient quantityof the selected salt with water, so thatthe excess undissolved crystals that re-main are saturated with water and ata level slightly above the liquid level. Itis very important that the bottle beabsolutely air tight. The temperatureof the solution and the room shouldbe as close to 75°F (24°C) as possible.and remain constant, to ensure ac-curacy.

Place the sensor in the air spaceabove the liquid. Caution: At no timeshould any liquid or salt be allowed totouch the sensor. Doing so coulddamage it. Allow a minimum of 1/2hour for stabilization; 1 to 2 hours isbetter. That time delay is required toallow the air space within the bottle toreach its final level of relative humidity.

Adjust the appropriate potentiom-eter until the display shows the valuespecified in Table 1, rounded to thenearest integer, for the selected saltsolution. For example, if sodium chlo-

(Con tniued on page 106)

Build a Remote -Control

Relay StationA one -evening project can help yourremotes' signals turn corners, orboost their power for well -lit orextra -long rooms.

BY JOHN YACONO

Ireally think remotes are great.They give you the freedom to con-trol a battery of entertainment

units and other equipment from a sin-gle location. To some that's a supportfor laziness, but to me that freedommakes it easier to switch to more en-riching programming if what I'mwatching or hearing isn't worth thetime.

Sometimes though, that freedomextracts an extra price. For example,you might have to draw a blind tokeep sunlight from interfering with aremote (a problem in one unfor-tunately shaped room in my house).Or perhaps you might have to placea couch or easy chair conspicuouslyin the middle of a very large mediaroom (a problem for a friend of mine).You are also confined to the roomcontaining the equipment-that'shardly full freedom.

Of course, a remote booster maysolve the first two problems, but withthree provisos: you must deal withadded bulk on the remote, thebooster must be strong enough towork from the viewing position, andyou must swap the booster betweenremotes or buy more than one. So abooster may be unsightly, tiring tohold, still inadequate for some rooms,and either a pain or expensive formulti -remote use. It is also unlikely tobounce signals around corners.

An alternative is to use an RF univer-sal remote. However, you must findone that is "universal" enough for allyour equipment, Furthermore, they

tend to lack buttons for mostequipment's special features(commercial skip, alternatechannels, a jog shuttle, etc.) whichmight have played a part in youroriginal purchase decision, Not tomention they're down -right frighten-ing to non -technical family members(believe me, I know)! Besides, mostfolks are looking to reduce theamount of RF around the home dueto the ELF health scare (see "Elec-tromagnetic Fields and Your Health,"Popular Electronics, March, 1993).

A third possibility is to use an IR-to-RFrelay station. They allow you to useyour own remotes from any location,but again, only provided your notconcerned about RF radiation. Ohyes, and you must plunk -down around$100 (almost the cost of a play -onlyVCR with its own remote).

I didn't like any of the mentionedalternatives, so I built the Remote -Control Relay Station. It looks for IR sig-nals, demodulates them (to ensurethey're from a remote), then re -modu-lates and broadcasts them at high -power. It can be placed anywherebetween the remote user and theequipment (say on a coffee table) toact as an unobtrusive booster, or in astrategic corner to rebroadcast sig-nals from a user down a hallway. Also,the units are so inexpensive to build, itwould be more economical to buildseveral to relay signals betweenrooms than to buy an RF remote or IR-to-RF relay station.

To add to its charm, the circuit is

simple to build, and it's a goad projectfor beginners. Its components arereadily available, and no special PCboard is required. The unit's simplicityis mainly due to the use of an all -in -one IR-demodulator module. Let's dis-cuss its operation and function first.

Demodulating IR. Remote controlsbasically transmit serial digital infor-mation. They encode digital pulses oran IR carrier by turning the carrier orand off. That is known as ourst-styledata transmission. To see what all thameans, take a look at Fig. *I. The top-most waveform represents a series ofdigital pulses that need to be trans-mitted by the remote control one at atime. The remote control transmitsthem by turning on a 40 -kHz oscillatorfor the duration of each pulse. Theoutput of the oscillator (representedby the bottom plot) drives an infraredemitter that pumps those 40 -kHzpulses into the room as IR.

As mentioned earlier, the Remote -Control Relay Station uses a specialmodule to detect and convert those40 -kHz IR pulses back into the digitalpulses they represent. It is called theGPIU52X infrared -detector module. Tohelp explain its operation, I havebroken it down into functional blocksthat are shown in Fig. 2. Refer to thatfigure as I explain further. 89

,MP IIIP "W. .111,

Fig. I. Some data pulses that must be encoded and transmitted by a remote are shownat the top. The data is transmitted as a series of bursts of infrared as shown in thebottom plot.

All IR is received by the module viaan IR photodiode. The diode is oper-ated in its reverse -bias mode. Theamount of reverse -bias current it per-mits depends on the intensity of thereceived infrared light.

The fluctuations in the reverse -biascurrent are then amplified by a high -gain stage. The output of the amplifieris then "limited" by the next stage. Thelimiter chops the extreme highs andlows off the amplified signal and theresult is a quasi -digital pulse train.

The simplified wave then passesthrough a bandwidth filter. It has itscenter frequency at 40 kHz and abandwidth of + mi4 kHz. The filter cutsout more noise and also preventssources of infrared operating at thewrong frequency (such as the lightbulb) from causing false activation.The filtered signal is then rectified forfurther processing.

At that point, the circuit has effec-tively retrieved the 40 -kHz remote car-rier. The reproduced carrier is thenintegrated. What that means is that it is

BANDPASSFILTER

RECTIFIER

0 V OUT

INTEGRATOR COMPARATOR

Fig. 2. The module stages looksomething like this. Together theyprocess infrared light to extract anypertinent waveform and then demodulate

90 it.

sent through a filter that respondsslowly to the changing signal-tooslowly to follow the 40 -kHz carrierpulses, but quick enough to rise andfall significantly with each burst andpause between bursts, respectively.

The nod stage takes advantage ofthose characteristics. It is called an"inverting Schmitt trigger" It will not golow unless the fitter's output signal sur-passes a certain amplitude, and willnot go high again until the signaldrops below a certain minimum. Thusthe Schmitt trigger only responds tolarge changes in the filter's output(caused by bursts and pauses) andignores small changes (caused by the40 -kHz carrier to which the filter can'trespond quickly). The Schmitt -trigger'soutput is thus low when a 40 -kHz burst

PARTS LISTFOR THE RELAY STATION

SEMICONDUCTORSU1-555 oscillator/timer, integrated

circuitQI-2N2222 general-purpose NPN

silicon transistorQ2 -2N3906 general-purpose PNP

silicon transistorLEDI-LED3-SY-IR53L IR-light-

emitting diodeMODI-GPIU52X IR detector/de-

modulator module (Radio ShackNo. 276-137)

Dl-D2-2.4-volt, 500-mW Zener di-ode

D3 -1N914 general-purpose small -signal silicon diode

RESISTORS(All resistors are V4 -watt, 5% units un-

less otherwise indicated.)RI, R2 -18,200 -ohm. 1/4 -watt, I%R3 -47 -ohmR4 -100,000 -ohmR5, R6-I0,000-ohm

ADDITIONAL PARTS AND MATERIALSC1 -0.001-µF, NPO ceramic -disc ca-

pacitorC2-0.1-1.LF bypass capacitorB1 -9 -volt transistor -radio batterySl-SPST on/off switchProject box, double -sided foam, perf-

board, wire, solder, etc.

$ R31' 470

0

T919V

$ R1718.2K 02

2N3906

8

R218.2K

1%

'xi D12.4V

D22.4V

C1

.001

D31N914

DISV R T

U1

665

THR

TRG OUT1

GND VCON

R4100K

MOD1GPIU52X

5

R610K

R510K

AIM

C2

Ti.

OUT

V+

GND )""

01

LED1SY-IR531

LED2SY-IR53L

LED3SY-IR631.

Fig. 3. In the IR-relay circuit all of the signal processing is performed by the infrareddetector/demodulator module MODI. The rest of the components regulate power andgenerate 40 -kHz IR pulses.

is received, and high during pausesbetween bursts. The resulting wave-form is an inverted version of thepulses that were modulated andtransmitted by the remote. Let's turnour attention to the project to seehow those signals are put to use.

The Relay Circuit. A schematic di-agram of the Remote -Control RelayStation is shown in Fig. 3. To summarizeits pertinent features, it consists of theIR-demodulator module (MOD1) con-nected to an inverter (made of Q2,


From time to time, articles onSCA communications have ap-peared in hobby -electronics

periodicals. The articles range in em-phasis from circuit experiments andconstruction projects to rather sensa-tional sounding descriptions of secretor mysterious signals not accessible tothe general public. Actually, there isnothing really that mysterious aboutSCA, which stands for Subsidiary Com-munications Authorization.

The technical basis for SCA is theFM -on -FM data -transmission tech-nique developed by range -instru-mentation engineers back in the1950's. Shortly afterward, the FCC au-thorized its use by FM stations to ex-pand their services. FM stationsbroadcasting SCA programs frequen-cy -modulate their main carrier (88 to108 MHz) with one or both of the com-monly used subcarrier frequencies of67 and 90 kHz. Those subcarrier fre-quencies are, in turn, modulated withprogram material. In order to preventinterference with the main program,both the amplitude and upper -fre-quency limit of the SCA programs areheld to relatively low values.

Since SCA program material is notmeant for reception by the generalpublic, the FCC protects it by prohibit-ing manufacturers from selling SCA-capable receivers on the consumermarket. For electronics experimenters,however, it is not hard to build a de-modulator that can be connected toany FM tuner to receive these trans-missions.

About the Circuit. A schematic di-agram for the SCA Adapter is shown inFig. 1. The circuit, for the most part, iscomprised of seven active elements:five assorted transistors and two IC's.The 67- or 90 -kHz subcarrier signal ispicked off the FM tuner (more on thatlater) as the input to the circuit. Afterpassing through a parallel -T network,the signal is applied to the input of aIwo -stage amplifier comprised of Q1and Q2. That amplifier boosts the sig-nal's strength by about 20 dB before itis applied to the input of U1 (anXR2211CP FSK demodulator/tone de-coder) at pin 2. That chip contains aphase -locked loop or PLL (which in itsmost basic form is comprised of ,aphase comparator, low-pass filter,and a voltage -controlled oscillator orVCO), a quadrature phase detector

HI

SCA OUTPUT

LOCK

SCAAdapterHear the hidden broadcasts

on the FM hand with thiseasy -to -build add-on unit.

BY DON McCORMICK

(which provides carrier detection), anFSK voltage comparator, and a pre -amp. It is specifically designed fordata communications, can operateover a wide supply -voltage range(4.5-20 volts), and can accommo-date analog input signals of from 2mV to 3 volts. Timing capacitor C7(connected between pins 13 and 14)and the total resistance connectedbetween pin 12 and ground deter-mine the circuit's lock or center fre-quency. Switch S1 allows the user toselect either 67 -kHz (lc) or 90 -kHz (H)SCA reception.

The voltage at pin 6 is held at zerowhen the input signal is either off -fre-quency or missing. When locked, pull-up resistor R16 causes pin 6 to go high(rising to about 7 volts), causing Q5 toturn on and LED1 to light.

That high also causes squelch tran-sistor Q4 (a 2N3820 P -channel JFET) toturn off. That allows the twice -demod-ulated signal, which is output at pin 11

of U1, to be fed through R18, C18, andR19 to the base of Q3 (the output am-plifier).

Power for the SCA Adapter is de-rived from the circuit consisting of T1,BR1, U2, and their support compo-nents. T1 can either be a standardpower transformer as specified in theParts List or a wall -plug type with anappropriate AC output.

Assembly. The SCA Adapter was as-sembled on a small printed -circuitboard measuring about 43/46 by 213/46inches. A full-size template of the cir-cuit -board layout is shown in Fig. 2.Once you have etched your ownprinted -circuit board and gatheredall of the required parts, constructioncan begin.

Note that all components are read-ily available and none are critical toproper operation of the circuit withthe possible exception of C7, Al-though a polystyrene capacitor is rec-ommended for C7, a polyester unitcan be substituted. Since the P -Chan-nel JFET might be hard to find, threechoices for it are listed in the Parts List.

Guided by the parts -placement di-agram shown in Fig. 3, install the pas-sive components first-beginningwith an IC socket for U1, followed bythe resistors and capacitors (makingsure that the polarized capacitors areproperly oriented). Normally, it wouldbe advisable to install the jumperconnections at this time, but becausethe jumpers will have to be snakedaround many of the board -mountedcomponents it is recommended thatthey be installed just before mountingU1 in its socket. When you reach thejumper phase of construction, leaveone end of the squelch jumper (JU1)disconnected; that will deactivate thesquelch function, thereby avoidingconfusing behavior during the circuit'scheckout routine.

In any event, once the passivecomponents have been installed, in-stall the active components, startingwith the diodes followed by the tran-sistors and U2. Do not install U1 in itssocket yet; it will not be installed untilthe checkout phase of the con-struction. If desired, LED1 can be tem-porarily mounted directly to theboard to facilitate the checkout.

Checkout. The circuit must bechecked out and aligned before it is 91

0

z

0,cr,

5 --

coco

0

0up

z0

construction. It is shown here full si:e.

I permanently connected to an FM re-rn or mounted in an enclosure.a)- Once the circuit has been assembledcf and you have double-checked your

work, set each of the trim potentiome-92 ters at the midpoint of its rotation.

1C182pF

+15V

R71K

01R2 R3 MPF102

220K 220K sosk 1, AA

C2 C4 _L IC347pF 82pF"

47pF

D1

1N914

R1

470K w D21N914

R4z100K Mik

R610K

R51000

1, C9

R9 ^ 10

1K#0-

C8Q2 47pF

2N3906

R81K

R11

5600 +9V

C7.001 -

C61

C5 R10

270pF 7 470K

14 13

+ C11

U1

XR2211

8 4 10

100

C12 -1 T

R1618K

AAA.

D31N4739

R27470K

052N3904

+18V

D41N4739

R263300

.4"

OLED1

11

12

C131

R177 27K

R12 R18100K 100K

tR13 C1410K T.001

J1

INPUT

PL1 J2

PL2*

T1'

`SEE TEXT

C19470pF

+18VSOURCE

1 R1410K7 --+I

LHI 0OW

kS1

4 11510K

C171

Fig. I. This SCA adapter is built around an XR22Il FSK demodulator, sometransistors, and a handlid of support components.

4 3/16 INCHES

Fig. 2. Although not strictly needed, thi.s. Jell pattern allows for neat and easy

JU1

R251K

042N3820

032N2222

AUDIOOUT

J3RIGHT

J4LEFT

R2450K

C16- 10

C15 C18 R19 R22.02 .47 6.8K 680K

1-*--N-Mcfc- R2310K

R201MEG

R21100K

+15VSOURCE

Power up the circuit and check the+ 18- and + 15 -volt DC voltages. Also,the voltage at pin 1 of the IC socketshould measure between 4 8 and+ 9 volts DC. If the DC supply voltagesare okay, remove power from the cir-

cuit, install the jumpers, and install U'in its socket.

Set -Up and Use. The best way toalign the board is with a signal gener-ator that can cover the range from 50to 100 kHz. With S1 set to the Lo position,feed a low-level 67 -kHz signal into theinput and adjust R14 until LED1 lights.Continue adjusting R14 to find thepoint where the LED goes off and trimthe adjustment to midway betweenthe on and off points. Set S1 to the HIposition, feed a 90 -kHz signal to theinput, and repeat the operation, thistime adjusting R15.

In selecting a tuner or receiver youhave two options. First, you can bebrave and resourceful and use yourfavorite equipment. If that is too muchexcitement in your life, you can golooking for an old clunker tuner orpull-out car radio at garage salesetc., keeping in mind that, while it canbe either stereo or mono, it must worproperly as an FM tuner. It should alsohave reasonably good RF and IF lin-earity to avoid cross -talk leakage be-tween the main program and the SCAoutput. Modern PLL/digitally tuned re

SEMICONDUCTORSUl-XR221 ICP FSK demodulator,

integrated circuit (EmilU2-7815 15 -volt voltage regulator,

integrated circuitQI-MPF102 N -channel WET

transistorQ2 -2N3906 PNP transistorQ3 -2N222 NPN transistorQ4-J 176. 2N3820, or NTE-326 P -

channel WET transistorQ5 -2N3904 NPN transistorDI, D2-IN914 silicon diodeD3, D4 -1N4739 9 -volt Zener diodeLEDI-red light -emitting diodeBR1-1.5-amp, 1(X) -Ply bridge

rectifier (Radio Shack 276-1152 orequivalent)

RESISTORS(All fixed resistors are VI -watt,

units unless otherwise indicated.)RI, RIO, R27--470,(X)O-ohmR2, R3 -220,0(X) -ohm

J

PARTS LIST FOR THE SCA ADAPTER

R4. R12, R18, R21 -100,000 -ohmR5 -100 -ohmR6, R13. R23 -10,()00 -ohmR7-R9, R25 -1000 -ohmRI I -560 -ohm. 1/2 -wattR1.1 -10,000 -ohm, potentiometerR15 -25,()00 -ohm. potentiometerR16--18,0(X)-ohmR17 -27,000 -ohmR19 -6,8(X) -ohmR20-I-megohmR22 -680,000 -ohmR24 -500)0 -ohm, potentiometerR26 -330 -ohm

CAPACITORSCl, C4-82-pF, ceramic -discC2. C3, C8-47-pF, ceramic -discC5-270-pF, ceramic -discC6 -1-µF, tantalumC7-.001-p.F, polystyreneC9. C16-10-4 16-WVDC.

electrolyticC10. C12, C13 -0.1-µF, ceramic -disc

J

i

C171 ,,,..tiii, C11

+i

s.....17- 19 ,..- '''''' +

-R6- Cv I

D3R11

-,-115-141 e

1 R14

I3

I

6U1

sl' R13I

I C14112-21

9 b I

C8 1 I

3 i I/.11,1 , 7

D1

ICrI I

4 r17 c

C10 I R1181

I I

I R12 I R23

C18

I_ C16' +

C13I Q5-R20- ICla3 e

i R7ae b615. A 9 1

D2Ril 1 -R26-

zC20

BR1+

C19

J2'

R15

LOS1Cer

HI

R2

\

s d lRf5

J1

INPUT

PL2*

-Jul.

*SEE TEXT

Fig. 3. Use 11 is parts -placement diagramnot install jumper JUl until the board has

ceivers seem to be best in this regard.If you have aligned your SCA board

as previously described, you may useit as a probe to find the basebandsignal take -off point in the receiver.The proper connection point is at theoutput of the FM demodulator before

4e.LE D 1

J4LEFTOUT

J3RIGHT

OUT

when installing parts on the PC board. Dobeen completely checked out.

any de -emphasis circuitry. Locate astrong station, one that you know isbroadcasting SCA signals. Connectthe output of the SCA circuit to an-other amplifier and speaker and lis-ten while you make trial connectionsto the receiver. Remember that the

C11-100-4 16-WVDC, electrolyticMylar

C15-.02-4 MylarC17 -1-11.F, l6-WVDC, electrolyticC18 -0.47-A MylarC19-470-pF. ceramic -discC20-2200111', 25-WVDC,

electrolytic


T1 -12 -15 -volt. 1(X)-3(X)-mA powertransformer (Radio Shack 273-1385or equivalent). see text

SI-SPST switchJ3, J4-phono jack

J2-coaxial power jackPLI-eoaxial power plugPL2-AC power plugPC board; enclosure (Radio Shack

270-238 or equivalent), see text:solder, wire, hardware, etc.

better the reception of the main FMprogram, the better the SCA recep-tion. Once you've located the correctconnection point, touch up the po-tentiometer settings by ear and en-able the squelch circuit by con-necting jumper JU1.

If you were unable to pre -align yourSCA circuit, the same basic hook-upprocedures should be followed, butthere is a lot more trial -and -error in-volved. Set R14 to its midpoint for start-ers and try various combinations ofhookup points and R14 settings untilyou find an SCA signal.

Now for the finished product. Thereare two ways to go. The easiest andmost obvious is to house the SCAAdapter in a separate enclosure. Ifyou choose to go that route, an ap-propriate enclosure is listed in theParts List. You will also want to use awall -plug type transformer ratherthan the one mentioned in the PartsList. Just make sure the one you selecthas an appropriate AC output.

The other approach is to house theadapter in the same enclosure as theradio with which it will be used. You willthen need to select the unit so tha+there is sufficient space for the boardand T1. The adapter controls can thenbe added to the front -panel of thereceiver or mounted on a small pieceof angle aluminum that is added tothe top or side of the enclosure. 93

The age of wireless ishere. Although the term"wireless" has until re-

cently implied radio that is byno means the only wirelessmedia. The Wireless Head-phone project described inthis article uses a differentform of wireless link- -infrared,Using a 100 -kHz frequency -modulated (FM) carrier, theWireless Headphone has A 4 -kHz bandwidth, making it suit-able for general-purpose lis-tening. Its high carrier fre-quency provides inter-ference -free operation, evenaround most consumer IR re-mote controls, which operatewith a carrier frequency of 40kHz. And it requires no specialconnections to the radio or IV.

The power requirements forthe system are modest: Thetransmitter requires a 7- to 14 -volt, 200-mA DC supply, whilethe receiver draws about 10mA (at minimum volume) froma 9 -volt battery. The receiverhas a volume control for con-venience, although the audiosource's volume control may also beused to that end.

The Transmitter Circuit. The trans-mitter for the Wireless Headphone(see Fig. 1) consists of a pair of infraredLED's and a CD4046 CMOS phase -locked loop (or PLL, which is com-prised of two phase comparators, avoltage -controlled oscillator or VCO,a source follower, and a Zener refer-ence) -coupled with a driver tran-sistor. Note that in this application, onlythe PLL's VCO is used.

The VCO's supply voltage is sta-bilized by the internal Zener refer-ence. The VCO input at pin 9 is biased

P. near the midpoint of the VCO's linearT- region. The VCO's programmable,6 sensitivity and high input impedance

eliminates the need for signal pre-( -2 conditioning.

Components C1 and R2 provide(.) impedance matching for low -imped-

ance speakers; those components4 should be eliminated if the audios source has a high impedance (600

ohms). The VCO frequency is set by R4,Li R5, and C4 for a minimum frequency

of 85 kHz, a maximum frequency of94 115 kHz, and a nominal center fre-

zI -

HeadphonesBY BRIAN MCKEAN

Listen to your favoritemusic unencumbered froma headphone cord withthis easy -to -build

transmitter/receivercombination

quency of 100 kHz, which yields a VCOsensitivity of 7.5 kHz/volt.

The VCO output at pin 5 of U1 drivesa saturated common -emitter circuit,built around Q1 (a 2N2222A general-purpose NPN silicon transistor). Al-though U1's Zener reference decou-ples the VCO from supply variations,the IR-emitter current is not regulated.Components shown in the schematicshould be suitable for most applica-tions. The number of IR emitters (onour circuit, LED1 and LED2) connectedto the collector of Q1 can be in-creased to provide increased roomcoverage, as long as the supply volt-

age is sufficient to drive theLED string. Allow 1.8 volt perLED, and select R7 so that thepeak current through the LED'sdoes not exceed the partspecification (typically 100mA).

The IR transmitter's supplyvoltage may be an unregu-lated DC source greater than7 volts. An internal Zener refer-ence at pin 15 of U1 regulatesthe supply to 5-6 volts for theVCO. The current through theIR LED's may be adjusted bychanging the value of R7 tosuit the diode ratings.

Receiver. The schematic di-agram of the receiver for theWireless Headphone -essen-tially a reverse -biased pho-todiode detector/amplifier -is shown in Fig. 2. That circuitconsists of a CA3237 high -gain IR remote -control pre-

amp (U1), a 4046 phase -lock-ed loop (U2), and an LM386low -voltage audio amplifier(U3).

Integrated circuit U1 -which is designed for 40 -kHz carriersystems but can provide limiting ac-tion to up to 1 MHz -contains two am-plifiers whose gain is set by C6/R4, andC7/R5. Together those componentsprovide DC and low -frequency block-ing, while setting the combined gainof the amplifiers to about 85 dB at 100kHz. The Schmitt -trigger section (pins 4and 6) of U1 is not used.

The IR remote -control preamp'soutput at pin 7 is AC coupled to thephase -locked loop, which operateswith a 100 -kHz center frequency. ThePLL's 15 -kHz capture range allows forconsiderable center -frequency mis-match with the transmitter whileproviding proper demodulationbandwidth and noise rejection. TheVCO of the PLL has a range of ± 30kHz around the center frequency,which also allows for transmitter/re-ceiver mismatch without unduly sacri-ficing loop performance.

The loop lowpass filter output con-tains the demodulated audio signaland is internally buffered at pin 10. Theaudio is filtered and fed through am-plitude control R12 to the non -invert-ing input of U3 (an LM386 low -voltageaudio power amplifier), which pro-

7-14V -0--ceroS1

SPST

5.5V- 6V

R82700 C5

.1

R1

PL1 1KAUDIO C2

IN 2.2

C1"50

'22Q

'SEE TEXT

R301.5K

C3

.047

16,15

ZENER

VCOIN U1 VCOwr

4040PLL

5,8,3,14

R4

270K

R7

+t

C61001-1 LED1

100

C725pF

-TIETLED2

7

11 12

t R5150K

C4100pF

R65.1K

\;4

Q1

2N2222A

Fig. I. The transmitter for the Wireless Headphone is built around a CD4046 CMOSphase -locked loop or PLL, coupled with a driver transistor, and a pair of infraredLED's. Although the CD4046 is comprised of two phase comparators, a voltage -controlled oscillator (or VCO), a source follower, and a Zener reference, only its VCOis used in this application.

C1

10

D1

F1L-3C

R1

2.2K+i C2

22

Wk.R2

10011

U1

CA3237E

C5680pF

R351K

'SEE TEXT

2 3

R547(11

5 7 9

1411----1111he INCHES

Fig. 4. The receiver's full-size printed -circuit layout is shown here.

require such a circuit to be enclosedin a metal case to provide elec-trostatic shielding. That may not benecessary, but is recommended forbest range.

Assembly. Most parts are quite non-critical. Many parts can be replacedwith near value(s) without affecting

t R93300

C3 +10

7k

C810

5.5-6V C10100pF

[6.-1 715,16

14

R668K

2

C13 I C14

.1 220

C2

10K .0047 100pF

cli

10

R8 c.4

9

D LI2484.

12 311

R7120K

4 58

AV*R11

100K - LM388R12 t100K?

3

S1

SPST

-1 I - -IB1

9V

C16100

+1(

t R10 --) 4F -24K C15

SPKR1*

10

Fig. 2. The receiver for the Wireless Headphone consists of a CA3237 high -gain IRremote -control preamp (UI, which is designed for 40 -kHz carrier systems,), a 4046phase -locked loop (U2), and an LM386 low -voltage audio amplifier (U3).

4- f

IIII 2 INCHES 311

Fig. 3. The transmitter's printed -circuitlayout is shown here at full size.

vides 26 -dB gain and will easily drivea paralleled pair of AC -coupled,low -impedance earphones.

Voltage regulation in the receiver isprovided by U1 and U2, with the as-sistance of R2 and R9. The receiver willoperate from supply voltages as lowas 7 volts, making battery operationfrom a rechargeable 9 -volt batteryhighly practical.

A final note regarding the high -gain receiver: Normal practice would

(SPKR2*

SPKR1 & SPKR2

8-320 SPEAKER

the circuit. The exceptions are notedon the schematic diagrams by as-terisks-those parts should not be re-placed with any other value. If youcan not find the specified pho-todiode, one salvaged from a com-mercial IR receiver should work well. Itshould have a capacitance of 30 pFor less at -5 volts, and be 0.1 -inch ct-ameter or less. The electrolytic ca-pacitors should be miniature radial -lead units.

96

PARTS LIST FOR THERECEIVER

SEMICONDUCTORSUl-CA3237E lR preamplifier

(NTEI682), integrated circuitU2-CD4046B CMOS phase -locked

loop. integrated circuitU3-LM386 low -voltage, audio -

power amplifier, integrated circuitDl-FIL-3C, FIL-5C, PIN-3CD,

PIN5D, or similar IR-detectordiode

RESISTORS(All fixed resistors are Vs -watt. 5%

units.)R1 -2200 -ohmR2 -100 -ohmR3 -51,000 -ohmR4 -20 -ohmR5 -47 -ohmR6 -68,000 -ohm

*R7-I20.000-ohm*R8 -10,000 -ohmR9 -330 -ohmR10 -24,000 -ohmR11 -100.000 -ohmR12 -100,000 -ohm PC -mount

potentiometer

7-14V

Ti:-05-

-C6- + R7

R8-

R6

C7 I

4

CAPACITORSCI, C3, C8, CI5-10-11E 16-,VVFX',

radial lead electrolyticor tantalum

C2 -22-µF, 16-Vv'VDC. radial leadelectrolytic or tantalum

*C4--0.0047-p.F, ceramic -discC5-680-pE ceramic -discC6. C7 -1-µF, 16-WVDC,

electrolyticC9. CII, C13 -0.111E2. ceramic -disc

*C10, C12-100-pF. ceramic discC14 -220-µ,E 16-WVDC,

electrolyticC16--100-p.E 16-WVDC, electrolytic


SPKR1-8-32-ohm speakerSI-SPST toggle switchBI -9 -volt alkaline, or rechargeable

batteryPrinted -circuit materials, enclosure.

earphone jack, 9 -volt batteryconnector, wire, solder, hardware.etc.

*Do not substitute

(11

4I LED1 I I LED2

R3-

C1

11

C2 1

R5 R2R4

C3

Fig. 5. The size of the parts used to build the transmitter are important due to spaceconstraints, thus. 1/4 -watt or smaller resistors are a must and miniature (radial leadelectrolytic or otherwise) capacitors are recommended.

Figure 3 shows a full-scale templateof the transmitter printed -circuit art-work, and Fig. 4 shows a full-scaletemplate of the receiver's printed -cir-cuit artwork. Printed -circuit assemblyis recommended, particularly for the

receiver, which has a very high gain.As can be seen from the size of thosefoil patterns, the most important fac-tor governing the use of a particularcomponent is its physical size. Due tothat factor, the smallest sized cornpo-

PARTS LIST FOR THETRANSMITTER

SEMICONDUCTORS1_11-C1)4046B phase -locked loop

(do not use the 74(' version)QI-2N2222A general-purpose NPN

silicon transistor (use TO -1g ifdriving LED at high current)

LEDI, LED'_-RS276-143 infrared,light -emitting diode (Radio Shack)

RESISTORS(All fixed resistors are ',-viatt.

units, unless otherwise noted.)R1 -1000 -ohmR2 -22 -ohmR3-- I500 -ohmR4 -270.000 -ohmR5-I50,000-ohmR6--5100-ohmR7 -100 -ohm. Y.4 -wattR8 -270 -ohm, !1 -watt

CAPACITORS(71-50-µF, 1-WVIX', radial -lead

electrolytic or tantalumC2---2.2-ILE non -polarized multilayer

ceramicC3-- 0.047-µE ceramic -discC4 -1(N) -ph ceramic -discC5-0.1-ja, ceramic -discC6 -1(X) -µF, 10-WVDC. electrolyticC7-25-pE, ceramic -disc


Si-sps-r switchPrinted -circuit board materials,

enclosure, 7- to 14 -volt DC, 2(X)-mA source, wire, solder, hardware.etc.

nents available should be used.Once you have etched your boards

and obtained all the parts, assemblethe board for the transmitter guidedby Fig. 5 and the board for the re-ceiver guided by Fig. 6.

The circuits, once assembled,should operate immediately withoutadjustment or alignment. The Zenerreferences can be checked at pins 15and 16 of the CD4046's and at pin 9 ofthe CA3237. The transmitter and re-ceiver VCO's can be checked at pin 4of the two CD4046's. The free -runningfrequency of the transmitter shouldbe 100 kHz t 10 kHz; and the transmit-ter and receiver VCO frequenciesshould be identical when the Iwo unitsare optically coupled and properlyfunctioning. The VCO tests should bemade without an audio input to en-sure a stable frequency reading.


SAFETY CIRCUITI ong ago, when we hadissjust a few simple tools towork with, "Murphy" wasclose at hand to see thatsome body part got be -

S1 S2

0 0

12-24V DC

TO

CONTROLCIRCUIT

K1

Fig. I. The simple two -hand safety -control switch shown here islittle more than two pushbutton switches connected in series sothat both must be depressed in order to energize the relay.

tween the tool and animmovable object. The re-sulting injury, in most cases,probably was not life threat-ening and, after a time,things returned to normal.But with today's high-techmetal -crunching machines,there's no place for eventhe slightest error. Your firstmistake could literally beyour last. Machines justdon't care what, or who,they mangle.

A SIMPLE SAFETYSWITCH

A good number of thesmall -parts stamping and

R1

47011

R24700

S2

C1

1000

Si

0I

k

r o

.1/ K2

K1

0-+12V

CONTROLOUTPUT

K3

Fig. 2. This safety control switch offers an improvement over theprevious on in that SI and S2 must be depressed at about thesame time to power the load circuit.

molding machines that arestill in use today are elec-trically controlled by theoperator. The operator putsa part in place and then,by operating hand -controlswitches, sends a signal tothe machine to completethe cycle. Since both handsare out of harm's way hold-ing down the hand -controlswitches, the machine op-erates safely.

Figure 'I shows a simpleIwo -hand safety -controlswitch that is still used insome manufacturing facili-ties. On the surface, thecircuit fulfills the basic safetyneed by requiring that Iwoswitches be closed beforethe machine will operate.That's just fine and dandy.But when an operator doespiecework, ingenuity gets inthe way of safety.

By placing somethingheavy on one of the switch-es, the machine can beoperated with one hand,leaving the other hand freeto speed up the operationby putting the next part inplace a little sooner. Hands,fingers, and other bodyparts have been lost thatway-certainly not good for

the operator, the company,or the insurance company.

IMPROVED SAFETYCIRCUIT

An improved safety cir-cuit, which eliminates theoperator's ability to defeatthe safety feature, is shownin Fig. 2. That circuit requiresthat each hand switch beoperated at about thesame time to produce amachine control output.The two large electrolyticcapacitors (C1 and C2) arecharged to 12 volts throughtwo current -limiting resistors(R1 and R2). If both S1 andS2 are closed at the sametime, the normally opencontacts of relays K1 and K2close momentarily, causingK3's (the control relay's)open contacts to close.

The control relay's on -time is controlled by Cl andC2, so fine tuning the out-put time period isn't exactlyan easy job. In addition, R1and R2 must be selected tolimit current to less than K1and K2's holding current,otherwise, the relay will re-main active as long as theirrespective control switchesare held down.

PARTS LIST FOR THESIMPLE SAFETY SWITCH

SI, S2-Normally open pushbutton switchKI-SPDT AC or DC relayWire, solder, hardware, etc.

PARTS LIST FOR THEIMPROVED SAFETY CIRCUIT

CI, C2-1000-ILF, 16-WVDC, electrolytic capacitorKI-K3-12-volt DC relay with 100 -150 -ohm coilRI, R2 -470 -ohm, I -watt, 5% resistorSI, S2-Normally open pushbutton switchPerfboard materials, 12 -volt source, wire, solder, hardware, etc.

5 pinning pinwheel -like devicesare frequently seen mountedatop the masts of ships, the

masts of weather and air -samplingstations, and atop tall towers locatedat airports and air fields. Those wind -driven spinning devices are mechan-ical sensors that are part of an instru-ment-known as an anemometer-that is used to measure the force ofthe wind and hence its speed. Hikers,campers, bikers, home owners, driv-ers, pilots, and sailors are interested inwind speed and force because it al-lows them to anticipate hazardousweather conditions and plan ac-tivities accordingly.

In this article, we'll show you how tobuild a simple, effective, and inexpen-sive anemometer. But before we getinto details of the circuit, it is wise toreview some background material.

Theory. Wind consists of air mole-cules in motion. When those mole-cules strike an object, they impart aforce (pressure) on the object that isproportional to their velocity. The totalforce on an object is the sum of thepressure per unit area multiplied bythe total effective area of the object.

Paddles or cups mounted to a rotorcan be used to convert the wind forceinto torque. That torque can then beused to turn a wire loop or coil that ispositioned between two permanentmagnets (a DC motor or generator) toproduce a voltage that is propor-tional to the torque or wind speed.

The resulting voltage can then be pro-cessed and displayed on a DC am-meter (calibrated to read miles perhour, mph, instead of amps) as windvelocity.

That is the essence of the ane-mometer described in this article. Theauthor used a simple DC motor as thegenerator. A rotor was then fabricatedfrom parts obtained locally, and com-bined with the generator.

To calibrate the generator/rotor as-sembly, it was mounted on a boardthat was, in turn placed on top of theauthor's van. On a windless evening,the van was driven at speeds rangingfrom 5 to 45 miles per hour (MPH) in 5MPH increments. The generator's out-put voltages were measured at vari-ous vehicle velocities, using a high -impedance voltmeter. The measure-ments were recorded, and data wasthen statistically processed and usedto develop the graph shown in Fig. 1.As shown, the generator's output volt-age varied linearly from 0 to 300 milli-volts (mV) over the vehicle's speedrange (0 to 45 mph).

A Look at the Circuit. Figure 2 is aof the Ane-

mometer. That circuit, which is shownseparated into two parts (the pro-cessor and the monitoring circuit) is

Build this simple and inexpensive wind -speedmeasuring device and become your

C)

own meteorologistO

i Build anI AnemometerL BY BOB SIMCOXrnrn

Li!

98

comprised of the generator (denotedMOT1 in Fig. 2), an LM324 quad op -amp (U1, whose individual op -ampsprovide pre -amplification, integra-tion, and buffering), a pair of light -emitting diodes (LED1 and LED2), a 0 -to 15-mA meter (M1), and a few sup-port components.

A reed relay (K1) and decouplingcapacitors (C1 and C3) were in-cluded in the processor portion of thecircuit to insure that the generatorvoltage was not applied to the inputop -amp before the ± 9 -volt supplyvoltages were present.

The input to the circuit (the voltagederived from the generator) is fedthrough K1 to the non -inverting inputof op -amp U1 -a. The output of U1 -a isthen fed to the inverting input of U1 -b,whose output is then fed to the invert-ing input of U1 -c. The output of U1 -c isthen fed to the final op -amp stage,U1 -d, which is configured as a voltagefollower. The output of U1 -d is fedthrough a voltage divider networkconsisting of three fixed resistors(R10 -R12) and a potentiometer (R13),to the DC ammeter, M1. That resistorstring allows the meter to be set forfull-scale deflection SwitchS1 (a momentary -contact DPDT unit) isused to initiate a measurement.

Power for the circuit is provided by a

350.00

300.00

250.00U)I-

d 200.00

150.00

100.00

50.00

0.00

0 3 5 10 15 20 25

MILES PER HOURFig. I. The generatorrotor assembly teas mounted on top of the author's van, whichteas driven at speeds ranging from 5 to 45 miles per hour intph) in 5-nu)11 incrementson a windless evening. The generator's output voltages-measured at various vehiclevelocities. rising a high -impedance mItmeter were recorded. and the data was used todevelop this graph.

pair of 9 -volt transistor -radio batteries(Bland B2), to produce a 9 volts DC.That dual -voltage arrangement al-lows the output of the op -amps to goto zero volts instead of half of the sup-ply rail. Two LED's, LED1 and LED2, areused to monitor the positive andnegative supply voltages (LED1 for B1and LED2 for B2).

Electronics Construction. Theelectronics portion of the project wasassembled in two parts called theprocessor and the monitoring circuit.The processor portion of the circuitwas assembled in a small section ofperfboard (approximately 11/,, x 2`/8

MOT1'

30 35 40 45

inches), and enclosed in the gener-ator housing (more on that later). Re-sistor R3 in the processor board isactually a pair (3.3k and 15k) of seriesconnected resistors. The monitoringcircuit was hard wired on a barrierstrip that was mounted in a 5- x 7- x2 -inch electrical breaker box. The twoportions of the circuit were then con-nected to each other through a 50 -foot length of 4 -conductor cable,

Mechanical Construction. The rotorhub was fabricated from a 3/4 -inch x11/2 -inch bolt head. The required drill-ing and cutting is shown in Fig. 3. Afterall holes were drilled and the set -

PROCESSOR BOARD

D1

1N4742

D21N4742

VNR1

4700

3

2

1/4 LM324

S R27/ 1K

4

U1 a

11

WitR3'

18.3KC1

7/64 -INCH DIAMETER THROUGH HOLE

13/64 -INCH DIAMETER

x 1/4 DEEP

(3 PLACES)

6-32 THREADFOR SET SCREW

CENTER LINE (4 HOLES)

4 1.1235

AFTER MODIFICATION,CUT OFF AND DISCARD

THREADED SHAFT.

0

3/4 x 1-1/2 BOLT

Fig. 3. The toter hub was jithricatedfrom a -1/4-inch x I1/2 -inch bolt head.The bolt head was drilled (as shown inthis diagram) and shaft of the bolt teascut (if l'he bolt head teas retained as therotor hub and the shaft was discarded.

screw hole threaded, the shaft of thebolt was cut off and discarded. Thebolt head was retained as the rotorhub. To assist in locating the holes, thefaces and top of the bolt head werecoated with a permanent marker

MONITOR

R422K

C210

R522K

R6220K

C3 -.-0-%

R71K

W

0

-WirdR91K

SEE TEXT L

2. The ancmemieler is comprised of the generator (denoted MOTI ),quad op -amp IC1, whase four op -amps- form a preamp, an integrator. and butler): apair of light -emitting diodes (1_17.I! and LED2, which are used to monitor batteryvoltage). an ammeter (MI), and a Jew support components.

W4R10

1000

1-4081-b

R11

1000

R12 t2200

B1

-9VDC=I=

R154700

1114

4700

LED2011 LED1

R1326.94

82 --+9VDC M1

- 0-15mA

99

PARTS LIST FOR THEANEMOMETER

SEMICONDUCTORSI-LM324 quad op -amp. integratedcircuit

Dl. D2 -1N4742 12 -volt, I -watt.Zener diode

LEDI, LED2-Light-emitting diode

RESISTORS

(All fixed resistors are 1/4 -watt, 5'4units.)

RI. R14, R15 -470 -ohm, 1/2 -wattR2, R7-R9-1000-ohmR3 -18,300 -ohm (see text)R4, R5 -22,000 -ohmR6 -220,000 -ohmRIO. R11 -100 -ohmR12-220 ohmsR13 -25 -ohm potentiometer

CAPACITORSCI, C3 -0.1-µF, ceramic -discC2 -10-µF. 35-WVDC, electrolytic


MOT1-9-18-volt DC motor (RadioShack #273-563), see text

MI -0-15 mA DC meter (SimpsonSK -525-T2) or 0 -15 -volt DC meter(Radio Shack #270-1754), see text

KI-I-amp, 12 -volt reed relay (RadioShack #275-233)

SI-DPDT 6 -amp momentary -contact, toggle switch

BI. B2 -9 -volt transistor -radiobattery

Perflx)ard materials, enclosure, ICsocket, pegboard hangers,electrical box connector, x 11/2

inch bolt, soup ladle, set screws,sheet metal screws. I1/2 -inchdiameter PVC pipe, 11/2-ineh PVCend caps, 1/2 -inch diameterelectrical conduit, 3 x I x 1/4 -

inch mounting plate (see text,epoxy. paint, 4 -conductor wire.battery holder and connector, wire.solder, hardware. etc.

0

and then scribed across opposingO corners to locate the center of each

face and the center of the top.

oEach of the three rotor spindles (see

Fig. 4) were fabricated from 31/2 -inch8 lengths of straight 1/4 -inch diameter

peg board hanger. Once the pegboard hangers were cut to length, the

`..9 spindles were beveled (using a grind-er) to match the slope of the wind

cr) cups.The three wind cups were tab -

ricated from 2'/a -inch diameter soupladles. The ladle handles were cut off

100 and discarded. The cups from the la -

120°TYPICAL

3.2173TYPICAL

BRAZE ALL JOINTS(6 PLACES)

6-32 SET SCREW

Fig. 4. Three rotor spindles were fabricated from 31/2 -inch lengths of straight 1/4 -inchdiameter peg board hanger, and the wind cups were fabricated from 27/x -inch diametersoup ladles.

R0.05502 PLACES

R0.2110

R0.1020 x 3/32 DEEP2 PLACES

DRILL 1/2 -INCH DIAMETER THROUGHHOLE

1/4 -INCH DIAMETER HOLE (BOTTOM)

4.1250 HOUSING SHAFT

JOIN WITH EPDXY

0to +

0 /

BOTTOM CAP

9.0000

MOUNTING PLATE

1.

3.0000

BRAZE JOINT j 0.

DRILL 4-3/16 -INCH DIAMETER HOLESEVENLY SPACED

Fig. 5. The generator housing was fabricated from a 40i -inch length of 11/2 -inchdiameter PVC pipe, a1/2 -inch length of electrical conduit, a x 3- x 1/4 -inch strap,and two 11/2 -inch PVC end caps.

dies were clamped face down on aninsulated surface. The spindles werethen clamped so that the bevelededges contacted the wind cups-coplanar with the mouth of the cupsand centered. A single setup wasused for all three spindles. The spin-

dle -to -wind -cup joints were then bra-zed and allowed to cool. The hubends of the three spindles were flat-tened slightly to form a force fit wheninserted in the 1/4 -inch holes in thehub. The spindles were arranged sothat all cups opened to the left when

viewed from the top and were at rightangles to the plane of the spindles.The three spindles were then brazedto the hub to complete the fabrica-tion of the rotor assembly.

Generator Housing. The generatorhousing (see Fig. 5) was fabricatedfrom a 41/8 -inch length of 11/2 -inch di-ameter PVC pipe, a 1/2 -inch length ofelectrical conduit, a 1- x 3- x 1/4 -inchstrap, and Iwo 11/2 -inch PVC end caps.

First, the 11/2 -inch PVC pipe was cutto length and a 1/2 -inch hole wasdrilled through one wall at the centerof the pipe (see Fig. 5). Nextthe 1/2 -inchelectrical conduit was brazed to the 1-x 3 x 1/4 -inch strap (as shown) to form

the shaft and mounting plate. Theshaft was then clamped to the hous-ing so that the hole in the wall of thehousing matched the internal diame-ter of the conduit. That joint was thencoated with epoxy putty so that asmooth 1/2- to 3/4 -inch radius fillet wasestablished between the shaft andthe housing. Next the top end capwas drilled and then cemented to thehousing with PVC cement. The gener-ator was then mounted to the topcap. The mounting screws werecoated with Elmers "Stix All" cementbefore installation. After mounting,the recessed counter bores werefilled with "Stix All" for waterproofing.

Once the generator housing wascomplete, the processor circuit wasconnected to the generator (MOT1)using a twisted wire pair. Sufficientslack was left in the cable and gener-ator wires so that the processor boardcould be removed from the housingfor maintenance. Next, a 4 -conductorcable was routed through the bottom1/4 -inch hole in the shaft, through theshaft, through the 1/2 -inch hole in thehousing to the processor board. Afterconnecting the 4 -conductor cable tothe board, the board was placed intothe housing behind the generator,and the bottom cap was pressedonto the housing.

Holes for two #6 x 3/8 -inch sheetmetal screws were drilled through thebottom cap into the PVC housing andthe bottom end cap was secured tothe PVC housing by the sheet metalscrews. The entire assembly was thenpainted with silver epoxy paint.

The Control Box. The control boxwas fabricated from a 5- x 7- x 2 -inch

electrical breaker box (from the au-thor's junk bin). However any metal orplastic enclosure with a minimum sizeof 4 x 43/4 x 2 inches will do.

The control box was prepared bydrilling four mounting holes for themeter using a template provided withthe unit. Three additional holes werethen drilled along the right side of theenclosure, next to the meter, to ac-commodate LED1 and LED2, and themomentary contact switch. Grom-mets were placed in the two LED holesto make a snug fit.

Next the meter has to be recali-brated for wind velocity. The rela-tionship between wind velocity inMPH and current in milliamperes isgiven by:

X MPH/45 MPH = Y mA/15 mA

That equation was used to determinethe new indices for the meter.

Once wind velocity conversion wascalculated, the cover plate was re-moved from the meter, the milliampmarkings were covered using correc-tion fluid ("White Out"), and windspeed in miles per hour were placedon the meter face with an indelible -ink marker. After that, the meter'scover plate was placed back on themeter, and the meter was thenmounted to the enclosure. Note thatwhile a Simpson SK -525-T2 0- to 15-mAmeter (also from the author's junkbox)was used in the prototype, a RadioShack DC voltmeter (#270-1754) canbe used if that unit's series resistor isremoved. After that, the LED's werepressed into the grommets, and themomentary -contact switch wasmounted just below the LED's.

A five -lug tie -point barrier strip wasmounted on the upper -right meter -

Most of the monitor circuit wasassembled inside an electrical breakerbox on a five -lug tie -point barrier strip,and connected to the meter, the powersource, switch and LED's via hook upwire.

mounting screw, and the resistor chain(R10, R11, and R12) was mountedacross the five -lug strip. The variableresistor, R13, was mounted through aVia -inch hole in a "Z -bracket." The Z -bracket can be nothing more than a1 -inch -wide strip of scrap metal bentat 90° angles in two places; the bendsshould be about an inch apart.

The battery clips were mounted onthe inside surface of the panel usingthe two 1/4 -inch holes and were fas-tened with 1/8 -inch pop rivets.

One end of a 4 -conductor cablewas brought into the box through a1 -inch electrical clamp. The meteringsection of the circuit was then con-nected to the sensor portion of thecircuit via the 4 -conductor cable.

Testing and Calibration. The pro-cessor board and control bqx werebench tested using a 300 -mV source.The 300 -mV source was applied tothe input of the circuit, producing anoutput of approximately 5.8 volts DCat U1 -a, approximately - 7.5 volts atU1 -b, approximately 7.1 volts at U1 -c,and approximately 6.8 -volts DC at theoutput of U1 -d. The generator wasconnected to the input of the pro-cessor board and a test run wasmade with the sensor mounted ontop of the author's van.

Data from the test run was analyzedto determine that the resistor networkshould have a total resistance 450ohms; R13 was then adjusted using adigital meter, to bring the network tothe required value.

Variations. Design of the ane-mometer can be varied to accom-modate variations in the size or typeof wind cups, the DC generator used,the meter used, etc. The critical issuesare: adjust the gain of the operationalamplifiers so that the dynamic rangeof the generator is within the linearrange of the op -amps, adjust the gainof the circuit to drive the combinedmeter and resistor string, and cali-brate your unit using a similar processto the one used by the author.

If you wish, ± 9 -volt DC suppliescould be used in place of the bat-teries to provide continuous, displayof wind speed. Also, the range of themeter could be increased or de-creased to customize the ane-mometer to measure wind speedsthat are typical at your location.

101

BUILD THE RELAY STATION(Continued from page 90)

R4, and R6), a 40 -kHz oscillator (the555 and its timing components), an IRtransmitter (R5, Q1, and LED1-LED3),and a power supply (the rest of thecircuitry).

Taking the power supply first, thecurrent source is a 9 -volt battery that isswitched on by S1. Its output is limitedby R3, and voltage regulated down to4.8 volts by D1 and D2. The regulationis necessary to reduce the voltage toa level that MOD1 can handle. It hasthe added advantage of allowing usto connect the three IR LED's in serieswithout using a dropping resistor,which would dissipate power but dono useful work.

While the oscillator circuit is basedon a 555 timer, that IC is used in a non-standard configuration. Normally, D3would not be present. Without it, C1(the oscillator's timing capacitor)would charge through R1 and R2, anddischarge through only R2. Therefore,it would discharge faster than it wouldcharge. Since the 555's output goeshigh as the capacitor charges andlow as it discharges, the output wouldnot be high for the same amount oftime that it is low. In other words, itsduty cycle would not be 50%, which isneeded for this application.

With D3 in the circuit, R2 is bypassed(or shorted out) while C1 is charging,but is in the current path during dis-charge. Therefore, by making R1 andR2 equal, the charge time equals thedischarge time, yielding an outputwith a 50% duty cycle. ComponentsR1, R2, and C1 (all precision, drift -freeunits) have been chosen to provide a40 -kHz output in this configuration.That output strobes the IR LED's via Q1.

However, the oscillator only func-tions when pin 4 of the 555 is high.Since that pin is connected to the in-verter circuit, the oscillator functionswhen the inverter's input is low. Theinverter's input is connected toMOD1's output, which goes low witheach remote burst received. So over-all, the circuit produces a 40 -kHz IRburst when it receives one.

Construction. Building this project isvery easy. That's because it uses veryfew components to do its job, and

102 they can all be purchased from Radio

Shack. My circuit was constructedusing point-to-point wiring of three as-semblies: the module plus inverter, theIR-LED chain, and the rest of the circuit(which was placed on a "mainboard").

The assemblies were installed in ametal cabinet to reduce RF inter-ference. If you'll just be using the unitas a repeater for long or well -lit rooms,the IR assembly and the moduleshould face out through opposite sur-faces of the cabinet. If you'll use theunit as a repeater to broadcast sig-nals around corners, the IR assemblyand the module should be on adja-cent surfaces of the cabinet. Keepthat in mind when laying out your ownunit. One unique way of accom-modating both situations would be tomake a swivel mount for the moduleout of an old film canister. It could beattached to a surface adjacent tothat of the LED's via a screw in thecanisters cap, and rotated to face anydesired direction.

The inverter's components (Q2, R4,and R6) were mounted directly ontothe module by soldering their leadsright to the module's pins. The mod-ule's ground pin was also connectedto its case. That is a must for reducingRF interference, which would makethe unit buggy. Three wires were thenattached to pertinent points in theassembly (V + , ground, and the inver-ter's output) and run to the mainboard. The module face was then at-tached to the case with a piece ofdouble sided foam, allowing it to peerthrough a hole made in the cabinet.

The IR LED's were mounted on apiece of vector board by themselves.They were wired together in seriesand the two remaining LED leadswere attached to wires that connectthe assembly to the main board. An-other piece of vector board was usedas a drilling template to make holes inthe case to accommodate the LED's.The LED assembly was pushed intoposition through the cabinet face.Then a hole was drilled through theboard and case to accommodate S1.The switch was passed through theholes and fastened, which holds theLED board in place. The switch wasthen wired to the battery clip andmain board.

The remaining components andbattery -clip lead were connected tothe main board (a small piece of ex -

perimenter's board). The board and afresh battery were mounted in the en-closure with double -sided foam. Thebattery clip was then snapped on thebattery and the case was closed.

Since there's nothing to adjust, ifyou've wired your unit properly itshould be ready to work as soon asyou switch S1 on. To test the circuit, tryto hide your remote control and thedetector side of the Relay under somenewspapers. Make sure the Relay is offand check to ensure that the remotecannot operate anything from underthe paper: If it still controls anything,shuffle the papers around until theequipment no longer receives re-sponses from the remote. Switch theRelay on and activate the remote. Ifeverything is working as it should, allyour components should respondnormally to the signals being re-broadcast by the Relay.

If something's amiss, disassemblethe unit. Fire a remote at the detectorand using a multimeter or os-cilloscope look for a digital signal atthe detector -module's output. If thereis none, you've got a faulty module, oryou've incorrectly wired it to the inver-ter. If MOD1 is fine, check the inverter'soutput. Its output signals should be thecompliment of the signals from MOD1.If not, check your inverter's wiring. Ifthat's fine, check pin 3 of U1 for 40 -kHzbursts while operating the remote. Alack of signal there indicates an os-cillator problem. If all is well, check forpulsed current flow through the IR-LEDchain. A lack of current there couldonly be the result of a bad Ql, a badLED, or miswiring.

Hopefully this circuit will add morefreedom to your use of remotes. Unlikemany of the manufactured alter-natives, its low-cost, safety, and ease -of -use should make it a really big hitwith the family, too.

e; 16.

40e)" 1 TV"' Cia

-r1.40".

"I'm a do-it-yourselfer, but I've never beena done-it-yourselfer."

WIRELESS HEADPHONES(Continued from page 96)

Implementation and Use. The re-ceiver may be mounted in a numberof ways, depending on the user andthe availability of parts. Perhaps themost obvious, but most difficult, is in aheadset. The small size of the receivercircuitry facilitates proportionallysmall packaging, but the added masson a light -weight headset can result inan unstable mount. Use a large -sizeheadset (full ear enclosure) andmount the receiver and battery aslow as possible to maintain a low cen-ter of gravity.

Placing the receiver and battery onopposite sides of the two speaker ele-ments is ideal or at least more bal-anced. Use shielded audio cablebetween the two headset speakers.

An alternative packaging idea is touse a lapel mount, in which the re-ceiver and battery are combined in asingle enclosure that is separate fromthe headphones. The photodiodemay be oriented on the printed-cir-

D1

C1

R1

CS

R3

-05 - c:14 116

R4 R1OFte_

R5

C:7 0

U1

-C9 -

R11 ,

C11

U2

R2

-C3-

C10C16

Cis

C14

U3

-C12-R12

Fig. 6. The receiver's printed -circuit board should be assembled using Vii -watt orsmaller resistors and miniature (radial lead electrolytic and ceramic -disc) capacitors.The volume control (R12) can be a miniature PC -mounted potentiometer orconventional unit, which would have to be mounted off -board.

cult board to permit side or front il-lumination relative to the printed -circuit board.

No external lens is necessary at ei-

ther the receiver or the transmitter-infact I recommend against it in orderto maintain a wide field of view at thereceiver end.

NOW Find the Right Part for Your VCR!The 320 -page, Fourth Edition of the VCRCross Reference contains both modeland part number cross references. Over1300 new parts and 360 new models havebeen added.

VCR's are made in a few factories fromwhich hundreds of different brand namesand model numbers identify cosmetically -changed identical and near -identical man-ufactured units. Interchangeable parts arevery common. An exact replacement partmay be available only a few minutes awayfrom you even though the original brand -name supplier is out of stock. Also, you maybe able to cannibalize scrap units at no cost.

withthe ISCET VCR CROSS REFERENCE

NEWT The Fourth Edition is contained on adiskette for IBM PC AT/XT compatibles,DOS 2.1 or higher. The disk software allowstechnicians to search by manufacturer formodel numbers and description of part num-bers. A parts editing sequence gives an on-screen view of all substitutes for parts en-tered. With the diskette, the technician canupdate files by adding model and parts

crosses of future models. The Fourth Edi-tion can be printed on pages completelyfrom the diskette.

The ISCET VCR Cross Reference, FourthEdition, is on 8/2 x 11 -in., pre -punchedpages and sells for $36.00. The 3'/ inchdiskette sells for $69.95 and you can viewlistings from a monitor or printed page.

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r Claggk Inc.VCR CROSS REFERENCE OFFERP.O. Box 4099Farmingdale, New York 11735Name

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Enclosed $36.00 for the ISCET VCR CrossReference, Fourth Edition.

Enclosed $69.95 for the diskette containingthe ISCET VCR Cross Reference, FourthEdition. Please specify:

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J_ 103

104

LOUDSPEAKER DESIGN(Continued from page 83)

Finally, you will be prompted for thetweeter's impedance at the selectedcrossover frequency. To find that val-ue, use the same strategy mentionedabove for the woofer, using the nomi-nal impedance of the driver if neces-sary.

Once all the above information hasbeen entered, the program will auto-matically calculate the inductor/ca-pacitor values and display them foryou. Copy the values down for futurereference.

Now that we have completed thislast in our series of loudspeaker de-sign programs, I hope that you allhave begun to feel a little more athome with speaker systems, and canstart to do a little designing and build-ing for yourselves without undo fear offailure. Admittedly, the speakers youcan design using these programs arefairly basic, but they are surprisinglygood performers that can give youyears of enjoyable listening.

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TELEPHONE SCRAMBLER(Continued from page 78)

and solder the green and red wire ofthat cord to the appropriate points onthe board as in Fig. 4. Label the othercord To TELEPHONE and solder its greenand red wires to the appropriate cir-cuit -board pads. Lastly, hook up thewires to the LED's, switches, and powerjack, and mount the board to thestandoffs.

Hook Up and Operation. Plug thecord labeled to WALL JACK into the tele-phone jack on the wall. Plug the cord

S1

0-

S2

LED2

-J-1

FinR42 C15

1 +- 1

R43

o

TO

TELEPHONEWALL JACK

H 1

MOV1 R41

T1

SCR1

R24 I R22

I R23 I

R44 ''R7 R6 I 6 R4

C61 I R5 I

1

R47 -R14- -R9-

41-

C9F U4

to the non -scrambled mode (LED1 off)and wait until the dial tone ceasesand is replaced by a recording prom-pting you to hang up and try again(approximately 30 seconds). As soonas you here the recording press S2,switching the unit into the scrambledmode and lighting LED1. The record-ing should now be unintelligible butstill present. Pressing S1 should returnthe recording to normal.

To use the Telephone Scrambler,each party must have their own unit.Dial the phone and establish contactwith the second party before engag-ing the scrambler. If you engage thescrambler first, the tones from a touch -

TO

TELEPHONE

GRNI !RED

D3

K1

TO 13 VACWALL TRANSFORMER

T2

CRit41 40 R Fil:U3

R36P40

R3 I R39 I R38 IC13

-R35-C8 +

1 1

4- C10

IR1131R17 1816113121R151R81 R8

-R29- C2- I R2

C3- XTAL1I R11

1C11F U5

R26 1 I1

I IR34 R3 R30

IIR25 IIR32I1 R33 C4

C21

C20

1

-R45-F146 _14112

1 D1

C18 C19

C16 C17

-R10- -J--R11-

U1

-R27--R28-

U2

Cl

Fig. 4. Use this parts -placement diagram when assembling the printed -circuit board.Don't forget to install the four jumpers.

labeled TO TELEPHONE into the tele-phone. Hook up the power cable,and plug in the transformer. If LED2does not light check for 13 volts AC atthe points where power enters theboard and check for the correct ori-entation of BR1. Pick up the telephoneand check for normal operation. If thephone is not working properly, checkthe orientation of U6 and the hookupof the telephone cables to the board.

Pressing S2 should light LED1, andpressing S1 should turn off LED1. If thatdoes not happen, check the orienta-tion of Q1 and SCR1, and for solderbridges on those components. Switch

tone phone will be scrambled, ren-dering them useless in dialing thephone. After establishing contact,both parties can engage theirscramblers by pressing S2 and enjoy acompletely private and secure con-versation.

Note: You'll be able to hear the3580 -Hz tone in the background veryfaintly. That is normal and can serve asa kind of reminder that your con-versation is secure from any third par-ty monitoring. Upon hanging up thetelephone, the scrambler will auto-matically switch back to the non -

scrambled mode.CIRCLE 21 ON FREE INFORMATION CARD

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L HHF4i

HUMIDITY GAUGE(Continued from page 88)

ride (table salt) is chosen for the solu-tion, the calibration is performed byadjusting R7 for a reading of 75% RH.

If a two -point calibration is desired,choose a salt from Table 1 that repre-sents a low value of relative humidity.Then repeat the above procedure,this time adjusting R4 to reflect thecorrect reading. That completes thecalibration procedure.

Once the circuit has been cali-brated, remove the temporary wiresfrom the board and install SENS1. Besure to observe correct orientation.Note: the humidity sensor is somewhatlight sensitive, therefore it must beshielded from light while in use. That isautomatically taken care of if an en-closure is used to house the instru-ment. A small opening can be madein the housing to allow the instrumentto respond faster to changing humidi-ty levels.

Use. Relative -humidity measure-ments can be made at any time by

ing will be produced in less than asecond. The use of a spring -loadedpower switch for battery operatedunits ensures long battery life. Shouldthe battery become exhausted, thedisplay reading will become erratic.For the best results, it is recommendedthat a fresh alkaline battery be in-stalled once a year.

The Digital Relative -HumidityGauge is designed primarily for in-door use where the temperature isrelatively constant, and assumes anambient temperature of 75°F How-ever, it can also be used outdoors,and at lower and higher tempera-tures. For variations in ambient oper-ating temperatures, refer to Table 2;that table gives correction factors thatcan be added to the relative -humidi-ty reading.

106

AUTOMOTIVE TESTER(Continued from page 74)

tinguished, indicating that the batteryvoltage is below normal.

Now, slowly raise the supply voltageuntil LED1 goes out (that should occurat about 9.0 volts). Further increasethe supply voltage until LED2 (BATTERY OK)

lights, which should occur at 12.4 volts.Slowly increase the power -supply volt-age until LED3 (NOT CHARGING) goes outand LED4 (INSUFFICIENT CHARGING) comes

on simultaneously. That should occurat 12.8 volts.

Continue to raise the power -supplyvoltage, and when it reaches 13.4volts only LED2 should be lit. Finally,raise the voltage until LED5 (OVER CHARG-

ING) comes on, which should occur at15.2 volts. CAUTION: Do not allow thepower supply's output voltage to ex-ceed 16 volts.

If you get the responses outlinedabove, the project is operating prop-erly. The voltage levels at which theLED's change state should be within0.1 volt of the specified voltages. Ifyour project falls out of that tolerance,you should adjust the value of one ormore of the resistors of the voltagedivider (R1 through R6) by about 1% tobring the circuit into spec.

If the project does not respondproperly at any one of the specifiedvoltage levels, check pin 1 of U2 to besure the 1.25 -volt reference voltage ispresent. If not, check the orientationof D1, C1, U1, and the LED's. Try anotherchip if possible. If only one LED mal-functions, troubleshoot the com-parator that drives that LED. Thenmeasure that output voltage as theinput voltage is varied about thespecified triggering level. Check theorientation of the inoperative LED,and replace it with a new one. If thesimultaneous switchover betweenLED3 and LED4 does not occur, checkthe orientation of D2.

Using the System. The followingsteps should be performed in se-quence when checking an auto-motive electrical system. If the projectis intended for stand-alone use, con-nect the project's input leads directlyto the vehicle's battery terminals be-fore starting the engine; be sure toobserve the proper polarity. Checkthe vehicle's alternator belt for proper

tension.1. Before the engine is started, the

BATTER' OK (LED2) should light. The NOT

CHARGING indicator (LED3) should alsobe illuminated since the alternator isnot operating at this point. Note: If theengine has just been turned off, LED4may be illuminated instead of LED3,since the battery may have a higherthan normal terminal voltage frombeing charged by the alternator.

2. Have an assistant start the enginewhile you observe the project's dis-play. If the WEAK BATTERY indicator (LED1)

comes on during cranking, the bat-tery terminal voltage has fallen to lessthan 9 volts. That indicates that thebattery is in a low state of charge or isnear the end of its life. Recharge thebattery and repeat the test.

3. With the engine idling, only LED2(BATTERY OK) should be illuminated, indi-cating that everything is normal. Notethat some vehicles will indicateINSUFFICIENT CHARGING (LED4 will be lit) be-

cause the battery's terminal voltagehas not exceeded 13.4 volts while idl-ing. That will be checked at the nextstep in the procedure. If the NOT CHARG-ING indicator (LED3) is lit, the chargingsystem of the vehicle is totally in-operative. That could be caused by adefective regulator circuit, an openalternator field, or a bad connectionsomewhere in the electrical harness.

4. To check the charging capacityof the alternator, turn on the heater/AC fan (highest setting), rear windowdefroster, windshield wiper (fastestspeed), and high -beam headlights.Race the engine to 1500 or 2000 RPM(moderate speed). Only LED2 shouldbe illuminated. If LED4 (INSUFFICIENTCHARGING) lights, the alternator cannotdeliver enough current to handle theaccessory load. That may be causedby a shorted or open diode, or ashorted winding in the alternator itself.Since the alternator is a three-phasedevice that uses a 6 -diode bridge cir-cuit, it will be able to deliver someoutput even though one of thephases is defective.

5. To check the regulator, turn off allaccessories and race the engine toabout 2000 RPM. If LED 5 (OVERCHARGING)

lights, the regulator is defective. Thatwill cause the battery to have exces-sive water consumption and shortenits life. In many vehicles, the regulatoris non-adjustable and must be re-placed.

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108

ADD A DMV(Continued from page 48)

Analog devices recommends aguard ring around the timing capaci-tor for demanding applications. Forthis project, careful placement shouldbe enough. Make sure the timing ca-pacitor is placed as close to theAD654 as possible. They also recom-mend that you do not use a groundplane.

To stuff the board of Fig. 2, look atFig. 3 and begin by installing the pas-sive components. Then add the ICsockets. You may wish to hold off fromsoldering LED1 to the board until thecase is prepared. This way you will besure to cut the leads to the properlength. While that LED is optional, itindicates that power is present. Ifpower is not present, you should notconnect a signal to the inputs. There-fore, LED1 also provides an indicationthat it is safe to use the adapter.

After you have prepared a circuitboard, you should build the interfacecable. The cable should be con-structed from a length of three -con-ductor cable. Keep the length toabout six feet or less. Strip one inch ofjacket from each end of the cableexposing the three wires. Strip one six-teenth inch from the ends of each ofthe wires. Solder the conductors topins 1, 2, and 4 of the DB15 connector.

Cut two lengths of wire to about twoinches in length. Strip 1/16 inch fromeach of the ends. These will connectthe input terminals to the board. Drilltwo 1/8 -inch holes in one end of thecase about one inch apart for mount-ing the terminals. Drill a 1/4 -inch hole inthe opposite end for the interface ca-ble. Feed the interface cable throughthe 1/4 -inch hole and solder the con-ductors to the appropriate pads onthe circuit board. I like to drill holes inthe board on either side of the cableso I can use a cable tie to secure thecable to the board.

Once everything is in place, I rec-ommend using a flux stripper on thesolder side of the board to be sure it isclean and free of contamination. Re-sidual flux can cause shorts and prob-lems that are difficult to debug at alater time.

With the power off, plug the inter-face cable into the game port. Turnthe computer on. Check for the prop-

er voltage and polarity on the board.Remember the button -0 input of thegame port has a pull-up resistor on it,so there will be + 5 volts present onthat signal line. If the voltages checkout turn the computer off, unplug theinterface cable and insert the AD654and LM324 into their sockets. Plug theinterface cable in and turn on thecomputer. If you have an os-cilloscope, check for a square -waveon pin 1 of the AD654. If you do nothave a scope, you can use a logicprobe or frequency meter. If there isno signal on pin 1 then there is a prob-lem with the board or perhaps theinterface cable. Turn the computer offand double check the circuit againstthe schematic and then check thewiring of the interface cable to findany errors.

If the circuit checks out then run theDVM program. If the program finds asquare -wave on the button -0 input, itwill begin to display values on thescreen. If the signal is not present, theprogram will display a message andabort.

Once the circuit is functioning cor-rectly you can secure the circuitboard in the case and connect theinput wires to the terminals. That's allthere is to it; your circuit is now readyfor use.

Going Further. There are 4 digitalinputs on the game port so you canhave up to 4 Analog Input Adaptersconnected at the same time. You caneasily construct a circuit board thatdistributes power and ground to theattached adapters. Remember toconnect the signal lines to differentbutton inputs on the port. Refer to thepinouts in Fig. 4 for the proper con-nections and input addresses for thegame sport connector.

..BACK TO VIRTUALREALITY IN A MOMENT,BUT FIRST THIS WORD

ABOUT TOOTHPASTE...

hiCtICIA

FLUIDCIRCUITS

By Charles D. Rakes

The ULN2429A was de-signed to be used as

an automotive coolant -level detector. The IC canalso be used to detect thepresence or absence of anumber of other liquids. It isavailable for less than $2from D.C. Electronics (PO.Box 3203, Scottsdale, AZ85271-3203; Tel.1-800-467-7736).

REFERENCE SIGNAL

DETECTOR INPUT

DECOUPLING

DECOUPLING

DECOUPLING

vcc

OUTPUT

A

10

11

12

13

14

vccr- - T13

i4D1

L-

B

R1

4700

05

R34.7K

R24.7K

Q6

D2 a/7V

Q2

The IC features a high -current square -wave outputthat can be used to drivean LED, an incandescentlamp, or a speaker. Theoutput may also be con-verted into a high -currentDC voltage to control relaysor solenoids. There's an in-ternal voltage regulatorthat allows the IC to oper-ate from a power source of

ULN2429A

DETECTOR

R44.7K

R52.5K

ULN2429A

+12V g

R1

1K

LED1

1,14 13

Fig. 2. Our low-level detector circuit

C21

11 10 8

U1

ULN2429A

S.:

10 to 16 volts. The IC is alsoprotected from reverse sup-ply -voltage damage and istemperature compensated.The IC is also not botheredby high -frequency noise.Ali -in -all the ULN2429A IC isan excellent candidate forour purpose.

OSCILLATOR CAPACITOR

OSCILLATOR OUTPUT

OSCILLATOR CAPACITOR

GROUND

GROUND

N.C.

OUTPUT

OSCILLATOR DECOUPLING

0 05 11

R9

R84.7K

4.7K

R6 D34.7K

R718K

3 60

GROUND OSCILLATOROUTPUT

8

Q1

OUTPUT

10 14 1

NM.R12 R11

22K 4.7KD5

Q4

Q3

R1322K

R142700

D4R 04.7K

R154.7K

REFERENCE

90

DETECTORINPUT

R167K

08

14012

DECOUPLING GROUND

NOTE: PIN 2 IS NOT USED

Fig. 1. As shown in A, the ULN2429A contains four basic circuits: a series voltage -regulator, asquare -wave oscillator, a detector/amplifier, and a load driver. The components of those circuitsare shown in B.

C1

.01

The IC's block diagram isshown in Fig. 1A and itsschematic in Fig. 1B. Thereare four basic circuits oper-ating within the IC. TransistorQ5 and the 7 -volt Zenermake up a series voltage -regulator circuit. DevicesQ1, Q2, and Q6 operatetogether as a sqUare-waveoscillator circuit. The os-cillator's frequency isdetermined by the value ofcapacitor connected be-tween terminals 5 and 7.Transistors Q3 and Q4 oper-ate as a simple detector/amplifier circuit. Units Q7and Q8 amplify the de-tected signal as either ACor DC to drive the loadconnected between Q8'scollector and the positivesupply.

If the circuit is to be oper-ated in the AC -detectionoutput mode no decoup-ling capacitor is neededbetween pins 10 and 11 orbetween pin 12 andground. For the circuit tooperate in the DC outputmode a capacitor isneeded at either or bothdecoupling locations. A rel-atively small capacitorvalue may be connectedbetween pins 10 and 11 toremove any AC contentfrom the output. A largercapacitor may be tied topin 12 and ground to add aslight turn on/off delay tothe DC output.

The circuit has two os-cillator outputs. Pin 6 offersthe maximum output ofabout 3 volts peak -to -peak

C3.1 PROBE

1FLUID

-n

Co

co

0coco

(7)

co

109

+12V

Fig. 3. The high-leveldetector keeps theLED on as long as theliquid level is abovethe probes. When theliquid level dropsbelow the probes, theLED turns off.

R1

1K

LED1

1,14

and pin 8's output is buff-ered by an internal 18kresistor.

Our first two circuits willshow the IC operating asintended in a level -detec-tor circuit. Let's look at themright now.

LOW-LEVEL DETECTORThe first level -detector

circuit (see Fig. 2) turns onLEDI when the liquid leveldrops below the probe.Configured as shown, theinternal oscillator is runningat about 2.4 kHz. That is theIC's recommended operat-ing frequency. With theprobe immersed in the liq-uid, the oscillator's signalfrom pin 8, which must trav-el through the internal 18kresistor, Is coupled toground through the liquid.When the level drops belowthe probe, the oscillator's

13

C2

1

U1

ULN2429A

C1

T01

C3.1

FLUID

PARTS LIST FOR THELOW-LEVEL DETECTOR

R218K

SEMICONDUCTORSLEDI-Light-emitting diodeUl-ULN2429A fluid detector, integrated circuit

CAPACITORSCI -0.01-µE ceramic -discC2, C3 -0.1-µF, ceramic -disc

ADDITIONAL PARTS AND MATERIALSRI -1000 -ohm, 1/4 -watt resistorProbe, perfboard, solder, wire, etc.

PARTS LIST FOR THE

HIGH-LEVEL DETECTOR

SEMICONDUCTORSLEDI-Light-emitting diodeUl-ULN2429A fluid -detector, integrated circuit

CAPACITORSCI-O.01-µF, ceramic -discC2. C3 -0.1-µ,E ceramic -disc

ADDITIONAL PARTS AND MATERIALSRI -1000 -ohm, 1/4 -watt, 5% resistorR2 -18.000 -ohm, 1/4 -watt, 5% resistorProbes, perfboard, solder, wire, etc.

110

signal is no longer taken toground and is coupled tothe detector's input at pin 9through a 0.1-g capacitor,C3. The signal is detected,amplified, and convertedto DC with the use of adecoupling capacitor, C2,between pins 10 and 11.The DC output turns on theLED indicating a low liquid -level condition.

HIGH-LEVEL DETECTORIn the next circuit (see

Fig. 3), the oscillator is againoperating at 2.4 kHz. Aslong as the liquid level isabove the probes, the LEDremains on, indicating anormal condition. The liquidbetween the two probescouples the oscillator's out-put to the detector's input.When the liquid level dropsbelow the probes, the sig-nal is lost and the LED turnsoff.

BUILD A TEST BENCH(Continued from page 39)

fiery should be considered during op-eration. The internal resistance of atypical dry -cell battery is between 10and 100 ohms. Considering the costof batteries these days, you may wantto use an AC wall adaptor as a powersource for lengthy operation. If the ex-ternal power supply is used, a resistor(R4) should be added in series withthe source to simulate the internal re-sistance of a dry -cell.

In the author's unit, a 22 -ohm, 1/2 -watt resistor was used for resistor R4. Toprevent charging B1, a reverse -bi-ased diode is used as an automaticchange -over switch. If external poweris removed, the battery automaticallytakes over. If the battery you use foryour unit is a rechargeable, you canomit diode D1 and choose a value forR4 to limit charging current.

If the Test Bench Amplifier is incor-porated into another project, andmade to share the same power sup-ply, such as a high -gain preamplifier,use a 100 -ohm resistor for R4 and in-crease the value of C4 to 100 RF toincrease isolation from other circuitryand prevent unwanted feedback.

The normal speaker load imped-ance for the LM386 is 16 ohms orgreater. An 8 -ohm speaker can beused, however, provided that the sup-ply voltage is clamped at 9 volts orless to prevent overheating U1. If youmust use a 4 ohm speaker, further re-duce the supply to 6 volts to keep U1happy.

Using the Project. The Test BenchAmplifier can come in handy in awide variety of applications. For in-stance, it can be used when checkingout a new circuit or repairing audioproducts. In addition, it can be usedaround the house with an inductivepickup coil to trace concealed (in -wall) AC wiring or to allow the wholefamily to listen in when a favorite rela-tive visits by telephone. It makes agreat extension amp for boosting theoutput of headphone -style personalradios. (Stereo sound, of course, re-quires that you build Iwo such ampli-fiers, one for each channel.) In anyevent, once you build it, you are sureto find many applications for this ver-satile and useful circuit.

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ADVERTISING INDEXPOPULAR ELECTRONICS magazinedoes not assume any responsibility forerrors that may in the index be-low.

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- Electronics Technology Today .. 15

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- Information Unlimited 1

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What Do These PrestigiousCompanies Have In Common?

AerovoxsDC Film and REI Suppression Capacitors. Aluminum

Electrolytic and AC Oil Capacitors, EMI Filters

AMP.Electrical/Electronic Connectors, IC Sockets,

PCB Switches

MRAD Engineering, Inc.

Motor Run Capacitors, HID lighting Capacitors,

Power Factor Correction Capacitors

Miniature and Subminature Coaxial

Connector and Cable Assemblies.

WAX CORPORATION

MEC, Tantalum and Thin Film Capacitors. Resistors,

Networks, Integrated Passive Components,

Trimmers, Oscillators, Resonators, Filters, Piero

Devices, and Connectors

BERG"ELECTRONICSHigh Density and Industry Standard

Connectors/Subsystems

CAROLElectronic and Electrical Wire and Cable and

Power Supply Cords

COLFLE

Tubing, Conduits, H se, Sleevings, Splices,

Insulation and (able Harness Products, Power

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4111-CommunicationsII Instruments, Inc.

CU Midtex

Relays and Solenoids

COOCERBussmann

fuses, Fuseholders, Fuse Blocks, and Fuse

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CORNELLDUBILIER

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Dale Electronics, Inc.

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Batteries: Computer, Cordless Phone, Laptop,

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Antennas: Cordless Phone and Scanner

DEARBORNWIRE ANI3 CABLE

Eaton Corporation, Commercial& Military Controls Operation

Switches, Relays, Displays and Keyboards

ELECTRONICSQuarts Crystals, Clock Oscillators, Surface Mount

Products, Programmable Devices

A

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Electronics Now Magazine

GUARDIAN

Relays and Solenoids

KEMETElectronics Corporation

Multilayer Ceramic and Solid Tantalum

Capacitors

[(OA SPEERResistors, SMT Tantalum Capacitors Inductors,

Resistor Networks, SMT Thermiston

MALLORYNorth American Capacitor Company

Tantalums, Aluminums, SonaltertsR Ceramics,

Films and AC's

Mtron

inlifiataFixed Ceramic Capacitors. Variable Capacitors and

Resistors. Crystal Oscillaton, Ceramic Filters,

Resonators, EMI Filters, Hybrid Circuits and more.

PanasonicIndustrial Company

Resistors, Resistor Networks. Ceramic, Film,

Electrolytic, Double Layer Capacitors,

Potentiometers, Switches, Inductors, filters,

Resonators. Vanstors, Thermistors

Philips ComponentsEre<T,ornc, North Amer Corporat.on

Resistors. Ferrite Components

Alluminum Electrolytic, Film & Ceramic Capacitors

They sellthfc3ugh

distributers.

They belc)ngtc3 the E.I.A.

They belcmig-cm your

vencic)r list.

QuamLoudspeakers and Commercial Sound Products

ROHIfflRohm Electronics Division

Resistors, Ceramic Capacitors,

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7:SEIECTASwitches, Relays, Terminals, Indicator/Pilot

Lights, LED Indicators, Test Clips, Test Leads,

Cable Ties and Heat Shrinkable Tubing

Tantalum Capacitors, Wet & Foil Capacitors,

Resistor Networks, Resistor Capacitor

Networks, Filters

SvailicheraireA Ilirilleimem Company

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CountersurveillanceNever before has so muchprofessional information on the artof detecting and eliminatingelectronic snooping devices-andhow to defend against experiencedinformation thieves-been placedin one VHS video. If you are aFortune 500 CEO, an executive inany hi -tech industry, or a noviceseeking entry into an honorable,rewarding field of work incountersurveillance, you mustview this video presentation againand again.

Wake up.' You may be the victim ofstolen words-precious ideas that wouldhave made you very wealthy! Yes, profes-sionals, even rank amateurs, may be lis-tening to your most private con-versations.

Wake up' If you are not the victim,then you are surrounded by countless vic-tims who need your help if you know howto discover telephone taps, locate bugs, or"sweep" a room clean.

There is a thriving professional servicesteeped in high-tech techniques that youcan become a part of But first, you mustknow and understand CountersurveilanceTechnology. Your very first insight intothis highly rewarding field is made possi-ble by a video VHS presentation that youcannot view on broadcast television, sat-ellite, or cable. It presents an informativeprogram prepared by professionals in thefield who know their industry, its tech-niques, kinks and loopholes. Men whocan tell you more in 45 minutes in astraightforward, exclusive talk than wasever attempted before.

Foiling Information ThievesDiscover the targets professional

snoopers seek out! The prey are stockbrokers, arbitrage firms, manufacturers,high-tech companies, any competitiveindustry, or even small businnesses in thesame community. The valuable informa-tion they filch may be marketing strat-egies, customer lists, product formulas,manufacturing techniques,- even adver-tising plans. Information thieves eaves-drop on court decisions, biddinginformation, financial data. The list is

unlimited in the mind of man-es-pecially if he is a thief!

You know that the Russians secretlyinstalled countless microphones in theconcrete work of the American Embassybuilding in Moscow. They converted

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Please rush my copy of the Countersurveillance TechniquesVideo VHS Cassette for a total cost of 553.95 each (Whichincludes 54.110 postage and handling).

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The professional discussions seen onthe TV screen in your home reveals howto detect and disable wiretaps, midgetradio -frequency transmitters, and otherbugs, plus when to use disinformation toconfuse the unwanted listener, and thetechnique of voice scrambling telephonecommunications. In fact, do you knowhow to look for a bug, where to look for abug, and what to do when you find it?

Bugs of a very small size are easy tobuild and they can be placed quickly in amatter of seconds, in any object or room.Today you may have used a telephonehandset that was bugged. It probablycontained three bugs. One was a phonybug to fool you into believing you found ahug and secured the telephone. The sec-ond bug placates the investigator whenhe finds the real thing! And the third bugis found only by the professional, whocontinued to search just in case there weremore bugs.

The professional is not without histools. Special equipment has been de-signed so that the professional can sweepa room so that he can detect voice -acti-vated (VOX) and remote -activated bugs.Some of this equipment can be operatedby novices, others require a trained coun-tersurveillance professional.

The professionals viewed on your tele-vision screen reveal information on thelatest technological advances like laser -beam snoopers that are installed hun-dreds of feet away from the room theysnoop on. The professionals disclose thatcomputers yield information too easily.

This advertisement was not written bya countersurveillance professional, but bya beginner whose only experience camefrom viewing the video tape in the pri-vacy of his home. After you review thevideo carefully and understand its con-tents, you have taken the first importantstep in either acquiring professional helpwith your surveillance problems, or youmay very well consider a career as a coun-tersurveillance professional.

The Dollars You SaveTo obtain the information contained in

the video VHS cassette, you would attenda professional seminar costing $350-750and possibly pay hundreds of dollars moreif you had to travel to a distant city toattend. Now, for only $49.95 (plus$4.00 P&H) you can view Countersur-veillance Techniques at home and takerefresher views often. To obtain yourcopy, complete the coupon or call.

GERNSBACK SPECIALTY SERIES...Great for special effects and remote fire starting. CAUTION REQUIRED!...

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