ti WVIR MAPLIN - World Radio History
36
\\ti WVIR MAPLIN PROJECTS WOK ELEVEN SERVO AND DRIVER MODULE 8 CHANNEL FLUID DETECTOR NOISE REDUCTION UNIT MK2 1VIAPM IX CAUTIOUS NI -CAD CHARGER MOTHER3OARD FOR BBC 'B' ANOTHER FIVE BOB'S WORTH XENON TUBE DRIVER ENLARGER EXPOSURE METER
Transcript of ti WVIR MAPLIN - World Radio History
\\ti WVIRMAPLINPROJECTSWOK ELEVEN
SERVO AND DRIVER MODULE8 CHANNEL FLUID DETECTORNOISE REDUCTION UNIT MK21VIAPM IXCAUTIOUS NI -CAD CHARGERMOTHER3OARD FOR BBC 'B'ANOTHER FIVE BOB'S WORTHXENON TUBE DRIVERENLARGER EXPOSURE METER
Ee k a AlHEVIIkPCILIN MAGAZINE 5PROJECT BOOK ELEVEN
This Project Book replaces issue 11 of 'Electronics'which is now out of print. Other issues of'Electronics' will also be replaced by Project
Books once they are out of print. For currentprices of kits, please consult the latest Maplinprice list, order as XFO8J, available free of charge.
CONTENTS PageServo and Driver Module 1
Ideal for Radio Control systems or Robotics.8 Channel Fluid Detector 4Monitor fluid level on an eight LED display.Mark 2 Noise Reduction Unit 6Stereo unit offering full Hi-Fi specification.Mapmix 12Six channel audio mixer with switched mono or stereo modes.Cautious Ni-Cad Charger 17Electronically timed charging and automatic discharge.Motherboard for the BBC Micro 22Easy access to the BBC's ports.Another Five Bob's Worth 24A Door Alarm, THD Filter, Cassette Processor, Volume Expanderand a Parametric Equaliser.Xenon Tube Driver 28Compact unit including trigger transformer.Enlarger Exposure Meter 31A must for photography enthusiasts.
Editor Doug SimmonsProduction Manager Hugh MoirTechnical Editors Robert Kirsch,
Dave Goodman
Art DirectorTechnical Artists
Peter BlackmoreRoy SmithJohn Dudley
Published by Maplin Electronic Supplies Ltd.P.O. Box 3, Rayleigh, Essex
Typeset by Eden Fisher (Southend) Ltd.Printed by Mayhew McCrimmon Ltd.
For Personal Service
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Copyright. All material is subject to world wide copyright protection, and reproduction or imitation in whole or part is expressly forbidden. All reasonable care istaken to ensure accuracy in preparation of the magazine, but Maplin Electronic Supplies Ltd. cannot be held legally responsible for its contents. Where errorsoccur corrections will be published as soon as possible afterwards. Permission to reproduce printed circuit board layouts commercially or marketing of kits mustbe sought from the publisher. © Copyright 1984 Maplin Electronic Supplies Ltd.
IRM7SADRIVERMOD N-Iff* Compact Lightweight Unit* Easy Construction -- Reasonably Priced* Ideal for Model Aircraft/Boats etc
by Dave Goodman
Electro-mechanical interfaces foruse in modelling, robotics and controlsystems can be difficult to produce withany accuracy, especially if facilities orfinances are limited; therefore this articledescribes construction of a completeservo mechanics kit and small drivermodule (37 x 25mm). The project is easyto build and the cost very reasonable.Both servo and PCB are small andlightweight, these being importantcriteria for use with such models asaircraft or small power boats, althoughthe PCB is not so small that constructionrequires a degree in micro -technology!
Robotics 'buffs' could find servo'suseful for producing arm lift and rotation-al movement or perhaps steering controlof wheels. The necessary electricalcontrol signals are generated from portscanning routines and FOR -NEXT loopsin BASIC, which is quite fast enough forsuccessful operation.
+ v01
2C4
IIIDO-112u2
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IR2150k
C3-
11
C11n5
13 12
14 IC1
ZN419I 409 CE
1 2
10 8
4
C2
11-0100n
_R1100k
1
VC
BC327
TF I
TP2BC327
Set doMechanism
40M1
R6330k
R5330k
R3
4k7
11 R4
5
0M2
6WO
2k2
PR1
M
8
PR02
5kCeramic
Proportional ControlA servo consists of an electric motor
with gear box, rotating arm, for transfer-rinc movement, and a feedback potentio-meter. Connecting a suitable voltageacross the motor causes high speedrotation which is geared down toproduce a final drive of one revolutionevery two seconds - at high torque. Thedrive arm continues rotating as long aspower is applied, this is not the requiredstate of affairs, as positional control of the
Figure 1 Circuit Diagram Figure 2 Artwork & Legend
arm is necessary. Positional or prop-ortional control is achieved by con-tinuously pulsing the motor in small steps.The gearbox drives the wiper of apotentiometer the resistance of whichvaries with each step, this variation issampled by the control module. Whenthe arm reaches the desired position,drive signals to the motor are inhibited,preventing further movement.
Circuit DescriptionIC1 requires a positive going, pulse
width modulated signal of between 0.5and 2.5mS repeated every 20mS (50Hz).This 20mS repetition, or frame rate, isstandard for most proportional radiocontrol transmitter/receiver systems (Fi-gure 7). Servo arm rotation (of 0 to 180degrees) is determined by the pulsewidth, with the centre position (90degrees) equal to half maximum pulsewidth, viz. 1.5mS. R1 and C2 aremono -stable timing components whichproduce a fixed time period, used forreference and comparison of the incom-ing signal. C I sets the 'dead band' or areaof non -movement, which corresponds toa centre loaded joystick used with radiocontrol transmitters. This area around1.5mS can be increased or decreased byaltering the value of Cl, but it should bekept below 2.2nF - otherwise the pulseexpansion timing becomes obscured. R2and C3 expandthe servo motor used, the values givenare correct for the system described inthis article. ICI output pins 7 and 8 bothsit at 1.75V DC under quiescent condi-tions. During operation one of theseoutputs pulses high and the other pulseslow, e.g. increasing pulse width from0.5mS to 2.5mS causes pin 7 to = OV (not
4001BE Pin14. +5VPin? .0V
1311
20mS Osc.Frame Generator
+5V
15k
220k
0.2 to 2.5mSMonostable
P.W.M.
PWM
0/P
Figure 3 Test Circuit
Q) and pin 8 = +V (Q). Decreasing pulsewidth from 2.5mS to 0.5mS causes pin 7 to= +V (Q) and pin 8 = OV (not Q). Puttingthe servo under a heavy load conditionwill produce supply current drains of150mA or more which the IC, with 7mAmax. output drive, is unable to cope with.Therefore TR1 and TR2 are used toswitch the +V rail to the motor, receivingbase drive from pins 5 and 9. A regulated+2.2V reference voltage level is derivedfrom pin 2 and connects to the servopotentiometer. As the wiper moves avoltage swing of +1.'7V to +2.2V isdeveloped on pin 3 this modifies themonostable timing thus increasing or
output drive to the motor. Apercentage of back EMF signals from themotor are connected via R5, along withthe controlling signal, to the monostablereference input; this helps to preventovershoot on faster servo mechanisms.The values of R5 and R6 can be altered byup to 10%, if necessary, to accomodatethis.
ASSEMBLY INSTRUCTIONSFOR SERVO MECHANISM(YG140)
Presetcontact
(tit onto Spacerby bending Lugs)
Presetbody
(clamp in posnusing screws)
Grommet A.(Fits into slot,
bring motor winosthrough)
I
C/S headscrews I
ContactSpacer
(Fit over spigotafter fitting
contact)
DriveLever
Roundhead
screws
Box Lid
Gear Mechanism
Gear
Motor
Sit
Box BaseRoundHeadScrew
PCB ConstructionInsert resistors RI to R6 and capaci-
tors Cl to C4. C2 to C4 are polarisedtypes and must be fitted correctly, withthe longest lead marked with a + sign tothe + sign on the PCB. Insert 8 Vero pins(if required) from the track side of thePCB and press home with a solderingiron. Solder these pins and components
r - -
Motor FeedbackPreset
Note: PR1 8 PR2 can be reversedit excessive jitter occurs.
M1PR2
PR1M2
Connectionsto PCBlas short aspos. 2"-3" I
Figure 5 Servo to PCB Wiring
Figure 4 Mechanics Assembly2
186 RotationPW.M.=0-5mS-2-5mS
Figure 6 Servo Operation
and remove excess wire ends. Fit IC1and both transistors and carefully solderthem in place. Clean the board andinspect for mistakes, short circuits, etc.
Servo ConstructionThe diagram in Figure 4 shows the
servo assembly, but a few points needexplaining in more detail. Snap off one ofthe contact spacers and remove thecasting piece. Fit the brass preset contactover the spacer so that the key fits intothe slot. Note that the contact mounts overthe face moulded with a small bushprotrusion - not the larger bush face!Gently bend both brass lugs over so thatthe contact is held firmly to the spacer.Next carefully press the assembly onto
spigot as shown, ensuringthat the wiper is facing outwards. This jobis a bit fiddly and great care must betaken to avoid damaging the wiper. Placethe cermet preset (terminals facingoutwards!) over the wiper and line up twoof the four available slots with two screwmounting holes. Insert the self tappingcross -head screws and tighten down justenough to grip the preset edges, over -tightening will break the body andobviously should be avoided. Place thesmall brass gear over the motor shah andpress home. Fit the motor onto themechanism housing by pushing andtwisting, the fit is made tight to preventthe motor from turning when in use.
Servo andPCB Wiring
Figures 2 and 5 show the fiveconnections between the servo and PCB.
P.W. M.
2-5mS-401 (1,..A)
inS
Max
20mS Frame
Figure 7 Control Signal
In fact motor connections MI and M2 canbe reversed as can preset connectionsPR1 and PR2 although the wiper Wconnection must be as shown. Keepwiring between the units as short aspossible -.o prevent excessive motor RFfrom being induced into the presetcircuitry, otherwise operation may beerratic. The PCB has been made thecorrect size for fitting onto the side of theservo box and quick stick pads can beused here to advantage - as shown in thephotograph.
TestingA suitable +V pulse transmitter/
receiver system can be used, or forconvenience the test circuit shown inFigure 3 can be constructed - toproduce a 20mS frame and variable 0.5 to2.5mS pulse width, using the values given.A suitable 4.2 to 6.5V supply wIl berequired, such as four AA Ni-Cads or a126 type dry battery. The power supply
+ 5V
used should be capable of delivering upto 1 Amp without the +V rail dropping,otherwise problems will be encountered.
Connect up the supply rails andswitch on, a slight 'glitch' may occur, butno:hing more. Input the PWM signal andmake a return path by connecting theservo ground (OV) to the signal sourceground. If using the test circuit (Figure 3)from the same power supply you willrequire a large de -coupling capacitorfitted across pins 7 and 14 of the 4001 IC,to prevent amplitude modulating thePWM signal; 470uF to 1000uF shouldsuffice. Move RV I or your transmitterjoystick from centre to full clockwise orfull anti -clockwise, whereupon the servoarm should follow suit. If the motor drivescontinuously or jitters excessively, re-verse PR1 and PR2 connections. In caseof malfunction various voltage levelsshould be checked with a high impe-dance voltmeter or oscilloscope, refer-ring to the circuit description as a guide.
SERVO & DRIVER MODULE PARTS LISTRESISTORS:- All 0.6W 1% Metal Film SEMICONDUCTORSRI 100k 1 (M100K) IC1 7.14419CE 1 (YH92A)R2 150k 1 (M150K) TR1,TR2 BC327 2 (QB66W)R3 4k7 1 (M4K7)R4 2k2 1 (M2K2) MISCELLANEOUSR5,6 330k 2 (M330K) Servodriver PCB 1 (GB68Y)
Veropins 2145 1 pkt (FL24B)CAPACITORS Servo Mechanism 1 (YG14Q)C1 1n5F Cerai.ic 1 (VirX70M)C2C3
100nF 35V Tantalum470nF 35V Tantalum
1
1
(WW54j)IWW58N) A complete kit of parts is available.
C4 2µ2F 35V Tantalum 1 (WW62S) Order As LK45Y (Servodriver Module Kit)
3
4
by Nigel FawcettIntroduction
This project, as the ti:le suggests, is avariation of the very popular fluddetector circuit, only here it has beantaken a stage further, and has therebyincreased the range of applications forsuch a device. when building adarkroom and workshop Into a garagerecently. it was deemed necessary tohave a sink with hot and cold runningwater. Getting tne water in was noproblem, but getting it out again was adifferent matter The garage wasconsiderably lower than the house. anddid not have immediate access to anymain drainage point.
The only solntior was to pump thewater back up to house level, andthereby into the normal domestic wastesystem. The waste frcm the garage sinkemptied into an expansion tank of thekind used in central heatinc systems, andwas pumped out again with aself -priming pump purloined from aredundant washing machine. It was here,.hat the need for a fluid detector lay. Ameans of determining the presence ofwater was required to switch on thepump. However, it was foreseen that agreater inflow of water than the pumpcould reasonably handle might occur. Toovercome this problem, eight separatechannels were incorporated to detect theincreasing level of water in the tank, andso indicate the effectiveness of the purr p.
Circuit DescriptionA: the heart of the circuit (see Figure
1) is the LM1830 fluid detector chip IC2.This is the type of IC commonly found indrinks vending machines, washingmachines, and a whole host of otherdomestic and industr.al appliances. It is awell designed IC wh:ch includes an A.C.currert :o the probes to alleviate theproblem of plating. The output is alsopulsed, and can be used to drive aspeaker or LED directly, but in thisinstance an 'on' or an `off condition wasrequired to interface with the CMOSdigital part of the design. This is achievedby the reservoir capacitor C4, whichsmoothes the oscillator output, and thepull-up resistor R2.
The IC detects the presence of waterby comparing the resistance across theprobes with an internal resistor. Oneprobe is connected tD ground, whilst theother is connected tc pin 10 of the IC. Inthis particular design, eight independentprobes are connected to the single8 -charnel analogue multiplexer'demulti-plexer [Cl. Each of the channels isscanned approximately once a second,and during the scan :ime IC2 checks forthe presence of water (conductive fluid).If water is detected then the output of IC2goes high and is written into the latchcorresponding to the input channel of the8 -bit aldressable latch IC3.
Both ICI ar.d IC3 have a three bitaddress bus to select the desired
* 8 LED's Indicate Fluid Level or* Monitor up to 8 Separate Levels* Based on LM1830 ICSimplifies Construction,
channel, and the addressing for the chipsis provided by half of the dual decadecounter IC5. The clock for the counter isformed from two of the dual input NANDgates of IC4, RI and Cl. The other twogates of IC4 and the other counter of IC5are used to produce a short pulse duringeach scan cycle, to ensure that data isonly written into the output latches whenIC2 has had time to sense the fluid andsettle down. The outputs from the latchare then used to drive the eight LED's andtheir associated circuitry. In theapplication described in the introduction,the LED's for channels 1-6 were greenand channels 7 and 8 were red. Thisprovided visual stimulation when thingswere getting dodgy. In practice thecolours chosen will depend on theapplication (see applications). It shouldbe noted here that a remote lead wastaken from channel one output to aseparate board which was used to switchthe pump on or off.
Construction DetailsAll the components are fitted on the
printed circuit board (see Figure 2). Stanby inserting, and soldering, the wire linksand resistors, proceed with the ICsockets, capacitors, PL1, the transistorsand LED's, and finally insert theintegrated circuits into their respectivesockets. Normal MOS handling precau-tions should be observed with the CMOSintegrated circuits, with care to ensurecorrect orientation. PL1 is a ten pinconnector, but only nine pins arerequired, and in fact there are only nineholes in the PCB, so pin one must beremoved from the plug before it can bemounted on the board. This is easilyachieved with a small pair of radio pliers.
A twelve volt power supply isrequired and, although no constructiondetails are described here, many of thecircuits shown in back issues of thismagazine will reveal a suitable design(i.e. Digital Enlarger Timer/Controller in
81
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PL187
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Figure 1. Circuit diagram
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13
the June to August 1983 issue, Vol.2 No.7).As far as the construction of a probe isconcerned, it would be beyond therealms of practicability to attempt thedescripton of a suitable design, since itdepends entirely on the application. Thereceptacle containing the fluid may besmall or large, shallow or deep. Theremay be one individual container or up toeight separate ones. There may not evenbe a container at all (see applications). Inmany applications however, a simplenarrow piece of copper strip Veroboardcan be employed, using the stripshorizontally, choosing appropriate stripsfor the panicular levels, and connectingthe ground terminal to the bottommoststrip.
MAPLIN8CHANNELFLUID IND.
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ApplicationsUp to this point, most of the
references to utilising this project haverevolved around using all eight channelsto monitor the fluid 'level in a container.In the previous para jraph it wassuggested that the various channels couldin fact be used quite independently or ingroups of any number. To explain thisfu_-ther, consider the Mowing threeapplications for which the circuit hasalready been gainfully employed. Caseore is for use in a car and is really rathera novelty idea. The pupose here is to usethe project to give a ccntinuous visualindication of the amount of water in thewindscreen washer bottle. When used inthis way, the red LED's should be
Continued on page 11.
o gN,
Figure 2. PCB track & layout5
.111.
=..111111.
.11111.
Based on an original design, by Dr. David Ellis
.111=.
* Simple Calibration* LED Peak Indicators* Full Hi - Fi Specification* Very Low Distortion Levels* Stereo or Four Channel Operation* 30dB Improvement in Signal -to -Noise Ratio
.
IntroductionThis improved design for the Maplin/
E&MM Noise Reduction Unit utilisesfewer components and is easier toconstruct, yet still offers the same highspecification. Calibration has been sim-plified, no test gear is required, althougha distortion analyser will enable thelowest distortion levels to be achieved.(0.06% typical).
The compander PCB's are fullycompatible with the original Noise Re-duction Unit. The Mark II Unit is designedfor stereo (2 channel) operation butpower supply outputs via a DIN socketenable two units to be connectedtogether, thus giving 4 channel operation.If required furthur units could be utilisedto give more channels.
N/R PrinciplesNoise reduction systems reduce the
irritating noise of tape hiss and so on byan encode/decode process. Quiet soundsespecially those at the top end of the
spectrum, are easily swamped by tapehiss, so an encoder is used to artificiallyboost these signals before they arerecorded. During playback the reverseprocess decodes the recorded soundback to its original state and rids themusic of tape generated noise. Untilrecently, noise reduction systems havefallen into three distinct types: Dolby B(domestic), Dolby A (professional) andDBX (professional). However there is nowa confusing proliferation of other systemsoffering various degrees of noise supres-sion, including: Toshiba's Adres system,Telefunken's Highcom & Telcom, Sanyo'sSuper D, Dolby's C & HX systems andTandberg's Dyneq. If there's any sense inthis race to the pinnacle of perfect musicreproduction, the hopefully there will besome common standards agreed upon!Table 1 gives the signal-to-noise ratiosobtainable from various recordingmediums with and without different typesof noise reduction.
The various systems available at
Recording medium Noise reduction S/N ratio Comments
Cassette - Dolby B(Sony TCK55 II) + Dolby B
+ Dolby C+ HighCom
Four -track tape(Teac 3440)
No noise reductionMarKII unit
Two track tape No noise reduction(Studer) + Dolby A
57dB67dB Above 4 KHz75dB Above 1 KHz75dB Above 1 KHz
55d B85dB Above 30 Hz
70dB80dB Above 20 Hz
= .1111=
.111111.
present basically work on the principle ofcomplementary compression of the on -tape signal and expansion of the off -tapesystem. Compression involves reducingthe dynamic range of the material that isbeing recorded, thus- with a 2:1 com-pression ratio, if the input to thecompressor increases by 12dB, then theoutput of the compressor (on -tape signal)will increase by only 6dB. Conversely,expansion involves increasing the dyna-mic range, so that an increase of 6dB inthe off -tape level will result in a 12dBincrease in the output from the expander,thereby restoring the original dynamicrange of the music. At the same time thenoise introduced in the recording chain,particularly tape hiss, is rendered inaudi-ble on expansion since this unwantedsignal was not subject to the initialcompression treatment and is thereforeexpanded downwards below the lowestdynamics of the music signal. Thisprocess is illustrated in Figure 1.
Another feature of the compression/
i/P
+ 20
OdBm
-20
100db
Coding0/P I P
Tape
+10
Decoding0/,
+20
-10
-20 50dB
Noise generated bytape and electronics
Expander
OdBm
-20
-00
-60
Table 1. Comparison of Noise Reduction Systems
6
Compressor
Figure 1. Operation of a Compression/Expansion System
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expansion process is that it allows therecording of signals with a dynamicrange approaching the limits of audibility,i.e.100 to 120dB.
CircuitThe circuit diagram for the com-
pressor and expander (compander) isshown in Figure 2. The power supplycircuit is given in Figure 3.
The compressor input is routed viaS la, either directly to the output in the'out' position, or to C1 in the 'in' position.IC1 and associated components form asecond -order high pass filter with a12dB/octave roll -off below 30Hz, Thisremoves sub -audible signals (infra-sonics) that might be generated from
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record warps or sub -octave trackingVCO's. The reason for this filtering is thatonce audio frequencies descend towardsDC, the response of tape recorders dropsoff dramatically, and on playback a signalcompressed in response to high level lowfrequency signals will be expanded,resulting in phantom modulation by themissing low frequency component lostduring recording. The output of the filteris AC coupled to a simple RC network(C4, R4) which forms a high frequencypre -emphasis circuit, providing a 12dBtreble boost. Without this pre -emphasisand corresponding de -emphasis in theexpander, a low level signal may beswamped by high level bass frequencies,typically resulting in a heavy breathing or
IC1 LF 3511C2 NE 570NIC3 LF 353IC5 3302
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pumping effect as the expander attemptsto adjust the gain accordingly.
The signal is then applied to theNES70 (IC2a) configured as a compressorusing an internal variable gain cell andfull -wave rectifier, as well as an externaloutput op -amp (IC3a). Thu variable gainceL is similar to a standard operationaltransconductance amplifier (OTA), ex-cept that, unlike OTA's, it is 'linearised'and therefore insensitive to temperaturechanges as well as offering low noise andlow distortion performance. The signal atthe output of IC3a is rectified and theresultant control voltage used to adjustthe variable gain cell. By placing the gainceL in a feedback loop with the op -amp,the variable current generated in propor-
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Figure 9. Compander PCB
s:gnal which is compared with referencevoltages derived from the potentialdivider network, R21, R22, and R23. Thefast attack/slow decay operation of thecomparators is determined by C13 andR19. IC5a and b respond to signal levelsof, respectively, -3dBm and OdBm.
The expander configures the other
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NOISE REDUCTION UNIT
half of the NE570, IC2b, with a differentarrangement of the various blocks. Oncethe off -tape signal has been routed viaSlc to C14, the signal is applied tocomparators, IC5c & d, to provide anindication of off -tape levels, and simul-taneously to the full -wave rectifier andvariable gain cell. The rectifier produces
C16
GA 30H
UC18
0 SK2
0 SK3
0 SK4
U
a control voltage that is used to adjust thegain cell, with a response time deter-mined by C51. An RC network (R30, C20)is connected in parallel with the op -amp,IC3b, to provide a treble cut a 12dB,therefore de-emphasising the pre -emphasised signal emerging from thecompressor via the tape recorder. When
8
tion to the input signal is used to adjustthe overall gain of the op -amp. A 6dBincrease in output level produces a 6dBincrease in the gain of the variable gaincell, since this is effectively an expanderinserted in the feedback loop, this resultsin a 12dB increase in feedback current tothe input of the op -amp. Consequently, anincrease in input level of 12 dB results inonly a 6dB increase at the output of theop -amp, thereby yielding the desired 2:1dynamic range compression.
The current from the full -waverectifier is averaged by an external filtercapacitor (C50) with the result that thegain control is made proportional to theaverage value of the input signal. Thespeed with which this gain adjustment ismade determines the transient responseof the compressor and is a product of thevalue of the filter capacitor and aninternal 10k resistor. The value of 2.2uFfor C50 yields good transient response ataverage signal levels.
Op -Amp Slew RatesThe RCR network (R8, C8, R9)
around the op -amp, IC3a, provides DCfeedback to bias the output at DC. C7 isan external compensation capacitor toprovide stable operation over the audiobandwith. It may seem curious to use anexternal op -amp when the circuit dia-grams indicate that the NE570 has itsown. This is because the op -amps in thisIC are equivalent to 741 types- with slewrate, noise, bandwidth, and output drivecapability that are not really adequate fordemanding audio applications. Withweak signals, the compressor circuitoperates at high gain and the NE570op -amp runs out of loop gain. Furth-urmore, a slew rate of 600mV permicro -second means that high frequen-cies will suffer. By using a J-FET op -amp,such as the LF351 with a slew rate of 13Vper micro -second, these problems areeliminated. Additionally, the output swingcan be larger since IC3a is powered by adual supply rather than from the single -rail supply required by the NE570.
The non -inverting input of the NE570op -amp is biased by an internal refer-ence voltage of 1.8V. In the case of the
external op -amp, IC3a, this is accom-plished by tying it to pin 8 via an RCdecoupLng network (R7, C5) which filtersout noise from the NE57C referencevoltage. Pin 8 also serves anctherimportant function; providing the meansfor trimming distortion generated byIC2a. Even harmonic distortion is pro-duced by voltage offsets in the variablegain cell, and RV 1 enables adjustment ofthe offsets for minimum distortion.
Comparator FunctionsThe function of R10 is to isolate the
output of IC3a from the potential capaci-tive load of a long length of screenedcable connected to the compressoroutput which could lead to oscillation. S lbselects the 'in' or 'out' mode of operation.
Comparators IC5a and b provide anindicatior of the signal level at the outputof the compressor. The inverting inputsreceive the half -wave rectified output
9
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Figure 5. Power Supply PCB
the input signal increases by 6dB, thegain cell control current is raised by afactor of 2, resulting in an increase in gainof 6dB. Since the input of the externalop -amp, IC3b, is derived from the gaincell, the output level increases by 12dB,thus giving the required 1:2 dynamicrange expansion. RV2 enables adjust-ment of gain cell offsets for minimumdistortion, as in the compressor. Finally,R32 isolates the output of IC3b fromsubsequent screened cable, and S Idselects the mode of use.
ConstructionThe unit is designed on a modular
basis so that each PCB provides simul-taneous compression and expansion forone channel. Single sided PCB's are used
IC1,LF351ICILF 353
a
IC5,3302 IC2 NE570N1 14 1 16
to keep the cost down. In order thatdecoding should be the exact inverse ofcoding, it is important that componentsare well matched.
PCB designs and component over-lays for the main board and PSU aregiven, respectively, in figures 4 and 5.Remember to fit the LED's to thecompander and PSU boards with theleads at full length - so that they may bebent at right angles to allow the LED's to
protrude through the front panel of theunit. The threaded phono sockets sug-gested for the unit have the advantage ofsmall physical size and compatabilitywith the connectors normally encoun-tered in Teacs, Revoxes and the like.These sockets are mounted on the rearpanel and connections to the signal pinsare made via short lengths of unscreenedwire from the relevant points on thePCB's. The phono socket earth connec-tions are linked together and connectedto the earth (OV) line on each companderPCB - again using short lengths ofunscreened wire.
The PSU is utterly standard, thoughit's important to note that mains earth isconnected directly only to the PSU PCBand then indirectly via a 1K resistor (R41)
Mark II Noise Reduction UnitCompander PCB Parts ListRESISTORS:- All 0.6W 1% Metal FilmRIR2R3,4,12,24,30,31R5,11,25,29R6,7,26R8,9,27,28R10,32R15,17,18,20,35,
37,38,40R16,19,36,39R21,51R22R23R50,52RV1,2
CAPACITORSC1,2C3,5,12,13,15,16,
18,21,23,24,52 1µF 50V MinelectC4,20 3n3F Polycarbonate
39k82k100k22klk47k5611
1k81M10k3k9510k62k22k Vert S -Min Preset
100nF Polycarbonate
1
1
6
43
4
2
84
2
1
1
2
2
2
11
2
C6,8,14,19,25,26 10/..if 40V MinelectC7,17 l)pF CeramicC9 13uF 16V TantalumCIO 130nF Mylar
621
1
(YY35Q)(WX44X)
(WW68Y)(WW21X)
(M39K) C50,51 2/22F 35V Tantalum 2 (WW62S)(M82K)
(M100K) SEMICONDUCTORS(M22K) IC1 LF 351 1 (WQ30H)
(M 1K) IC2 NE 570N 1 (QY1OL)(M47K) IC3 LF 353 1 (WQ31J)(M56R) IC5 3302 1 (QH48C)
D3,4,7,8 1N4148 4 (QL80B)(M1K8) D13,15 0.2in LED Green 2 (WL28F)(M1M) D14,16 0.2in LED Red 2 (WL27E)
(MIOK) D50 BZY88C3V9 1 (QH04E)(M3K9)
(M510K) MISCELLANEOUS(M62K) P.C.Board 1 (GA30H)
(WR72P) D.I.L. Socket 8 Pin 2 (BL17T)D.I.L. Socket 14 Pin 1 (BL18U)D.I.L. Socket 16 Pin 1 (BL19V)
(WW41U) Si Latchswitch 4 -pole 1 (FH68Y)Latchbutton Black 1 (BW13P)
(YY31J) SKT1-4 Threaded Phono Socket 4 (YWO6G)(WW25C) N.B. Two Compander P.C. Boards are required for the Stereo Unit.
10
Power Supply PCB Parts List Miscellaneous Parts ListRESISTORS:- All 0.6W 1% Metal Film S2 DPDT Toggle Sub -Min E 1 (FHO4E)R41,42 lk 2 (M1K) FS1 25CmA Fuse 20mm 1 (WRO1B)
CAPACITORSC27,28 2200/AF 25V Axial ElectrolyticC29,30 100nF Minidisc CeramicC31,32 10µF 40V Minelect
2
22
(FB90X)(YR75S)
(YY35Q)
T1Safaseholcier 20Transformer 15V/15VHook-up wireMains Cable Black 3AmpGrommet Strain Relief
1
1
3m2m1
(RX96E)(LY03D)(BLOOA)(XR01B)(LR48C)
SEMICONDUCTORSIC6 µA7815UC 1 (QL33L) OPTIONAL ITEMSIC7 I.LA7915UC 1 (QL36P) 3 Pin DIN Socket 1 (HH32K)D9-12 1N4002 4 (QL74R) 3 Pin DIN Plug (HH25C)D17 0.2in. LED Red 1 (WL27E) RVA,RVB 10k Hor S -Min Preset 2 (WR58N)
Case 1 (XG37S)MISCELLANEOUS Printed Front Plate 1 (FJ35Q)
P.C.Board 1 (GA3IJ) Stick -on -feat 1 pkt (FW38R)
A complete kit of parts (excluding optional items) for a stereo Mark II N/R Unit is available.Order As LK38R (Mark II N/R Unit)
to the OV line. This should prevent thebuild up of any hum loop when using thenoise reduction unit with earthed equip-ment. Power line buses are connectedfrom the PSU to each compander PCB.
The power supply and two compan-der PCB's can be mounted in the optionalcase using the bolts and spacers, to forma stereo noise reduction unit.The frontpanel can be drilled using the optional,self adhesive, face plate as a template. Iffour channel operation is required powersupply outputs are connected to a 3 pinDIN socket, a second N/R Unit consistingonly of two compander boards can thenbe connected using a 3 pin DIN plug.
Setting -up and UseThe unit requires very little setting -
up apart from adjustment of RV1 and RV2which are simply set to mid -travel, thusensuring a low distortion level of well
within 0.1% typical. Furthur adjustmentwith a distortion analyser will allowminimum levels to be reached (0.06%typical).
If the unit is being used with a mixerand a tape recorder with variable lineoutput, the mixer output is adjusted sothat the compressor OdBm LED's fire atpeak sound levels. The record level is setto match the optimum for the tape beingused. Playback levels are then adjustedso that the expander OdBm LED's fire atapproximately the same level as thecompressor OdBm LED's. When the noisereduction unit is used with an amplifier ortape recorder where the line outputlevels are not adjustable the OdBm LED'sshould fire at peak sound levels, provid-ing the equipment is to Hi-Fi specifica-tion. However, this level isn't critical sincethe level -adaptive response tine circuitstake care of possible mistracking, but it
does ensure really accurate decoding ofthe encoded signal.
In order to adjust output levels andavoid overloading the input to equipmentnot to Hi-Fi standards, it may benecessary to insert preset potentiometersof 10k (RVa & RVb) in the output of theexpander and compressor circuits (be-tween RIO & S lb and between R32 & S Id)
A couple of points to note: the unitwill not reduce the noise present in anoisy signal applied to the compressorinput (this is territory best served bydynamic noise limiters), and any differ-ence in the signal between compressoroutput and expander input introduced bythe recording process will be exagger-ated by expansion, including such hor-rors as common -or -garden dropouts.Therefore to get the best out of the unitscrupulous attention should be paid toalignment and cleaning of tape heads!
8 CHANNEL FLUID DETECTOR Continued from page 5.
inserted in the positions for channels oneand two, as a warning condition is nowrequired when the water content isgetting low. If you are using the idea in anestate car or any other car with a rearwindow washing facility then use twoprobes; channels 1-4 for one bottle andchannels 5-8 for the other, and this timeinsert one red LED in channel one andthe other in channel five. The strip ofVeroboard was found to be ideal in thisapplication.
Case two was for an installationwhich had a number of large tanks.These gradually drained over a period oftime but when they got down to apredetermined level they were to berefilled by opening an electronicallycontrolled valve. Here two channels wereused for each tank, one channel openingthe valve when fluid dropped below theminimum level, and the other closing thevalve when the tank was full. Four tankswere able to be controlled by the oneboard.
Case three was for use in anurseryman's greenhouses. The growerin question used mist spayers in his
houses which gave the plants a goodspraying whenever the water hadevaporated from the surface of theprobes, which were placed at regularintervals between the plants. The mistwas turned off again when enough waterhad fallen to bridge the gap on the probeand therefore detect the presence ofwater again. As he grew a large numberof different plants at different tempara-
tures and humidity, he was able to useeach channel separately to giveindividual monitoring and control for allthe environments he required.
There are obviously a great manymore ways in which this circuit could beused, and these suggestions are onlyhere to demonstrate the wide range ofuses in which this project may be put towork.Continued on page 16. 11
r11CAtIZ 811ZbyDave Goodman
* Twin VU Meters* Battery Operation* Master Volume Control
IntroductionThe Mapmix is a versatile six input
mixer, in the stereo mode it has threeinputs connected to the left channel andthree inputs connected to the right chan-nel. For mono use all six inputs areconnected to both output jacks via themode select switch. Both left and rightchannels have separate post -mix send/receive facilities, for connecting externaleffects units. Tonal balance can be mod-ified with Bass and Treble controls. Thetwin VU meter gives an indication of finaloutput levels although it is unaffected bythe master volume control, which is con-nected to the output. All input and outputconnections are made with standard 1/4inch mono jack sockets, while send andreceive connections utilise 1/4 inch stereojack sockets. The unit is powered by a 9Vbattery - so current consumption hasbeen kept at a very low level, to prolongbattery life. However, external DC powersupplies can be connected using the2.1mm power socket.
* Switched Mono/Stereo Modes* Bass and Treble Equalisation* Six Microphone or Instrument Inputs
Circuit DescriptionLow power IC's are used to keep
power requirements to a minimum -approximately 1.75mA quiescent currentat 9V. ICla and b are configured asvirtual earth mixers with inverted outputsand can be referred to as 'adders'. Forthe left channel, input signals are appliedto IC1b via SKI to 3 with signal attenua-tion, or volume control, being performedby RV1 to 3. Resistors RI to 3 and R7 havethe same value, a signal applied to SK 1only, will appear at C7 +V in invertedform, but at the same amplitude as theinput. Thus the mixer exhibits a unity gaincharacteristic under this condition. Ifsignals are now applied to all threeinputs, the total current flowing in R7 willbe equal to the sum of the input current sand the output voltage at C7 will be equalto the sum of the input voltages. Forinstance, a 100mV signal applied to allthree inputs, with RV1 to 3 set to max-imum, will produce the sum product of300mV at C7; the signals being effectively
'added' together.Blocking capacitors Cl to 3 isolate
IC1b from possible DC level changespresent at the input jacks; the inputimpedance of each channel is set by thevolume control resistance at 100k ohms.R11 carries the mixer output to the sendterminal, this being the 'tip' connection ofa stereo jack plug. Without a plug in-serted into SK4, the switched connectionsdirect the signal path to SI and IC2a,another inter -stage unity gain mixer,which re -inverts the input signal andprovides a low impedance drive to thetone control stages which follow. SI isshown operated, which is the monomode, thus IC2b receives the same inputsignal as IC2a.
When an external device is insertedinto SK4, both 'tip' and 'ring' are discon-nected by the internal switching andreceive inputs are connected to IC2a vialevel preset RV 10. Effects units such asecho, phase, reverb or perhaps anothermixer can be inserted here and mixed
12
SK
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Rear view of meter
Join -V pinstogether
Figure 3 Meter Wiring
into the rest of the system. IC la and IC2bfunction in exactly the same way aspreviously described for the left channel.Switching SI out of circuit establishes thestereo mode, where left and right chan-nels become independent of each other.
IC3c and d form the active section ofthe treble control RV8, which is a dualpotentiometer. Both channels, althoughelectrically independent, can be set forflat response by keeping the wiperscentral. A boost of up to 10dB at 10kHzcan be applied by turning the wiper ofRV8 clockwise and a cut of 10dB byturning anti -clockwise. Similarly IC3band IC3a form the active section of basscontrol RV7, another dual potentiometer.This control gives up to 10dB boost or cutat 40Hz when rotated clockwise or anti-clockwise respectively. Separate activefilters are used to keep interaction to aminimum, for improved performance andto lessen the effects of distortion whichcan be noticeable in multi -feedback typesystems. R37 (38) is located in the outputstage to prevent IC3b from drawingexcessive supply current if the outputconnecting cable is shorted out, whilstRV9 (master volume control) is set atmaximum. This raises the output impe-dance slightly, to about lk ohm, or 10kohm at low output volume settings - thisis. however, adequate for most audioamplifier input stages.
Emitter follower TR1 (TR2) chargescapacitor C27 (C28) to produce a meanDC average from the outgoing AC signal.this capacitor also dampens the meter
2 1mmPower Skt
Break switchdisconnectsbattery whenplug inserted
Insulator
-VCentre +V Sleeve
Figure 4 Power Plug & Socket
response otherwise it would be un-readable, with the needle bouncingaround on its mountings! Switched poten-tial dividing stages have not been in-corporated with the meter, so low levelinput signals applied to the mixer will notbe registered on the scale. Zero on thescale corresponds to approximately 0dB(+ or- 1d3) or 775mV at IkEz applied toone input, maximum volume, mono modeand tone controls set flat. The maximumscale reading corresponds to an outputlevel of 1.6V RMS (4V peak to peak),which is some 2dB down on the absolutesignal handling capability of the mixer.Because the input levels can be con-tinuously variable it is possible that sig-nals of a few millivolts to a few dozenvolts can be connected to the system. Themaximum signal that any one channel canhandle is 500mV - with the volume set tomaximum and sufficient margin allowedfor bass and treble boost. Of coursehigher input signal levels simply requirethe volume control to be turned down.
Prototype Specifications:Power Requirement: 1.75mA with 9V battery
(eg PP3). Or fully regul-ated external PSUmax 13.5V DC.
Frequenzy Response:2SHz 30kHz 1dBBass Control:Treble Control:Meter Response:LHORHC Tracking:Signal TD Noise:Distortion:Input Impedance:Output Impedance:
if 10dB at 40Hz10dB at 10kHz
50Hz ± 1dB± 1dBBetter than 65d13<0.05% at 1kHz flat100k ohm each channellk ohm at max. setting
2 1mmPlug
+13 5V maxDC Supply
Body
PCB AssemblyRefer to the parts list for component
values and Figure 2 (legend overlay) fordesignations. Begin construction by in-serting each of the 25 links, using 24SWGB T.C. Next insert the 40 resistors intotheir respective positions, the PCB holespacing is set at 13mm and each resistorlead must be bent to fit and then pushedfirmly onto the board. Mount both 47kpresets, RV 10 & RV11, and diode DImaking sure of correct polarisation. Nowfi: IC's 1 to 3, these must be fittedcorrectly - pin 1 is usually marked with asmall hole or indentation, occasionally a'D' shaped concave slot is cut into oneend of the body; if the IC is held with thisslot facing to the left, with the pins facingdown, then pin 1 is the first on the bottomrow.
Solder the part assembled board atthis stage and remove all excess wireends. Next the capacitors can be fitted.Cl to C6 are polyester types and mountir.-line across the centre of the board. C7to 10 and C13,14,27,28 are polarised typeswith long + V leads and short -V leads,ensure they are mounted correctly to thePCB legend. C23 to 26 are PCB mountinge.ectrolytics, which are of course pola-rised, only the -V lead is identified socare must be taken to fit these compo-nents correctly. Now fit TR1 & TR2 andpower socket (SK11) along with the fiveVero pins which are inserted from thetrack side of the PCB, finally solder thesecomponents. The ten PC mounting jack
15
mtganwre
sockets can now be inserted and sol-dered in position, noting that SK4 and SK8are stereo jacks with six terminals. Fitboth latch switches with the sprung endsprotruding over the PCB edge and solderin place. Now fit the single potentio-meters RV1 to RV6 with the spindlesprotruding over the same edge of thePCB and solder in place; finally dolikewise with the three dual potentio-meters RV7 to RV9.
Closely inspect all solder joints, forexceJs solder, shorts, dry joints etc, andclean the PCB track with a suitablesolvent. Re -check all components, valuesetc and when satisified connect the PP3type battery clip with the red (+V) to pin1 and the black (-V) to pin 2.
If the dual VU meter is being usedthen it must be wired to the PCB pins 3,4& 5 (see Figure 3) using hook-up wire.Both -V terminals on the meter move -
TipRing
Insulators Ring
TipSleeve
4,11 dlLr111011
Sleeve
Left & Right ChannelsSendlO/PI :Tip [Effects IIPI
ReceivelljPl :RinglEffects OJPICommon: Sleeve I -V Gnd
Body
Figure 5 Stereo Jack Connections
ments should be joined together andconnected to -V supply pin 5, as shown.
Using the MixerDetails of connection to an exter-
nal power supply are shown in Figure 4.
The inner terminal of SK 11 is connectedto -V and the outer spring contact to +V.Do not exceed 13.5V as some componentworking voltages may be exceeded, notethat when a plug is inserted into SKI1 thebattery is disconnected.
The connections to the send andreceive jacks are shown in Figure 5, sendoutputs (tip) carry the signal from themixer to external equipment and receiveinputs (ring) carry processed signalsfrom the equipment to the mixer. Thesleeve terminal is for screen connectionor earth return.
It should be borne in mind whenusing the mixer that amplification is low,the unit does not act as a pre -amplifier.Thus when mixing microphone, musicalinstrument or line output levels, as re-commended, an amplifier with integralpre -amp or a power amp and suitablepre -amp should be used.
MAPMIX PARTS LISTRESISTORS:- All 0.6W 1% Metal Film
SEMICONDUCTORSD I IN4002 I (QL74R)
R1-8,22,23,27,28, IC1,2 LF442 2 (QY30H)
30,31,35,36 100k 16 (M100K) IC3 1 (QY31J)R9,19 68k 2 (M68K) TR1,2 BC547 2 (QQ 14Q)
R10,20,39,40 10k 4 (M1OK)R11,12 lk 2 (M1K) MISCELLANEOUS
R13-18 47k 6 (M47K) SK1-3,5-7,9,10 PCB Jack Skt Mono 8 (FJO0A)
R21,24,25,26 18k 4 (M18K) SK4,8 PCB Jack Skt Stereo 2 (905F)R29,32,33,34 15k 4 (M15K) SK 1 1 PC Mtg Power Skt 1 (RK37S)
R37,38 27011 2 (M270R) S1,2 Soft Latchswitch 2 -Pole 2 (BW 11M)
RV1-6 47k Log Pot 6 (FW24B) MI Dual VU Meter 1 (Y04713)RV7,8 100k Lin Pot Dual 2 (FW88V) Small Latchbutton Black 2 (BW 13P)
RV9 10k Log Pot Dual 1 (FX09K) Knob K7A 9 (YX01B)
RV10,11 47k Hor Sub -Mm Preset 2 (WR6OQ) Veropin 2141 1 pkt (FL21X)Battery Clip 1 (HF28F)
CAPACITORS Mapmix PCB 1 (GB60Q)
C1-6 22011F Polyester 6 (BX78K)C7 -10,I3,14,27,28 4µ.7F Tantalum 8 (WW64U) OPTIONAL
C11,12 22pF Ceramic 2 (WX48C) Mapmix Case 1 (XG38R)C15-18 4n7F Polycarbonate 4 (WW26D) Pritted Front Panel 1 (1736P)
C19-22 47nF Mylar 4 (WW2OW)C23,24 10/./F 16V Minielect 2 (YY34M) A kit of parts (excluding optional items) is available.C25,26 100µF 10V PC Electrolytic 2 (FF I OL) Order As LK49D (Mapmix Kit)
8 CHANNEL FLUID DETECTOR Continued from page 11.
8 CHANNEL FLUID DETECTOR PARTS LISTRESISTORS: All 0.6W 1% Metal FilmRI IM 1 (M1M) IC1 4051BE (QW34M)R2,3,5,7,9, I1, IC2 LM11130 (YY99H)
13,15,17 lkO 9 (M1K) IC3 4099BE (QW571VI)
R4,6,8,I0,12, IC4 4011BE (QX05F)14,16,18 10k 8 (M1OK) IC5 4518BE (QX32K)
CAPACITORS MISCELLANEOUS
C1,2 47nF Minidisc 2 (YR74R) PL1 RA Minicon Latch Plug 10 -Way 1 (RK68Y)
C3 la' Ceramic 1 (WX68Y) DIL Socket 14 -pin 2 (BL18U)
C4 22µF 16V PC Electrolytic 1 (FF06G) DIL Socket I6 -pin 3 (BL19V)PC Board 1 (GB66W)
SEMICONDUCTORS Strapping Wire 1 roll (BL 13P)
D1-6 Shape LED RI Green 6 (YY46A)D7,8 Shape LED RI Red 2 (YY45Y) A complete kit of parts is available for this project.TRI-8 BC547 8 (00140) Order As LK48C (8 -Channel Fluid Det Kit)
16
- CAUTIOUS -NI -CAD CHARGER
IntroductionThe accepted life for most Ni-Cad
cells is five hundred charge/dischargecycles, however this sort of life can onlybe achieved if some care is taken overthe treatment of the cells. A number ofchargers are available commercially andmany High Street shops are now sellingboth batteries and chargers. Thesechargers are only able to charge a limitednumber of cells at one time (usually 4),have a fixed charge time of about 15hours and no provision for high speedcharging of scintered cells.
The overcharging of cells is de-trimental to their useful life, most manu-facturers state their cells must not becharged for more than 14 to 16 hours atthe recommended charge current.However, no commercial chargerappears to offer an automatic timing
by B. Puttock & D.J. Silvester
1.2V-1V -
Voltageacrosscell
Single cell
I FullyI charged
I Time..Charge discharge atconstant current.
Figure 1 Single Cell Discharge
system. Partially used cells are normallytreated as though they were completelydischarged and will consequently besubstantially overcharged by a commer-cial unit.
The discharge of a single cell at aconstant current is shown in Figure 1. Thevoltage across the cell remains constantat about 1.2V until the remaining charge
* Battery Polarity Sensor* Constant Current Charging* Fast Charge for Scintered Cells* Electronic Timing of Charge Cycle* Will Accept up to 6 Cells* Will Accept 6 AA (Typ. 5OmA),
is below 10% of the full charge - then thevoltage drops rapidly. Recharging theceL raises the voltage across the cell tobetween 1.3 and 1.4 volts very rapidly,where it remains until the cell becomesovercharged, the voltage then risesslightly before becoming constant again.
In order to charge a variety ofbatteries this design uses the dischargevoltage of 1V per cell to initiate thecharge cycle, which is then carried out atconziant current for either 23/4 or 15 hoursdepending on cell type. To satisfy anumber of interests, including photogra-phy and amateur radio, the unit is able tocharge a range of batteries, from a singleAA cell to a bank of up to 6 cells -which will provide a 12V supply. Conse-quently switched reference voltages areused to detect the end of the dischargecycle and various charge rates are
6 AM (Typ.19OrnA), 6 C (Typ. 180mA),3 D (Typ. 38OmA), 6 SC (Typ. 130mA)
* Trickle Charge to Maintain Cells inFully Charged Condition
* Discharge Facility for Part ChargedCells to Prevent Overcharging
917
QE
T1240VAC
0 -Y-6110Y-Y1
R110k
0
C210uF
C11000uF
S1Start
TR7BC 177
R3210k
R313906
IC
T
dis-charge
R3010k
-77
Yel D4
R29390R
yRed TR4D5 80140TR5BCI77
TR6BC177
IC6741
6
10RR28
TR3 TR2BC107 BD131
Charge
6
2
7
I 1
R14 R13 R12 R11 RIO R9 R82k0 1k0 1k0 2k0 2k0 IkO 1k0
17 I- JE- JE- IL
R27
I -117Z7 6 5 4 3 2
2L4
R26 R1510k 1k8
R16 4k7
R18 R20 R22 R2424R 24R 6R8 5R6
TIT TTITTR17 R19 R21 R23 R25
120R 13R 5R6 24R 3R9
TR1BC107
R739OR
D3Grn
Di C02 +VE
D1
Batterypola ity
IC4 b
2 6 IC3
0)SI 1
1 4 10 2
334
S2Time 15select
3
IC2
IC 1 4047 14 7,8,912.IC 2 4020 16 8IC3 4013 14 3,5,7.IC 4 4023 11.12,13,14. 7
IC 5 4081 14 1,2,5,6,7.
Figure 2 Circuit Diagram
offered. A discharge only facility is alsooffered for cases where a new batterypack is to be made up from old cells invarious charge conditions. After dis-charging each of the cells individually,they can then be recharged as a batterypack so that each of the cells is equallycharged.
To accommodate the charging ofvarious types of batteries the output isconnected to a PP3 type clip, which canthen be connected to the requiredbattery pack (although for certain singlecells the connection to the battery holderwill need to be soldered). For full detailsof battery holders see page 42 of the 1986Maplin Catalogue.
Circuit DescriptionT1, BR1 and Cl are used to convert
AC mains input to 16.5V DC, the voltageused to drive all of the logic circuitry andto charge the batteries (see circuitdiagram Figure 2). The circuit consistingof DI, R3 to R6 and IC7 is used to detectthat the battery to be charged isconnected correctly. Normally when abattery is regarded as being dischargedit in fact still produces a small potentialdifference across its terminals. R3 to R6form a potential divider so that if the twobattery contacts are shorted together theinputs to the voltage comparator IC7 willbe the same, ignoring resistance toler-ances. To ensure that in this condition 1C7produces an error signal, i.e. IC7 pin 6 islow voltage, DI is introduced to unba-lance the divider chain. The off -set isextremely low and will be overcomewhen a battery is connected to thecharging terminals in the correct manner.18
8IC5a
11
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6
R5 10k
'4**** ---"4R3 10kIC7
741
R6 R410k 10k
Battery
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Plug intoBattery skts- +
PP3 clip
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106
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0 0 . 0:) 0 0 CAUTIOUS Ni CADCHARGE GB65V
MAPLIN
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Single celldischarge only
IDI
S4
Figure 3 PCB Track Legend & Wiring Diagram
IC7 pin 6 will then become high. Anincorrectly inserted battery will reinforcethe off -set introduced by DI.
TR1, R7 and green LED D3 are usedto indicate the voltage level, i.e. logicstate, of the output of IC7 - when D3 isilluminated the battery has been insertedcorrectly. The output from IC7 is alsoused to disable the charge/dischargelogic (IC4a and IC4b) thus preventing
S1
D5IRI D4IYI
damage to an incorrectly inserted bat-tery.
IC6 with its associated resistors R8,R15, R16, R26, R27 and the resistor chainR9 to R14 which is connected onto theswitch S3 (see S3 diagram in Figure 2)form a second voltage comparator sys-tem. This provides the voltage standardagainst which the voltage of the battery ischecked. The total resistance across the
Red
Black
S2
k eD3IGI
supply is the sum of R8 to R16 whichequals 16.5k ohms. The current throughthe chain is 1mA and therefore a lkresiscor will produce a 1 volt potentialdifference. Hence position 1 of S3 gives a1 volt input to IC6 pin 2, against which thebattery voltage is compared. Similarlyswitch positions 2 to 7 give 2,4,6,7,8 and10 volts respectively, for checking largerbattery packs. If the battery voltage is
19
MAPLINCAUTIOUS Ni CADCHARGERGB65VISS 2
L0
"t:,&
as t.>kt
0,c.
greater than 1 volt per cell the output ofIC6 pin 6 will be low and if less than 1 voltper cell it will be high.
The timing and control logic receivethe outputs of IC's 6 and 7 which are usedto initiate the charge/discharge opera-tions.
IC1 is a free running multivibratorthe output cycle time of which iscontrolled by the values of R2, RV1, C3and C4; it requires low leakage non -polarised capacitors across pins 1 and 3,as a high value of capacitance is requiredin this case, back to back tantalum beadtypes are used. The resistance neededacross IC1 pins 2 and 3 is provided by R2and RV1, the variable component is usedso that the output square -wave from pin10 can be made as close as possible to6.59 seconds per cycle - to allowreasonably accurate charge times.
IC2 is a 14 stage binary counterwhich is used to divide down the 0.152cycles per second from IC1; the outputvoltage at pin 1 becomes high after 211input pulses, i.e. 33/4 hours and that at pin3 high after 213 input pulses, i.e. 15 hours.IC5b and the associated logic loop of IC2and IC3'prevent the timer restarting after212 and 214- counts respectively, thuspreventing a second charge cycle.
Consider the situation where apartially charged battery is connected tothe charger. S3 must be switched to thecorrect number of cells. S4 must be20
switched to the charging current req-uired and S2 to the time needed. fithe
-Polarity is correct the output -of rcs willbe low and as there is more than 1 volt
per cellOhe output of IC7 will be high,:which will illuminate D3, The output ofIC7 is also connecledTo-IC4a and IC4b,when this output is low the outputs of bothIC4a and IC4b are forced high, disablingthe charge/discharge circuitry. With acorrectly inserted battery IC4a and IC4bare enabled.
Pressing and releasing S 1 or switch-ing on the mains supply forces the resetinputs of IC2 and IC3 high and then lowagain, in a time period controlled by thevalues of RI and C2. The outputs Q of thedual flip-flop, IC3, become high whilst Qof flip-flop (F/F$) becomes low. This lowoutput passes ivia IC5a and IC5b todisable the counting of IC2 during thedischarge _cycle. The Q signal of F/F2'ipasses to IC4b, the output of which forcesthe output of IC4a high. This then turns offthe charge circuit. The low output of :C4benables the discharge circuit consistingof R28,R29,R30,TR4,TR5 and TR6 thusilluminating the red LED D5.
The discharge circuit remains onuntil the voltage across the battery dropsbelow 1 volt per cell. This voltage dropcauses the output of IC6 to go high thuscausing the outputs of F/F2' in IC3 tochange.
When Q of F/F% becomes low, the
output of IC4b becomes high, disablingthe discharge sequence and enabling thecharge circuit via IC4a. Since Q of F/F2becomes high and Q of F/F1 is still high,the counting of IC2 is enabled by the highinput to IC5b derived from IC5a. IC2 nowbegins to count the pulses from ICI, withS2 selecting whether the high signal ispassed on to IC3 after 211 or 213 pulses.During this period all inputs to IC4a arehigh and its output is low, thus turning onthe charge circuit consisting of R17 toR25, R31,R32,TR2,TR3,TR7 and illuminat-ing the yellow LED D4. The voltage dropacross the illuminated D4 is about 2.4volts and as each of the base -emitterjunctions of TR2 and TR3 produce avoltage drop of 0.7 volts, 1 volt is appliedacross the switch (S4) selectable resis-tors, R17 to R25. The range of currentspassing through TR2 is quite large andtherefore a variable voltage drop actuallyoccurs across the base -emitter junction ofthis transistor. The values of the resistorsare chosen so that a constant currentsuitable for charging the cell selected,will be passed through themselves, TR2and the battery.
At the end of the charge timeselected by S2 the input of F/F1 becomeshigh. This causes Q to become low andvia IC4a turn off the charge system. Inaddition, via IC5a and IC5b, furthur timingis prevented. The unit now remains in thisstate and the battery receives a verysmall trickle charge via R3 and R4. D2 isincluded so that when the charger isdisconnected from the mains the batterycannot discharge through the rest of thecircuitry.
ConstructionBefore starting the PCB construction
the bottom of the box should be markedwith the positions of the transformer, thePCB mounting screws, the mains inputcable and the charger power outputsockets. All these holes should be drilledand the components (excluding the PCB)mounted into position, not forgetting tolocate the mains cable through thegrommet. Next mark the positions of the 3switches and the LED's on the lid of thebox, drill the holes and mount thesecomponents.
Assemble the PC board in thefollowing order: first locate the positionsof the IC sockets and carefully solderthem onto the PCB. Do not insert the IC'sat this stage! Next mount and solder inturn, the resistors and capacitors, thebridge rectifier and the transistors exceptTR2. Once these components are fittedTR2 can be attached to the heat -sink andthe two items screwed to the PCB - afterbending the transistor leads to passthrough the holes in the board. Finallysolder TR2 and fit the wires connectingthe PCB to the sockets and transformer,and also the switches and LED's in the lid(see Figure 3, PCB track legend andwiring diagram), then fix the PCB into thebox.
Now wire the switch S3 with theresistor chain as shown in Figure 3, andensure that 4 switch positions only are
available. If not there is a small movablestop under the mounting nut of the switchwhich should be placed in the positionmarked 4. Incidentally S4 should rotatethrough 6 positions and should beadjusted if required. Before testing, allwiring should be checked and the wiresbetween the PCB and other componentssecured with cable ties.
TestingHaving completed construction the
supply voltage to each of the IC's shouldbe checked - there should be a readingof 16 volts between pin 14 (positive) andpin 7 of IC's 1,3,4 & 5, and between pin 16(positive) and pin 8 of IC2. For IC6 and 7the 16 volt supply should be across pins 4and 7 (positive). If these readings aresatisfactory plug in all the IC's.
With the output sockets discon-nected, the green and possibly the yellowLED's should light. Shorting the socketsshould turn off all the LED's - proving thatthe polarity checker is working.
Connect a part charged cell to thecharger and check that the unit transfersfrom discharge to charge when thevoltage drops to 1 volt and that thecharge current is correct for the cellbeing used. Each time a new size of cell ischarged the charge current should bechecked.
Finally check that the time for 10cycles from the output ci IC1 (pin 10) is asclose to 65.9 seconds as possible. RV1can be adjusted to alter the cycle time theaccuracy of which wil: obviously affectthe charge time.
OperationSet S4 to the correct battery type to
be charged. Set S3 to the correct numberof cells. Set S2, for charge time, to 15hours - or 33/4 hours fcr scintered cells.
NI -CAD CHARGER PARTS LIST
Fit the cells to be charged in the correcttype of battery holder and connect to thecharger via the PP3 clip. Pressing thestart button or switching on the mainssupply will initiate the discharge/chargecycle.
When a battery pack is to be madet:p of cells in varying states of discharge,the cells must first be discharged singlyby setting S4 to the discharge only posit-ion and S3 to 1 cell. Once the cells are alldischarged they can be fitted into therequired battery holder for recharging.
RESISTORS: All 0.6W 1% Metal Film unless otherwise stated. MISCELLANEOUSR1,3,4,5,6,26,27,
30,32 10k 9 (M1OK)S1 Push Switch 1 (FH59P)S2 Sub -Min Toggle Switch 'A' (FHOOA)
R2 220k 1 (M220K) 53,4 13-1-6az..)--"Rotary Switch 12B 2 (FF730)R7,29,31 39011 3 (M390R) 04.-31.1 '4, Min Transformer 6V (WBO6G)R8,9,12,13 lk0 4 (M 1K) He atsink 'Janed (FL59P)R10,11,14 2k0 3 (M2K) LED Clips 3 (YY40T)R15 1k8 1 (M1K8) DIL Socket 8 -pin 2 (BL 17T)R16 4k7 1 (M4K7) DIL Socket 14 -pin 4 (BL 18U)R17 12011 1 (M120R) DIL Socket 16 -pin 1 (BL 19V)R18,20,23 2412 3 (M24R) Knob K7C 2 (YX03D)R19 1311 1 (M13R) Feet Stick -on 1 pkt (FW38R)R21,24 5126 2 (M5R6) Grommet Small 1 (FW59P)R22 6118 1 (M6R8) Tie Wrap 92 4 (BF91Y)R25 3119 1 (M3R9) Cable C6A Mains White 2m (XR04E)R28 1011 7W 5% Wirewound 1 (L1OR) We I pkt (BLOOA)RV1 100k Hor S -Mitt Preset 1 (WR61R) Ve:opin 2145 1 plct (FL24B)
Socket Black 2mm 1 (HF44X)CAPACITORS Socket Red 2mm 1 (HF47B)Cl 1000µF 25V PC Electrolytic 1 (FF I8U) Plug Black 2mm 1 (HF38R)C2,3,4 10µF 25V Tantalum 3 (WW69A) Plug Red 3mm 1 (HF41U)
SEMICONDUCTORS Printed Circuit Board 1 (GB65V)
D1,2 1N4001 2 (QL73Q) PP3 Battery Clip 1 (HF28F)
D3 LED Green 1 (WL28F) Bolt 6BA 1 pkt (BFO6G)
D4 LED Yellow 1 (WL3OH) Nut 6BA 1 pkt (BF18U)
D5 LED Red 1 :WL2'7E) Spacer 6BA 1/4in. 1 pkt (FW34M)
TR1,3 BC107B 2 (QB31J) Tag 6BA 1 pkt (BF29G)
TR2 BD131 1 (QFO3D)TR4 BD140 1 (QF08J) OPTIONAL
TR5,6,7 BC177 3 (QB52G) sos -sqs" e--71 Case Verobox 305 1 (LH51F)
IC1 4047BE - 1 (QX2OW) Battery Hclders - see Maplin CatalogueIC2 4020BE - 1 (QX11M)IC3 4013BE 1 (QX07H)
A ccmplete kit of parts (excluding the case and battery holders)IC4 4023BE - 1 (QX12N)IC5 4081BE 1 (QW48C) is available.IC6,7 i.LA741C 8 -pin DIL 2 (QL22Y) Order As LK5OE (Cautious Ni-Cad Charger Kit)BR1 W005 1 (QL37S)
21
* Gives Easy Access to User -port, 1MHz Bus andAnalogue Input.* Provides Standard Edge Connectors forDevelopment Purposes.* +5V and +12V Switching and Indication.* Fused External 12V Input.
by Robert KirschThe Acorn BBC computer is one of
the most popular and versatile of the vastrange of microcomputers at presentavailable and is ideally suited fordevelopment work. The one drawback isthe location of the 1MHz Bus and UserPort underneath the computer. Thisproject describes a Motherboard thatbrings both these ports as well as theAnalogue Input out to 4 parallel doublesided edge connectors on a board thatcan be located either in front of or behindthe computer when in its workingposition. Power switching and protectionare also provided. Figure 1 shows thecircuit diagram and pin functions of themotherboard (only one of the 4 identical-ly connected edge connectors is shown).Note that the pins are configured to keepthe individual ports from the computergrouped together with power supplies ateither end.
ConstructionThe construction of this project is
fairly straightforward although careshould be taken to ensure the correctpolarity of the capacitor Cl and LED1 andLED2. Note also that the location guide onthe edge connectors is towards the sideof the PCB away from the external powerinput socket. The headers of the ribboncables should be carefully insertedthrough the PCB to prevent bendingunder any of the pins during insertion.There are three wire links to be providedon the board and these can be made ofany odd lengths of tinned copper wireabout 24swg. Careful inspection of thecompleted project is recommended toensure that there are no short circuits orunsoldered pins before connection ismade to the computer.
NOTE: Always turn off the powerbefore any connections are made to thecomputer ports or cards inserted into themotherboard. With the motherboardconnected and no cards inserted thecomputer should function in the normalmanner. This project is the first of severalwe hope to include for the BBC computerand we would be interested to hear fromanyone having projects for the BBCparticularly if they could be adapted touse the motherboard system shown here.22
1
+5V
+5V
SKS 4-7S1
+5V In from:1
183
+5VSK 1 pinSK 2 pins+5V
+12V FS 1JK 1+12V
+12V +12VD -
nc
v.In+12Vnc - -
R W1MHz
IRO R2LED 2
##
C1100uFNMI
1MHzSK 3
FD FC
RST1k0A in 1#
BD1 BOOR1
270RLED1CBI BD 2
110V DO CB2
PORT VD2 VD1
VD 3SK 2
V D4
VD6 VD 5
AiGND VD 7
Analogue CH 1 CHO
PB 0SK 1CH2
VREF CH 3
LPSTB
nc
PB 1
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Figure 1. Circuit diagram
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MARLIN GB39N ISS.3BBC MOTHERBOARD 0 o 0 PL
Figure 2. Track and overlay Continued on page 2723
Atwater FIVE
808s WORTHFrom Robert Penfold
This circuit is a slightly moresophisticated alternative to the populardoor -chain alarms. Units of this type havea microswitch that is operated when anytension is placed on the door -chain, andthe microswitch in turn operates an audioalarm generator. With this circuit a reedswitch or microswitch is directly operat-ed when the door is opened so that thealarm is activated, even if the door isopened by just a few millimetres.Furthermore, once activated, the alarmlatches in the ON state so that closing thedoor again will not silence it.
IC1 is a CMOS 4011BE quad 2 inputNAND gate, but in this circuit two of thegates are used as simple inverters andthe other two are unused. The twoinverters are connected to act as abistable circuit, and C2 ensures that theoutput of the bistable always goes low atswitch -on. If SI should close, evenmomentarily, the output of the bistablewill trigger to the high state, and latch inthat state. TR1 is then biased hard intoconduction, and it provides the alarmgenerator with virtually the full supplyvoltage.
The alarm generator is based on IC2and IC3. The latter is a 555 astable circuitoperating at a fairly high audio frequencyand having its output coupled to aloudspeaker by C6. The 555 can providea strong output current into a lowimpedance load such as LSI, and this
Equipment for the measurement oftotal harmonic distortion (THD) tends tobe quite complex and expensive. How-ever, anyone who has a high quality(sinewave) audio signal generator and anAC millivoltmeter has the basis of a THDmeasuring set-up. The only other majoritem of equipment required is a highquality notch filter, such as the one shownin the accompanying circuit. To measurpTHD the signal usea tosupply a sinewave signal to the amplifierunder test, and the output of the amplifieris fed to the millivoltmeter via the notchfilter. Initially the filter is bypassed andthe millivoltmeter is used to measure theoutput signal level. Then the filter is usedto notch out the sinewave signal, leaving24
DOOR ALARM
C l10uF
S1Reed ormicroswitch
RI4k7
C2100nF
R210k
IC1pins8.9,12,13.14
2 6ICU ICIb
ICI 40118E
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R610k
3
2
4
IC2
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BC 337
gives quite a loud alarm signal.The alarm is made even more
effective by frequency modulating thetone generator. IC2 is a 741C operationalamplifier used in a standard relaxationoscillator circuit, and the low frequencysquare -wave output from pin 6 is looselycoupled to pin 5 of IC3 by R10. This givesan increase in the output frequency of thetone generator when the output of IC2 islow, and a reduction in frequency when itis high, giving a two-tone output signal.
The unit is reset by switching off andthen switching on again. On/off switch S2should ideally be a key -operated switch,and the unit should be housed in a fairlytough case such as a diecast aluminiumtype, so that there is no quick and easyway for an intruder to silence the unit.
The most convenient type of switch
THD FILTER
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to use for SI is a reed switch havingchangeover contacts. The unit can thenbe arranged so that the activating magnetis next to the switch when the door isclosed, but the pair of contacts thatprovide a normally closed action areused, so that the alarm is not activated.When the door is opened the magnet andreed switch become separated, thecontacts close, and the alarm is activated.
As the unit will be left switched onfor long periods of time it is obviouslyessential for it to have a low stand-bycurrent consumption. Under quiescentconditions the only supply current thatflows is the leakage currents through Cland TR1, plus the supply current of IC1.This is not likely to total more than a fewmicroamps, and in practice each set ofbatteries will give months of use.
Re363
IC2a
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7
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out
only the noise and distortion, which ismeasured using the millivoltmeter. Theratio of the output signal to the noise anddistortion level gives the distortion factor(which is normally expressed as apercentage). The millivoltmeter can thenbe used to measure the output noise levelof the amplifier. Deducting this from thenoise and distortion figure gives the THDlevel, and again this is normally compar-ed with the output signal level andspecified as a percentage.
The filter is based around two phaseshifters (IC lb and IC2a). At a certainfrequency these provide a 180 degreephase shift, and mixing the phase shiftedand unshifted signals at IC2b thereforeproduces a cancelling effect and a notchin the response of the circuit at thisfrequency. RV1 and RV4 are adjusted toprovide precise cancelling so that a highdegree of attenuation is provided. Withcareful adjustment more than 80dB ofattenuation can readily be achieved. IC3is an output buffer stage.
A problem with this basic filter is thatit provides significant attenuation atdouble the notch frequency, and there-fore tends to reduce any second har-monic content that is generated in theamplifier, and consequently a slightly lowTHD reading is produced. This isovercome by using a small amount ofoverall negative feedback. This tends toflatten the frequency response of thecircuit, but near the centre of the notchthe degree of attenuation is far too highfor the negative feedback to have anysignificant effect. RI and R2 provide theoverall negative feedback and give thecircuit a nominal voltage gain of unity.The reduction in gain at twice the notchfrequency is less than 1dB.
RV3 enables the notch frequency tobe varied from about 100Hz to approx-imately 1kHz, which is the frequencyband that is likely to be of prime interest,but the values of C2 and C3 could bechanged to provide operation at otherfrequencies. Changes in the values of
these components have an inverselyproportional effect on the band offrequencies covered (e.g. a value of 5nFgives coverage from about 200Hz to2icHz).
RV2 is the fine tuning control, andtogether with the fine balance control(RV1), need to be adjusted very carefullyin order to accurately notch out thefundamental signal. SI is the bypassswitch, and this renders the first phaseshifter inoperative so that the notch iseliminated and the fundamental signalcan pass unhindered to the output of the
A supply voltage of between 9 and33 volts is needed, and in order to giveoptimum large signal handling ability, ahigh supply voltage is preferable. If amains power supply is used it shouldhave a low noise and ripple content on itsoutput. Bifet operational amplifiers areused in the filter so that it has minimalnoise and distortion levels.
CASSETTE PROCESSOR
Loading programs from a cassetterecorder seems to be a major problemfor many home -computer users. While RI
some machines, such as the Dragon 32 orU
18k
64, seem to operate well with practicallyany cassette recorder over a wide rangeof volume and tone control settings, some Cl
others seem to be far less co-operative. 100nF
There are various ways of processing the 2R3
output of a cassette recorder to (hope-fully) provide more reliable results. The
JK1In
10k
R2
circuit shown here is quite simple andinexpensive to construct, but providestwo types of signal processing. Of course,how well (or otherwise) any cassette
18k
processor operates depends on theprecise nature of the problem, andwhether or not the applied processing isappropriate for that problem, but thecircuit described here will effect animprovement in most cases.
ICI is a CMOS 4001BE quad 2 -inputNOR gate, but here only two of the gatesare used, and each of these has its twoinputs connected together so that it
functions as a straightforward inverter.The two inverters are connected in
series, and R1 plus R2 are used to bias theinput to about half the supply voltage. R4provides DC positive feedback over thecircuit, which operates as a sort ofSchmitt trigger. The signal from thecassette recorder is capacitively coupledto the input of the circuit by Cl, andprovided an input of around 1 volt RMS ormore is provided, a good quality square -wave signal is produced at the output ofthe circuit. This type of signal seems towork much better with some home -computers than the direct output from thecassette recorder which has much slowerrise and fall times, as well as a highernoise content. RV1 is used to set theoutput level of the circuit for optimumreliability.
The second processor is based onIC2, and is simply a highpass filter having
IIC IPin14
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3 5
IC I l130113EIC2.14 58C
6IC1a IC 1b
4
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C3iOOuF.
702uF
JK 2out
31.9RIO
22k
In v1I IIR7 2C26nF C622
47t nF.4
:4IC OuF
RET3k9
R85k6
.9tO12V
RV2100k
tin
7
JK4out
1-vIC2b .1C05oF
-Ve
a cut-off frequency of about 700Hz,followed by a voltage amplifier stage.The filter is a third order (18dB peroctave type), and it removes any 'mainshum' or any other low frequency noise,which can often prove troublesome.Noise of this type can be caused by humloops, stray pick-up in the connectingleads or me computer itself, and can bevery difficult to prevent. However, anactive filter of this type should attenuateany low frequency noise to an insignif-icant level.
With RV2 at minimum resistance,IC2b operates as a unity gain bufferamplifier, but advancing RV2 increasesthe voltage gain up to a maximum ofabout 5 times. It can sometimes bebeneficial to advance RV2 to the pointwhere the output signal becomesclipped, as this can give a more regularwaveshape having a reduced risetime.However, in many cases results will bebest with RV2 set for minimum resistance,and it is probably best to initially try outthe circuit with RV2 at this setting.
VOLUME EXPANDERAlthough modern digital recordings
are capable of reproducing the fulldynamic range of even the most demand-ing music, most recordings cannotachieve the required 70dB or so dynamicrange. In order to prevent the signal fromeither dropping down into the back-ground noise level during quiet passagesor producing overloading on volumepeaks, most recordings have to besubjected to a degree of compression.
A volume expander can be used toincrease the dynamic range of a signal,and restore some of the 'impact' that islost during the recording process. There
is no way of exactly counteracting theoriginal compression, since the com-pression characteristic will vary con-siderably from one recording to another.However, in many cases the use of acertain amount of volume expansion willprovide a worthwhile improvement inresults with an apparent increase in thesignal to noise ratio as well as theboosted dynamic range.
This volume expander is based onan NE570 compander (compressor-ezpander) device. This is primarilyintended for use in noise reductionsystems with one of the identical sections
25
of the device used as a compressor andthe other configured as an expander. Inthis case both sections are used asexpanders, with one being used toprocess each stereo channel. Only onechannel is shown in the circuit diagram,but the numbers in brackets show theequivalent pin numbers for the otherchannel, which is in other respectsidentical.
There are three stages in eachsection of the NE570; a voltage controlledgain block, a precision fullwave rectifier,and an operational amplifier. When usedas an expander the input signal iscoupled to the rectifier and gain blockstages by C2 and C3 respectively. C4 isthe smoothing capacitor for the rectifier,and this has a value which gives suitablyfast attack and decay times, withouteither being so fast that distortion iscaused. The output of the rectifier is usedto control the gain block. As the inputsignal level is increased, the outputvoltage from the rectifier rises, the gainblock provides increased gain, and theexpansion is obtained. R3 is used in thebias and feedback circuit of the operat-ional amplifier, which is utilized here asjust a buffer at the output.
The NE570 has a 2 to 1 expansion
56k
R2
2 2kRV1 56k
C1100nF
C210uFuta
1151
C3 1144u7
8 191 13
ICINE570
1161
C4111uF
5R315k
1101
i S1a1S1b1
low high
R4220k
10uFclout
T
R5270k
+9to
12V
R6330k
C6100nF
-ve
characteristic. In other words, a rise inthe input level of (say) 20dB (10 times)gives a 40dB (100 times) increase in theoutput. This gives far too much expansionfor this application, but a bias registerfrom the positive supply to the smoothingcapacitor of the rectifier can be used togive reduced expansion. In this circuitthere are three switched bias resistors(R4 to R6). With a maximum input signalof about 500 millivolts rms, these provide
expansion levels of about 6, 9, and 12dB.12dB is about the maximum that can beused in practice without the expansionbecoming too obvious.
RV1 is adjusted to minimise distor-tion, and the NE570 has a typical trimmedTHD of only 0.05%. If RI, R2 and RV1 areomitted, the typical THD is still only 0.3%,which is adequate for most purposes.Input levels of up to about 1 volt rms canbe handled before clipping occurs.
PARAMETRIC EQUALISERA parametric equaliser is a versatile
form of tone control which is usedprincipally in the production of electronicmusic, but circuits of this type can also beused in hi-fi systems. Both lift and cut canbe provided, like an ordinary bass ortreble tone control, but it is a frequencyband somewhere in the middle of theaudio range that is controlled by this typeof filter, rather than one end of the audiospectrum. The centre frequency is tun-able (usually over a fairly wide frequencyrange), and the filter is really a bandpassand notch type, with the type of filteringprovided depending on whether thecircuit is set for lift or cut. Circuits of thistype invariably have variable 0, so that avery narrow range of frequencies, abroad frequency range, or anything inbetween these two extremes can becontrolled.
In electronic music, a parametricequaliser can obviously be used to radic-ally alter the sound of an instrument, andcan modify the sound in a variety of ways.When used with a hi-fi system it could beused to counteract a resonance or otherirregularity in the frequency response ofthe system.
Although quite simple, the designfeatured here has a respectable level ofperformance with a tuning range whichextends from about 200Hz to approx-imately 4kHz. Up to about 15dB of boostand cut can be provided and the Q canbe varied over wide limits.
In common with other designs of thisgeneral type, the circuit is based on astate variable filter. This is formed byIC la, IC2a and IC2b, and it is thebandpass output at pin 1 of IC2a that isutilised here. The frequency of the filter is26
3k9
-[R310k
R23k9
R8 100k
IC1,2pin 8
R4R1 100k RV1a
to -1 j__ 220k R7lin 12k
nc
R6
- 39k
R5
tune
C22n7
nc
RV1b220klin R9
IC2a
C5 2n7
12k 5
47k
ncRV22M2lin
C3470nF
II---I 1
R11 6k8
RV3 47k I.3g_ f__.lin 100kj_
JK1:V-M 4,
In C4 R10 5 +011--vr4u7 47k IC1b C6 .
10uF
r- R1347k
IC2b
C1.,220
uFIC1,2pin 4
lilt cut
IC1,21458c
+9to
30V
C7100mnF to
JK 2out
-ve
governed by the values of C2 and C5.plus the series resistances of R7 plusRV la and R9 plus RV lb. By making theresistive elements variable, the operatingfrequency can be adjusted over thenominal range specified above, withminimum resistance corresponding tomaximum operating frequency.
The signal is not actually handleddirectly by the bandpass filter, butinstead passes through inverting ampli-fier IC lb. The bandpass filter is effect-ively used as a sort of frequency selectivenetwork in the negative feedback circuitof IC lb. The point of doing this is that itenables the notch response to beobtained in addition to the bandpassresponse, with the type of filtering
obtained depending on whether RV3 isadjusted for lift or cut. Feedback over thefilter (and hence its Q value) is controlledby R5, and R6 plus RV2. The Q cantherefore be controlled using RV2, withminimum resistance corresponding tomaximum Q (and a narrow response).
The circuit will operate on anysupply voltage in the range 9 to 30 volts,but a supply of around 15 to 30 volts ispreferable as it enables high outputlevels to be handled without clipping andserious distortion resulting. Bear in mindthat when set for maximum boost thecircuit provides a significant amount ofvoltage gain at the centre of the response,and it is then more vulnerable tooverloading.
DOOR ALARM PARTS LIST CASSETTE PROCESSOR PARTS LISTRESISTORS:- All 0.6W 1% Metal FilmRI 4k7 (M4K7)R2,4,5,6,8 10k 5 (M1OK)R3 3k9 (M3K9)R7 100k (M100K)R9 47k (M47K)R10 82k (M82K)
CAPACITORSC1,3C2C4C5C6
10uF 35V PC Electrolytic100nF PolyesterluF 100V PC ElectrolyticlOnF Polyester100uF 63V PC Electrolytic
SEMICONDUCTORSIC 1 4011BE1C2 741C 8 pin DILIC3 555TR1 BC337
MISCELLANEOUSSI Reed Switch MiniatureS2 Key SwitchB1
LS1
HP7 BatteryL/S Lo-Z 768Magnet small
THD FILTER PARTS LISTRESISTORS:- All 0.6W 1% Metal FilmR1,2,10,11 10kR3,4,6,7R5,14R8R9R12R13RV1,4RV2RV3
CAPACITORSC1C2,3C4CSC6C7C8
100k18k3k33k98k233kPot Lin 47kPot Lin 4k7Dual Pot Lin 220k
4u7F PC ElectrolyticlOnF Polyester220uF 63V PC Electrolytic220pF I% Polystyrene10uF 35V PC Electrolytic100nF Polyester100uF 63V PC Electrolytic
SEMICONDUCTORSIC1,2 LF353IC3 LF351
MISCELLANEOUSJK1,2 1/4" Jack Socket
1/4" Jack PlugsSi Sub -Mm Toggle A
2 (FF04E)(BX76H)(FFO1B)
(BX70M)(FF12N)
6
(0X05F)(QL22Y)
(QH66W)(QB68Y)
(FX70M)(FH40T)
(YW53H)(FX7 IN)
4 (M1OK)4 (M100K)2 (M18K)
(M3K3)(M3K9)(M8K2)(M33K)
2 (FW04E)(FWO1B)
(FW89W)
(FFO3D)2 (BX70M)
(FF14Q)(BX49D)(FF04E)(BX76H)(FF12N)
2 (WQ31J)(WQ30H)
2
2
(HF90X)(HF85G)(FHOOA)
VOLUME EXPANDER PARTS LISTRESISTORS:- All 0.6W 1% Metal FilmR1,R101,R2,R102 56kR3,R103 15kR4,R104 220kR5,R105R6,R106RV1,RV101
270k330kHor Preset S -Min 22k
CAPACITORSC1,C101,C6,C106 100nF PolyesterC2,C102,C5,C105 10uF 35V PC ElectrolyticC3,C 103 4u7F 63V PC ElectrolyticC4,C104 luF 100V PC Electrolytic
SEMICONDUCTORSICI NE570
MISCELLANEOUSJK I JK2 1/4" Jack Socket Stereo
1/4" Jack Plug Stereo PlasticS1 Switch Rotary SW3B
4 (M56K)2 (M15K)2 (M220K)2 (M270K)2 (M330K)2 (WR59P)
4 (BX76H)4 (FF04E)2 (FFO3D)2 (FF01B)
(QY1OL)
2 (HF92A)2 (HF88V)
(FF76H)
RESISTORS:- All 0.6W 1% Metal FilmR1,2 18kR3 10kR4 100KR5,6 3k9R7 47kR8 5k6R9 3k3R10 22kRV1 Pot Lin 4k7RV2 Pot Lin 100k
CAPACITORSCl 100nF PolyesterC2,8 10uF 35V ?C ElectrolyticC3,4 100uF 63V PC ElectrolyticC5,6,7 22nF Polyester
SEMICONDUCTORSIC 1 4001BEIC2 1453C
MISCELLANEOUSJK1,2,5,4 1/4" Jack Socket
1/4" Jack Plug
PARAMETRIC EQUALISERRESISTORS:- All 0.6W 1% Metal FilmR1,2R3R4,8,12R5,10,13R6R7,9R11RV1RV2RV3
CAPACITORSC1C2,5C3C4C6C7
3k910k100k47k39k12k6k8Dual Pot Lin 220kPot Lin 2P42Pot Lin 4Tx
220uF 63V PC Electrolytic2n7F 1% Polystyrene470uF Polyester4u7F 63V ?C Electrolytic10uF 35V ?C Electrolytic100nF Polyester
SEMICONDUCTORSIC1,2 1458C
MISCELLANEOUSJK1,2 1/4" Jack Socket
1/4" Jack Rug
2 (M18K)(M 10K)
(M100K)2 (M3K9)
(M47K)(M5K6)(M3K3)(M22K)
(FWO1B)(FWO5F)
(BX76H)2 (FF04E)2 (FF12N)3 (BX72P)
(QX01B)(QH46A)
4 (1-1F90X)4 (HF85G)
PARTS LIST
2 (M3K9)(M1OK)
3 (M100K)3 (M47K)
(M39K)2 (M12K)
(M6K8)(FW89W)(FWO9K)(FW04E)
(FF14Q)2 (BX61R)
(BX80B)(FFO3D)(FF04E)(BX76H)
2 (QH46A)
2 (HF90X)(HF85G)
Continued from page 23
Motherboard for BBC Parts ListRESISTORS: All 0.6W 1% Metal FilmRI 2701/ 1
R2 lkO 1
CAPACITORSCl 100/AF 25V Axial Electrolytic
SEMICONDUCTORSLED12 LED Red
MISCELLANEOUSSKI Analogue Port CableSK2SK3SK4-7FS1
JK1Si
I/C Port Cable1MHz Port Cable2 x 23 Way PC EdgeconFuse 20m -nn 250mAFuse ClipPC Mtg Power SktDPOT Slide SwitchMctherboard PCBStd Power Plug 2.1mmStrapping Wire 20 SWG
(M270R)(M1K)
1 (FB49D)
2 (WL27E)
1
1
1
4
2
1
1
1 roll
(924B)(026D)(925C)
(RK35Q)(WRO1B)
(WH49D)(RK37S)(FH36P)
(GB39N)(HH60Q)
(BL13P)
A complete kit pf the above parts is avalable.Order Az LK47B (Motherboard for BBC Kit)
27
* Driver Module for Xenon Tube* Complete with Trigger Transformer
by Dave GoodmanIntroduction
The Xenon Tube, along with theTrigger Transformer required to operateit, are regular subjects of enquiry bymany of our readers, therefore to put thebooks straight, a tube driver module withexternal triggering and 'on board' strobeoscillator is offered. The module can beused for photography, roadside hazardindication, navigation, distress beaconsor perhaps underwater communications,and is ideal for further experimentation.Xenon tubes are glass envelopes filled
17-
T16V OV
OV
SEC
6V
001
* External Triggering or* Internal Strobe Oscillator
with a gas which emits blue/white highintensity light when energised. A highvoltage potential of 210 to 400V must beapplied across both anodes, Al & A2,(see Figure 4g) which will allow the gas to'strike' when a 3 to 5kV pulse is applied tothe trigger electrode strip, located alongone side of the tube. To generate the EHTtriggering voltage, a pulse transformer isused which is similar in action to the wellknown car ignition coil (see Figure 4f),stepping up the primary (B,C) voltage tothe required secondary (B,A) voltage.
Circuit DescriptionTo generate the xenon strike voltage
a simple inverter system is employed.Each half of transformer T1 secondary isconnected to a power transistor (TR3 &TR4) and the common centre tap isconnected to +V supply. By alternatelyswitching each transistor on and off, onehalf of Ti is grounded at a time, andmaximum current flows through eachwinding in turn. By inductive effect a 50Vpeak pulse develops between TR3 andTR4 collectors (across Ti secondary)
1N 4001
2
PRI.
240V
30
131
47k
LK C
0-
4 OVr
05
C1
SeeText
R32M2
R24M7
N1
C210
A
X1
k.1/4-2) T2
07
TR1BC328
_L R4IMO
10k
R747k
R8100k
LK A
R5
4k7R610nF
C3
12
IC1e
I
IC1dtt 10
R9100k
RVIIMO
C4 tuF
R1010k
R121k0
LICEr
TR 2 1
2N3053
2 061N4148
+Ve IC1t
14
5
_r +Ve
ICla2
R1110k
I
6
IC1c
2-8
RV 2470k
C5 1nF
IC1b
5R15
R14 5k65k6
80 Q9Ext
trigger "0- 0-
TR3BD711
C6470uF.I.
100 011
TR4BD711
1
ICI=4049
R131k0
TR 52N
3053
Figure 1. Circuit Diagram28
Supply OV
X1 Mountedexternally oron PCB
Add link for9-12V supplyonly
.-11=1... D2-r-s- 03
P1
PRIMARY 240V\--03 6ck--{=R- D5- Dal
O10
T1
6V
SECONDARY
OV I6V11
T
+ve Supply
4-5V to 12V 1;1 T R 5 c c YR2-C=.3- R12
D1 -CI- RT3
I C1
r' 3 Corecable
Length determinedby insulation properties
C6MAPLING861R
Al
C212
A
KlyX1 A2
6 8Trig,
k a g TH1C3D6
Remcte trigger.Push tc make switch
lor camera shutter INorm.open
Screen wire
lifT5:e7[11) co
C4
0
Link for remotetrigger only
,Link for strobetrigger only
cR8
XENON TUBE DRIVER -=-R11
RV1
R10
11V2
C1see text
Set wiper hereforte 2kHz.
MAPLIN XENON TUBE DRV
Figures 2 & 3. PCB Track Legend and Wiring Diagram
which is stepped up by the primarywinding approximately 20 times to pro-duce a lkV peak signal at pins 2 and 3. T1is in fact a normal 240 to 12V mainstransformer connected the reverse wayround; instead of applying 240VAC forstepping down to 12VAC, we apply12VAC and step it up to 240VAC, or inthis case 1kVAC.
The alternating signal for switchingTR3 & TR4 comes from a CMOSinverter/oscillator IC la,c and f. IC lc has avariable resistance RV2, and R11 con-nected across it, which maintain the inputvoltage level close to the output level onpin 6. If IC la output, pin 2, is assumed to
be low (OV) capacitor C5 will start tocharge via RV2 and IC lc pin 7 input willbe momentarily pulled low. By inverteraction IC lc pin 6 will go high (+V)maintaining IC la pin 2 in the low state. AsC5 charges, the voltage across it in-creases until a point is reached whenIC lc input pin 7 is potentially highenough to flip the output pin 6 low, IC lapin 2 will then change state from low tohigh. At this stage the voltage across C5is reversed and a discharge path via RV2& R11 gradually drops the potential atIC lc input until the switching level isreached and the oscillation cycle repeats.RV2 determines both charge and dis-
charge times which can be varied from25uS to 650uS, or between frequencies of4CacHz and 1.5kHz.
IC if buffers the oscillator and drivesthe emitter follower driver transistor TR2.With output high, TR3 is turned on, IC lbgoes low and TR4 is turned off. WhenIC .f output goes low TR3 is turned off,IC .b output switches high and TR4 turnson. C6 decouples the +VE rail and DIhelps prevent component damage in thecase of supply reversals.
Once oscillation is established, D2 toD5 form a full wave bridge rectifier forcharging Cl. This capacitor must be of a
voltage rating, in this case 450V29
working, and to keep the voltage withinlimits, RI can be connected across T1primary by inserting link 'C' if necessary(see Testing).
Neon lamp Ni indicates when the Clcharge voltage is high enough to strikethe xenon tube, but as neons normallyconduct at around 90V, a high impedancepotential divider (R2, R3) is required toset this threshold. Resistor R4 charges ahigh voltage capacitor, C2 via the pulsetransformer primary winding (T2, c & b).By discharging C4 to ground a fastrise -time spike of several hundred voltsis generated in the primary of T2 which isstepped up to some 5kV in the secondarywinding thus triggering the tube. CIdischarges a high current pulse throughthe tube to ground and is then re -chargedby the inverter.
Connecting link 'A' allows an exter-nal make switch to momentarily connectD6 to ground, TR1 base potential islowered via R7 and R8, TR1 conducts sothat a positive gating voltage appears atR5, R6. Thyristor TH1, which can beviewed as a switched diode, conductsand C2 is discharged to ground from theanode to the cathode. Immediately afterdischarging, C2 re -charges via R4 so thatthe anode voltage rises positively, underthis condition TH1 would remain in apermanently conducting state, even with-out further control gate signals! This isobviously not what is required andsomehow the thyristor must be reset to anon -conducting high impedance state.Fortunately the effect of expanding T2primary, by discharging C2 through it,results in the coil contracting back again,thus producing a high, negative voltage,spike in the reverse direction. This isapplied via C2 to TH1 - taking the anodemore negative than its cathode. Theconducting state is thus prevented byreverse biasing the anode/cathode junc-tion and TH1 resets to the high impe-dance state, under gate control.
A second CMOS oscillator runs at alower frequency than the inverter clockand with link 'B' inserted can be used tostrobe the xenon tube from approximate-ly 0.5Hz to 6Hz. If required links A and Bcan both be fitted for repeat and manualtriggering.
ConstructionRefer to the parts list and begin by
bending the resistor leads for fitting intothe PCB. Do the same with diodes DI toD6 referring to Figure 4a for orientation.Mount both presets (RV1 & RV2), IC1,TR I and Cl to C5. Figure 4c, d and eshows lead connections for TR2 and 5,TR3 and 4, also TH1 which must be fittedcorrectly to the legend. Next fit pulsetransformer T2 with the primary lead Cexiting on the left towards C2. Now fitvero-pins P1 to 11 from the track side ofthe PCB and push home with a solderingiron. All components may now besoldered and excess wire ends cut off.Clean the tracks with solvent and a brush,then inspect for solder splashes, dryjoints, short circuits etc. Neon Ni can befitted either way round, but X1 must be30
D1-6ka/land
0
TR1
e
b pin
TR2,5 /viewtab
b
TR3,4
OHeatsink
0 k
cU
EPrimary
PulseTransformer T2
Al
XenontubeX1
TriggerA2 electrode
Figure 4. Component Reference
fitted with the double wire end to theright of the board. For test purposescarefully solder the anodes Al and A2 topins 5 and 7 respectively, and the triggerelectrode directly to the component sideof the PCB (Figure 3). Mount the min.mains transformer Ti with the primary(thick wires) to pins 2 and 3 and thesecondary (three thin wires) to pins 10and 11. The centre tap (middle wire)connects to pin 1 ( +V). Finally re -checkthe construction and when completelysatisfied, proceed with testing.
TestingConnect a suitable power supply of
from 4.5V to 12V with +V to pin 1 and OV(- V) to pin 4. Adjust RV1 wiper to abouthalf -travel and RV2 wiper to the arrow onthe legend. Turn on the power whereup-on a slight buzzing sound should beheard, after a few seconds the neonshould start to glow. Now take a length ofinsulated wire, connect one end to OVand momentarily touch the other end ontopin 12. The xenon tube should flash and aloud crack may be heard as the airaround the tube expands; Ni will go out.If using a 9 to 12V power supply connectlink 'C' to prevent excess charge acrossCl and connect link 'A'. Re -apply power,wait for the neon to glow, then touch pins8 and 9 together, once again the tube will
flash. Switch off the power, discharge thesystem by grounding pin 12, remove link'A' and connect link 'B'. Re -apply power,the tube should flash at approximately 1second intervals. Adjusting RV1 will varythe flash rate slightly, but not a lot. Switchoff the power, discharge pin 12 to groundand remove the +V PSU lead, leave Ticentre tap in place. Now connect anammeter between the +V supply leadand pin 1 on the PCB, set the range to 0.5or IA and switch on. The final currentreading will be dependant on the supplyvoltage, on average it should be around80mA for a 6 volt supply. Slowly adjustRV2 clockwise or anti -clockwise until thelowest reading is found, link 'B' may haveto be removed before doing this check. Ifa frequency counter or 'scope is avail-able, monitor the inverter clock on IC1pin 15, it should be close to 2kHz atminimum current setting. Also an oscillo-scope connected across Cl with a10M.ohm probe should read below 450VDC with a 12V supply and link 'C'inserted. Note that link will not benecessary when using a power supply of4.5 to 9 volts.
Strobe RateAdjustment
Capacitor CI is supplied as 47uF butmay be reduced in value providing itsworking voltage is kept at 450V or more.Because the inverter source is highimpedance, the charge rate for Cl isslower for larger capacitance valuesand faster for smaller values. The finalvalue chosen will depend upon the use towhich the module is to be put. Thus fasterstrobe oscillator times will require Clbeing lower in value, say 10uF or less, toincrease the oscillator frequency stillfurther, C5 can be reduced in value.
One major effect of reducing Cl invalue is a reduction in discharge currentthrough the tube, hence a reduction inlight output, so this must be borne in mindwhen selecting Cl. If it is required to usethe 47uF value for Cl, but light intensityneeds to be variable, link 'C' can beinserted and the value of RI decreased to
Continued on inside back cover
EnkaExpourMeter* Over Six Stops Range* Simple & Inexpensive Design* Battery Operated - Low Consumption
by Robert PenfoldA common way of determining the
optimum exposure when making en-largements is to make a test strip, but it isquicker and more convenient to use anenlarger exposure meter. With the aid ofan exposure meter of this type only onetest strip needs to be produced for eachbox of paper. The correct exposure foreach negative is then quickly and simplyobtained using the meter to indicate thecorrect aperture.
A unit of this type can be very simpleand inexpensive, and the enlarger expo-sure meter featured in this articlecertainly falls into this category. It isperhaps a little misleading to refer to it asa 'meter' since it does not actuallyincorporate a meter movement of anykind. Instead, the unit has a calibratedpotentiometer and a LED indicator. Areading is obtained by adjusting thepotentiometer to the point where the LEDswitches on and off, and then taking thereading from the potentiometer's scale.This scale is only in arbitrary units from 0to 10, but it is perfectly adequate for thisapplication.
The meter has a usable range of sixstops or more. It is completely selfcontained with power being obtainedfrom an internal 9 volt (PP3 size) batterywhich has a long operating life. A simplebattery check facility is included so thatmisleading results due to an inadequatesupply voltage can be avoided.
Operating PrincipleThe circuit is based on an operation-
al amplifier which is used as a voltagecomparator. Figure 1 shows the basiccircuit of the unit.
An operational amplifier amplifiesthe voltage difference across its twoinputs, and at DC it has an extremely highvoltage gain of typically about 200,000times. Therefore, only a very smallvoltage difference at the inputs is neededin order to send the outputs of the device
+Ve
Figure I. Voltage Comparator
fully positive or negative. The output goespositive :f the non -inverting (+) input isthe one at the higher potential, ornegative if the inverting (-) input is at thehigher voltage.
The input to the non -inverting inputis prcvided by RV1, which is thecalibrated potentiometer. The voltage atthe inverting input is produced by thepotential divider which is comprised ofload resistor RI and photocell PCC1. Theresistance of PCC1 varies in sympathywith the light level to which it is
subjected. The higher the light level thelower the resistance of PCC1, and thelnwer the voltage fed to the invertinginput of the operational amplifier. If RV1is adjusted for maximum slider potential,and then gradually backed off, the outputof the operational amplifier will initiallybe high, but will switch to the low state astae slider voltage falls below the potentialproduced by the photocell circuit. Inother words, by adjusting RV1 to thisswitch over point its scale reading willreflect (in arbitrary units) the voltageproduced by the photocell circuit, andtherefore the light level received by thephotocell. Due to the high gain of theoperational amplifier a high degree ofprecision can be obtained with thissystem, and the accuracy is limitedlargely by the degree of precision withwhich the potentiometer's position can beread, rather than by any electricallimitations. In fact, in practice the outputof the operational amplifier will only behigh or low, and it will not be possible toadjust RV1 for an intermediate level.
LED indicator DI is used to show theoutput state of the operational amplifier,and this switches on when the output is
Out
RI1M2
C1'OOnF
RV122klin
mmC2
lOnF
TR1ME L 12
C3E lm 100nF
CA3130
IC2
check0
78L05
Corn
0-1C4
100 nF
Si D2BZY88C5V6
R24'OR
131
TIL 209
S2
LtBI9V
PP3
Figure 2. Practical Circuit31
high. In theory the unit covers anextremely wide light level range, sinceRV 1 can be adjusted to match any voltageproduced by the photocell circuit. Inpratice the usable light range of the unit isfar more restricted as a very wide rangeof light levels are covered by a very smallsection at each end of the scale. Thescale is only usable over the centralsection where a comparatively small lightrange is covered. The range coveredhere is wide enough for this applicationthough, and the value of RI is chosen tobring the appropriate light level rangeinto this usable area.
Practical CircuitFigure 2 shows the full circuit
diagram of the Enlarger Exposure Meter,and this has obvious similarities with thebasic circuit. However, there are a fewimportant differences.
One of these is the use of aphotodarlington transistor as the photo-cell, rather than a photoresistor. Aphotodarlington device has the advan-tage of a relatively fast response at lowlight levels and it is also inexpensive. Adisadvantage is that is does not provide atrue resistance, and changes in thesupply voltage may not cause a prop-ortional change in the output voltage ofthe photocell circuit, This results inchanges in supply voltage slightly chang-ng the reading produced by a given lightlevel; a stabilised supply therefore has tobe used. IC2 is a small monolithic voltageregulator which gives a well stabilised 5volt supply that ensures good accuracyand consistent results. The circuit has tooperate at very low light levels (far lowerthan an ordinary exposure meter), andthis is reflected in the high value of loadresistor RI. IC1 is a MOS operationalamplifier which has an extremely highinput resistance and operates well at asupply potential of just 5 volts. Most otheroperational amplifiers will not work in thecircuit.
The circuit is very sensitive to straypick-up of mains 'hum' and other electric-al noise, due to the use of the operationalamplifier with its full voltage gain. C2 andC3 help to minimise this unwantedpick-up which could otherwise prevent awell defined switch over point from beingobtained, and could seriously impair theaccuracy of the unit.
A simple battery check circuit isincluded, and the only additional compo-nents used in this are Si and D2. With Siin the 'normal' position the LED indicatorDI and its current limiting resistor R2 areconnected across the output of IC1 so thatthe unit functions normally. In the 'check'position the LED indicator circuit isconnected across the non -stabilised 9volt battery supply via zener diode D2.With about 5.6 volts dropped across D2and just under 2 volts needed across D1before it will switch on, around 7.5 volts isneeded across the battery check circuitbefore DI will pass any current at all, andabout 8 volts is needed before it will lightup reasonably brightly. Therefore, if D2lights up brightly when SI is set to the'check' position the battery voltage is
21
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Dimensions in mm53
Figure 3. Front Panel Drilling
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satisfactory. If DI lights up only dimly thebattery is nearly exhausted, and if DI failsto light at all the battery should bereplaced immediately. Incidentally, thebattery check facility only functions whenthe unit is switched on.
The current consumption of thecircuit is only about 5 or 10 milliamps(depending on whether DI is switched onor off) and a small (PP3 size) 9 volt batteryis quite adequate to power the unit.
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Figure 4. PCB Layout and Wiring
EXPOSURE BD.GB64U
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32
ConstructionThe recommended case for this
project is a plastic type having analuminium front panel and approximateoutside dimensions of 111 by 71 by 48millimetres. As the printed circuit boardhas been specifically designed to fit thiscase it is strongly recommended that thisparticular type should be used. If all thecomponents are to fit into place properly,especially TR1 and DI, it is essential thatthe mounting holes in the front panel aredrilled in the correct positions. Figure 3gives drilling details for the front panel,and once again, it is strongly recom-mended that this layout should be used.
Details of the printed circuit boardand wiring are provided in Figure 4. ICIis a MOS input device and it shouldtherefore be mounted in an 8 pin DILsocket. Do not fit ICI onto the board untilall the other components have beenmounted, and leave it in the antistaticpackaging until then. Handle ICI as littleas possible. DI and TR1 are mounted atright angles to the board, and are made toprotrude slightly over the edge of theboard. When the completed board hasbeen wired up to the rest of the unit it isslotted into the vertical set of guide railson the extreme left hand end of the case,with the component side of the boardfacing inwards. With DI and TR1 suitably
positioned they will fit into their mountingholes in the front panel when this ispushed into place.
Either RV 1 must be fitted with acalibrated control knob, or it must befitted with a pointer knob and a scalemust then be marked around this. Theformer is by far the easier option, and isthe one adopted for the prototype. Anindicator line must be marked on thefront panel next to RV1.
In UseIn use the unit is simply placed on its
back on the enlarger baseboard with thephotocell facing upwards towards theenlarging lens. In order to find thecorrect scale setting for a particular boxof paper it is necessary to first determinethe optimum aperture and exposuretimes for an average negative. This isdone in the usual way by producing a teststrip. With the negative and the diffuserin place and the enlarger adjusted for theappropriate aperture, position the expo-sure meter on the baseboard and adjustRV1 to the switch over point. Make a noteof the scale reading and the exposuretime on the box of paper.
The procedure for finding the cor-rect exposure for a new negative is thenquite straightforward. Place the exposuremeter on the baseboard and set it at thereading marked on the box. With the
negative and diffuser in position theaperture of the lens is adjusted to bringthe meter to the switch over point. Thisthen gives the correct aperture for theexposure time marked on the paper'sbox. The same exposure time is alwaysused for a given box of paper, and onlythe aperture is varied to suit eachnegative.
As the photocell has only a verysmall sensitive area it is possible to usethe unit as a spot meter, reading either ahighlight or a shadow tone as desired, orit can be utilized as an integrating meter ifa diffuser is fitted under the enlarginglens while metering (as describedabove). An important point to keep inmind is that a different scale reading for agiven box of paper will be obtained foreach of these three methods, and if usingmore than one of these you must note thecorrect readings for each method on thebox (and then be careful to use the rightone each time).
The unit should give satisfactoryresults without any modifications beingmade, but it is just possible that the rangeof light intensities that you will use maytend to be in a cramped portion at oneend of the scale or the other. If necessaryRI can be raised in value to broaden outthe low light level end of the scale, or itcan be reduced in value to broaden outthe opposite end of the scale.
ENLARGER EXPOSURE METERRESISTORSRI 1M2 '/3W 5% Carbon FilmR2 4701! 0.6W 1% Metal FilmRVI 22k Pot Lin
CAPACITORSC1,4C2C3
100nF DisclOnF Polyester100nF Polyester
SEMICONDUCTORSDI Mini LED RedD2 BZY88C5V6TR1 MEL12
(B1M2)(M470R)
(FWO3D)
2 (BX03D)1 (BX70M)1 (BX76H)
1
(WL32K)(QH081)
(HQ61R)
ICIIC2
CA3130kiA78L05AWC
MISCELLANEOUSS1,2 Sub -MM Toggle A
Printed Circuit BoardMetal Panel Box M4004Knob FlODIL Socket 8 -pinWireVeropins 2145Battery Clip (PP3)
1 (QH28F)1 (QL26D)
2 (FH0OA)1 (GB64U)1 (WYO1B)1 (RW78K)1 (BL 17T)lm (BLOOA)1pkt (FL24B)1 (HF28F)
A Kit of all the above parts, including the case, is available.Order As LK44X (Enlarger Exposure Meter)
XENON TUBE DRIVER Continued from page 30.
XENON TUBE DRIVER PARTS LISTRESISTORS: All 0.6W 1% Metal Film unless otherwise statedR2 4M7 '/3W 5% Carbon FilmR3 2M2 '/3W 5% Carbon FilmR4 IMOR5,10,Il 10kR6 4k7R1,7 47kR8,9 100kR12,13 1k0R14,15 5k6RV I IMO Hor. Sub -min PresetRV2 470k Hor. Sub -min Preset
CAPACITORSCl (See Text)C2C3C4CSC6
47µF 450V Axial Electrolytic0.1,uF Interference Supp.lOnF Disc
PolycarbonateI rtF Polycarbonate470,LIF 16V Axial Electrolytic
1 (B4M7)1 (B2M2)1 (M IMO)3 (M10K)1 (M4K7)2 (M47K)2 (M100K)2 (M IKO)2 (M5K6)1 (WR64U)1 (WR63T)
1 (FB43W)1 (FF56L)1 (BX00A)1 (WW53H)1 (WW22Y)1 (FB72P)
SEMICONDUCTORSD1 1N4001D2-5 1N4007D6 1N4148TR I BC328TR2,5 2N3053TR3,4 BD711TH1 C106DIC 1 4049UBE
MISCELLANEOUST1 Transformer 6V Sub. MM.T2 Trigger TransformerNI Neon Bulb Wire EndedX1 Xenon Tube
Veropin 2141Printed Circuit Board
1 (QL73Q)4 (QL79L)1 (OL80B)1 (QB67X)2 (QR23A)2 (WH15R)1 (QH3OH)1 (QX21X)
1 (WBOOA)1 (YQ63T)1 (RX70M)1 (YQ62S)1pkt (FL21X)1 (GB61R)
A complete kit of parts is available.Order As LK46A (Xenon Tube Driver)
31
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