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Er.trutiicsOCTOBER 1990

TECHNOLOGY - VIDEl

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Digital readout gives asecond-by-second playbackof your rocket's flight

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ROCKET

ALTIMETER

THE SPORT OF MODKI. ROCKE-

try allows hobbyists to manage their own miniature spaceprogram. Small-scale rockets,usually constructed from paper,plastic, and balsa wood, are routinely launched with commerciallymade solid-fuel motors. Reaching altitudes between 100 feet and severalmiles, model rockets are safely recoveredby parachute to allow repeated flights and toreduce trie risk of personal injury.

In NAR (National Association of Rocketrycontest events, a visual tracking system usingtriangulation is used to determine the peak altitude of each model. The contestant who launcheshis rocket out-of-sighl. or through the clouds, willreceive a "track lost" rating instead of altitude points.Visual tracking, dependent upon weather conditionsand operator skill, can often be difficult, and the sport flyerwho wants to know how high his model went will rarely takethe time to set up and operate visual trackers.

Our rocket altimeter was developed to help contest and sportrocketeers determine their models* altitude without tracking.

his airborne "flight-recorder" is an all-CMOS microcomputerthat is coupled to an atmospheric pressure sensor via signal-conditioning circuitry. Powered by a 9-volt battery, the unit is smallenough to be launched in a D-, E-, or F-motor powered model rocket.(The letters indicate the relative power of each engine: in alphabeticalorder, each engine is twice as powerful as the previous one.) The unit takesa pressure sample every lA second and stores 1000 data values in memoryduring the flight.

The completed system contains two sections: the flight-recorder section thatgoes up in the rocket, and an LCD module that's used to display flight data backon the ground. When the rocket returns to Earth, the LCD module is connected tothe flight recorder and the peak altitude achieved can be displayed in 50-footincrements, along with a '/i-speed "playback" of the entire flight. Rocketeers now

Foun.Jhree...two...one..*

ignition! Model rocketrycatches up with

technology with thiselectronic altimeter.

JOHN FLEISCHER

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37

have a reliable and accuratemeans to measure the altitudethat a model reaches. The dataobtained can then be used to calculate the speed and accelerationof the rocket.

Figure 1 shows the construction and pinout of theSCX15AN pressure sensor usedin the altimeter. The sensor,manufactured by Sensym (1255Reamwood Ave.. Sunnyvale, CA94089), is a low-cost (about $42)piezoresistive IC in a strain-gauge bridge configuration. Themonolithic circuitry inside thesensor (see Fig. 2) is deposited ona silicon chip that has a cavityetched out to form a diaphragm.A port is on top, and a vacuumreference cavity is on the bottom.The result is a sensor that measures absolute barometric pressure. Output voltage (VI-V2)ranges from 10-50 mV, and isproportional to atmosphericpressure—which, of course, varies with altitude. Although theentire unit is not temperaturecompensated, the sensor itself is,by means of two built-in thermistors. Best accuracy for the al-timeter is achieved in the55-75°F range. Outside thatrange, a shift of 2% for every 10°Fwill occur.

Figure 3 shows the block diagram of the system. The pressure sensor is buffered with anLM324 op-amp to feed an LM331voltage-to-frequency (V/F) converter. At ground level, a signal ofabout 3.7 kHz will be output bythe V/F converter. As the atmospheric pressure decreases(with increasing altitude), thatfrequency also decreases; at15,000 feet, the signal is about2.9 kHz. An RCA 1802 microprocessor calculates the altitudedata from the frequency input.

The entire system is made upfrom three separate PC boards,although only two ever leave theground. The pressure sensor, theLM324 buffer, the V/F converter,and other support circuitry is located on an "analog" PC board,and the microprocessor anddata-logging circuitry are on a"CPU" board. The two boards areheld together with screws, andelectrical connections are jumpered between the two. The display module is built on a separatePC board, and it stays on theground; the module must be con-

ALUMINUM PLATE

ALUMINUM BASEPLATE

PRESSURE MEDIA(B) PRESSURE MEDIA(A)

FIG. 1—THE MONOLITHIC CIRCUITRY inside the SCX15AN pressure sensoris depositedon a silicon chipthat hasacavity etchedout to form adiaphragm. A portis on top,andavacuum reference cavity is on the bottom. The result is a sensor that measures absolutebarometric pressure, which is then converted to altitude.

PIN DTEMPERATURE OUTPUTS)PIN2)VS

PIN3)0UTPUT(+)PIN4)GR0UND

O)p|N5)0UTPUT(-)PIN6)TEMPERATURE 0UTPUT(-)

BOTTOM VIEW

FIG. 2—THE SENSOR'S OUTPUT VOLTAGE (V1-V2) ranges from 10-50 mV, and isproportional to atmospheric pressure.

FIG. 3—THEBLOCK DIAGRAM OFTHESYSTEM. The pressure sensor is buffered with anop-amp to feed a voltage-to-frequency (V/F) converter. At ground level, a signal of about3.7 kHz will be output by the V/F converter; at 15,000 feet, the signal is about 2.9 kHz.

nected to the other two boardsvia a ribbon cable to play backflight information.

Figure 4 shows the schematicof the CPU board; it gels its inputfrom the analog board and logsthe data every lA second. The cir-cuit consists of the microprocessor which calculates thealtitude, the EPROM containingthe operating software, and theRAM where the altitude data is

stored. Figure 5 shows the schematic of the analog board; thepressure sensor is located on thisboard. The output from the sensor is buffered and fed to the V/Fconverter, which provides the frequency input for the microprocessor. Figure 6 shows theschematic of the display moduleboard. It is basically made up ofthe display driver and the displayitself, but also contains the con-

FIG. 4—THE SCHEMATIC OF THE CPU BOARD. It logs the data to be read back when therocket returns to Earth.

FIG. 5—THE ANALOG BOARD outputs a frequency that's proportional to altitude. Thesensor (IC1) is located on this board.

trol switches. Power for the display module comes from theother two boards via the ribboncable.

The software for the altimeteris available on the R-E BBS—

516-293-2283. (For those whoprefer to type, the machine codefor the EPROM is shown in List

ing 1.) The flowchart for the program is shown in Fig. 7. Thesoftware handles data logging(the sample LED flashes every '/isecond), mode switch input, andLCD interfacing.

39

40

'9 II 1e 1d lclbtagg_2< 2e 2d 2c 2b 2a 3g 31 3» 3d 3c3b 3a -«g 41 Ats 4d 4c 4b 4a 5f

?3|:'4| 1?|I6|15 14 13 12 I1|10 9 8 7I6 5 4 3 ?I40l 39| 38l 3?| 3f,| 35 34 33J 3?|3l|30l 291 ?8| 27, 26BPOBPINbl b2 b3 b4 bS b6 b/ b8 b9bl0b11bl2b13b14b1Sb 6b1/b'8b 9b?0b21b22b23b24b25b?CbVb'8b 9bDATA

IC1L0AD MM5483NCLK

J1-2.9 JM < J1-8o J1-6o

OFF" i°N,& urn •—™ lO' h|> °fFPVSAMPLE .,,.,.,0^ 15 -AS J?•i ZERO

S2 V S3

2 5 PLAYBACKPEAK MODE

FIG. 6—THE DISPLAY MODULE stays on the ground; when the rocket is retrieved, thedata from the CPU board is displayed on this module.

INITIALIZE REGISTERS

RESET

MEMORYPOINTER

,YES

YhS,STORE PEAK

YESt

YESSET-UP DATA

'

DELAY

<Clas^am7le?^>.NoT

INCREMENT COUNT

LCD DISPLAY

DELAY

FIG. 7—THE SOFTWARE FOR THE ALTIMETER HANDLES data logging, mode switchinput, and LCD interfacing.

Construction

Three printed-circuit boardsare used. The pressure sensorand analog section are combinedon a single-sided PC board. TheCPU board is double-sided, as isthe board for the display module.If the holes in either of the double-sided boards are not plated-through, feed-through wiresmust be used instead. All pads onthe top and bottom of the boardsmust be soldered to the compo

i

nent lead or feed-through wire.(The boards available from thesource mentioned in the partslist are plated-through.)

To assemble the CPU board, follow the'parts-placement diagramshown in Fig. 8. Install the resistors, capacitors, connector,switch. LED. and transistor. Thecrystal may be fastened to theboard with foam tape or RTV silicone cement. IC sockets should

be used to ease any future repairs

CPU BOARD

All resistors are TA-watt, 5%, unlessotherwise noted.

R1—10 megohmsR2—1 megohmR3—100,000 ohmsR4—22,000 ohmsR5—10 ohmsR6—-2700 ohms, SIP resistor (cut to fit

board)CapacitorsC1-C3—0.1 (j.F, monolithicC4—1 |xF, 25 volts, tantalumC5—100 (xF, 16 volts, electrolyticSemiconductorsIC1—CDP1802CE microprocessor (GE/

RCA)IC2—HM6116LP-4 RAM

IC3—27C16 CMOS EPROMIC4—4013 dual D-type flip flopIC5—4520 dual synchronous up counterIC6—4081 quad 2-input AND gateIC7—4584 hex Schmitt triggerIC8—MM74HC374N octal tri-state D-

type flip-flopLED1—-red light-emitting diodeQ1—2N4401 NPN transistorOther componentsXTAL1—2 MHz crystalS1—PC-mount slide switchJ1—10-pin headerMiscellaneous: 10-conductor ribbon ca

ble, 3 Vi-inch#6 spacers, 3 #6-32 x Vz-inch screws and nuts, PC board, ICsockets, wire, solder, etc..

ANALOG BOARD

All resistors are Vi-watt, 5%, unlessotherwise noted.

R1—50,000 ohms, 25-turn trimmerpotentiometer

R2—not used

R3, R4—40,200 ohms, Vfe-watt, 1%, metal-film

R5, R16—8060 ohms, Va-watt, 1%, metal-film

R6—2000 ohms, '/n-watt, 1%, metal-filmR7, R8—1 megohm, Vs-watt, 1%, metal-

film

R9, R13—100,000 ohms, V's-watt. 1%.metal-film

R10—20,000 ohms, '/e-watt, 1%, metal-film

R11—100,000 ohms, 5% resistor, 1/4WR12—-47 ohmsR14—1000 ohms

R15—6800 ohms, '/a-watt, 1%, metal-filmCapacitorsC1, C5—4.7 m-F, 16 volts, tantalumC2-C4—1 ,xF, 25 volts, tantalumC6—0.01 |xF, 50 volts, 5% film-typeSemiconductor

IC1—SCX15AN pressure sensorIC2—LP2950CZ-5.0, 5-volt regulatorIC3—LM324N op-ampIC4—LM33IN voltage-to-frequency

converter

D1—1N4002 diode

Miscellaneous: 9-volt battery and clip,PC board, IC sockets, wire, solder, etc..

GROUND FROM J_ANALOG BOARD-

INPUT FROMANALOG BOARD

, +9Vd> TO

RS ANALOG

° V# FR0MB1

>+5\TFR0MANALOG BOARD

.+9V

FIG. 8—WHEN ASSEMBLING THE CPU BOARD, the crystal should be fastened to theboard with foam tape or RTV siliconecement to prevent damagedue to vibration.

+9V +5VTO CPU BOARD TO

+9V FROM CPU BOARDf ? oCPUJOARDir

AGROUND TO CPU BOARD

ftB1

SOLDER LEADSr*jVjBATTERY TERMINALS!

MOUNT B1 ON FOIL |Ltop |̂|:&DEp_TAPE_

FIG. 9—THEANALOG-BOARDPARTS LAYOUT. Carefully install the pressure sensor and,if youeverclean the PC board, do not allow any solventormoisture to enterthe sensorport.

DISPLAY MODULEAll resistors are Vfc-watt, 5%, unless

otherwise noted.R1—1 megohmR2—22,000 ohmsR3—10 ohmsCapacitorsC1—470 pF, ceramic discC2—1 |iF, 25 volts, tantalumSemiconductorsIC1—MM5483N display driverDSP1—LCD009 LCD moduleLED1—red light-emittingdiodeQ1—2N4401 NPN transistorOther componentsJ1—10-pin headerS1, S2—momentary pushbutton switchS3—SPST toggle switchMiscellaneous: 4 #6-32 x 1-inch screws

and nuts, 8 Vi-inch #6 spacers, case,clear plastic sheet for display window,40-pin wirewrap socket strip, PC board,wire, solder, etc..

Note: The following items are availablefrom Transolve Corporation, 13361Shady Lane, Chesterland, Ohio44026 (216) 341-5970: Pressure sensor, $42; PC board set, $35; completekit (except case), $135; EPROM only,$15; machined case and customEPROM's available on request. Forlarge-scale rocket kits contact NorthCoast Rocketry, P.O. Box 24468,Mayfield Heights, Ohio 44124. Formore information on model rocketryin general, contact the National Association of Rocketry, 2140 ColburnDrive, Shakopee, MN 55379.

6 / R,8ijsR16

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PLAYBACKMODE

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^

IC1 MOUNTSUNDERNEATH DSP1

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FIG. 10—DISPLAY BOARD parts-placement diagram. The LCD module isplugged into wire wrap socket stripsabove IC1.

(Transolve Corp. will not serviceany non-socketed unit).

Follow the analog-board partslayout shown in Fig. 9, and install the resistors, jumper wire,diode, and capacitors. Note thatC6 must be a film-type capacitor—a disc capacitor will causeexcessive drift with temperature.Next install the trimmer potentiometer and the IC s. Carefully install the pressure sensor as

shown. If you ever clean the PCboard, do not allow any solvent ormoisture to enter the sensorport, or you'll damage it. Youmust test the analog board beforeattaching it to the CPU board.

The display board parts-placement diagram is shown in Fig.10. Install the resistors, capacitors, connector, LED, and IC1.The LCD module is plugged intowire-wrap socket strips aboveIC1. Space the top of the strips lA-inch from the board. Make surethe LCD pins are perfectlystraight, and press the displayinto the socket strips. Thefinished analog/CPU assembly isshown in Fig. 11, and the displaymodule in Fig. 12.

Testing and calibrationConnect a 9-volt battery to the

+ and - battery input pads onthe analog board. Connect yourDVM and scope ground leads tobattery -. The regulator output(IC3 pin 4) should measure 5volts. Set Rl to midpoint. Connect scope probe to IC4 pin 3;this output signal should be ashort, negative-going pulse, repeating at about 3.7 kHz. AdjustRl to obtain that value. Use a frequency counter if one is available. Apply suction to sensor portA (draw a vacuum with yourmouth) and verify that the signalfrequency decreases slightly.

If the analog board is functioning, it can now be attached to theCPU board. The wire attachmentpoints are designated in the threeparts layouts. The 9-volt battery'spositive lead connects to the CPUboard, and the 9 volts from theCPU board is jumped over to theanalog board. Also remember toconnect ground, +5 volts, andthe analog output between thetwo boards. A rocket is a veryhigh vibration environment, sothe 9-volt battery snap must betaped on, or the leads must besoldered to the battery. After theelectrical connections are madebetween the two boards, the analog board is fastened to the CPUboard with three screws, spacers,and nuts.

Wire the pushbutton and toggle switches to the display PCboard as shown in the display-module parts-placement diagram. One of the normally openpushbuttons is used to select

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peak, and the other zero. The toggle switch selects the playbackmode (when closed). Install theheader connectors on the ribboncable that goes between the CPUand display boards, and connectthe two boards together.

Turn the power switch on andopen the playback switch. Thesample LED on the CPU and display boards should be flashingfour times per second. A value ofseveral thousand feet should bedisplayed. Adjust Rl on the analog board for a reading of 100 feet,then 50 feet, and the unit willthen be calibrated for groundlevel. Do not adjust past thatthreshold, or the altitude mea-

MICROPROCESSORPOWER

ON/OFFEPROM

ANALOG

BOARD

FIG. 11—THE FINISHED ANALOG/CPU ASSEMBLY. It fits in the payload section of arocket.

LISTING 1

000000 cO 00 c5 f8 00 b3 f8 fa a3 7a e3 65 7b 7a 7b 7a000010 f8 20 b5 bl f8 06 a5 f8 02 al f8 00 b2 f8 fO a2000020 f8 02 55 el 82 f4 a2 e5 02 f2 32 31 f8 fb a3 30000030 34 f8 fa a3 7a e3 65 7b 7a 05 fe 55 3a 27 11 81000040 fb 06 3a la f8 fb a3 7a e3 65 7b 7a 65 65 f8 00000050 a9 b9 19 99 fb 10 3a 52 cO 00 db f8 00 a4 b4 3c000060 5f 34 61 14 3c 63 f8 00 a5 f8 05 b5 cO 01 85 fb000070 00 3a 6c cO 01 Oe f8 05 al f8 20 bl el f8 00 73000080 73 73 51 c4 c4 f8 05 al 01 32 90 f8 00 51 30 95000090 f8 05 51 30 bb 21 01 fb 09 32 al f8 01 f4 51 30OOOOaO bb f8 00 73 01 fb 09 32 af f8 01 f4 51 30 bb f8OOOObO 00 73 01 fb 09 32 bb f8 01 f4 51 24 94 3a 85 84OOOOcO 3a 85 cO 00 03 f8 00 a6 b6 a7 b7 f8 20 ad bd cOOOOOdO 01 cO c4 c4 c4 c4 c4 c4 c4 c4 c4 35 e7 37 e4 36OOOOeO ea cO 01 45 cO 01 07 cO 01 00 cO 01 78 c4 c4 c4OOOOfO fc 60 da f2 66 b6 be eO fe e6 00 01 02 00 c4 c4000100 96 b4 86 a4 cO 00 76 f8 00 a7 b7 f8 20 ad bd cO000110 00 df c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 94 bf000120 84 af 96 be 86 ae 9f 3a 2c 8f 32 36 2f 9e 3a 33000130 8e 32 26 2e 30 26 9e 3a 3c 8e 32 3e 30 42 94 b6000140 84 a6 cO 01 aO f8 00 a4 b4 3c 49 34 4b 14 3c 4d000150 f8 00 a5 f8 05 b5 94 3a 61 84 fb 01 3a 61 cO 01

000160 70 24 25 95 3a 56 30 5e c4 c4 c4 c4 c4 c4 c4 c4

000170 94 5d Id 84 5d Id 30 le 4d b4 4d a4 f8 00 a9 b9

000180 19 99 fb 30 3a 80 30 aO c4 c4 c4 c4 c4 c4 c4 c4

000190 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4

OOOlaO 97 fb 03 3a af 87 fb fO 3a af 2d 2d cO 00 76 17

OOOlbO 30 ac c4 c4 c4 c4 c4 c4 c4 c4 c4 C4 c4 c4 c4 c4

OOOlcO f8 20 a5 b5 f8 00 55 15 f8 01 55 15 95 fb 28 3a

OOOldO c4 cO 00 db c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4

OOOleO c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4

OOOlfO c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4 c4

000200 c4 • • •-

•6'/4 INCHES -

THE ANALOG BOARD USES THIS FOIL PATTERN.

FIG. 12—THE DISPLAY MODULE can be

housed in any kind of project case you canfind.

surements may be inaccurate. If not cycle. Ifa 50-foot reading can-the potentiometer adjustment is not be achieved, your altitudefar off, the microprocessor may above sea level may be excessive.

CDAtP.

THE COMPONENT SIDE OF THE CPU BOARD.

J^l^MtedJrO^

-4a/« INCHE8-

THE SOLDER SIDE OF THE CPU BOARD.

r.Lz"EBBB

olo 0000000009066 pjo 6-o

0 00 0

ooelo 660000000000000

oololo 000000Q0

-3V* INCHES-

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In that case, simply adjust thevalue of R16 on the analog board:increase it by IK to decrease thereading by 1000 feet, or decreaseit by IK to increase the readingby1000 feet. Note that the potentiometer has enough range to allow for "simulated flights" ofthousands of feet.

If the unit doesn't run, checkfor correct parts placement, solder bridges, and other defects.Verify that a 2-MHz signal existson IC1 pin 1. The crystal circuithas a very high impedance. Anymoisture or contamination mayprevent oscillation (rosin fluxwon't hurt). Touching pins 1 or39 of the microprocessor cancause the program to crash!Spraying the crystal area withclear lacquer is recommended, lbreset the program, turn thepower off for 5 seconds, thenturn it back on. Removing thebattery power will erase data.Switching the playback switch tooff will resume data logging atwhatever sample was last displayed. The unit must be reset(turn off for 5 seconds, then on)before the next flight.

Prepare for launchMount the flight-recorder in

the rocket payload section. Packit securely with foam or someother support so that it will notrattle during flight. Punch several 'A-inch holes in the bodytube near the sensor. An access

port may be cut out to allow theribbon cable to be attached.

•37* INCHES-

DISPLAY MODULE COMPONENT SIDE. DISPLAY MODULE SOLDER SIDE.

45

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Activate the unit with the display connected and verify groundcalibration (a 50-foot reading).Unplug the display, verify thatthe sample LED is flashing, andsecure the hatch.

Before launching, however, itis important to observe certainsafety precautions in order toavoid unnecessary injury orproperty damage. First of all, always use properly constructedrockets, launchers, and factory-loaded NAR-certified rocket engines. A model rocket should always have a parachute recoverysystem. Never launch a rocketwith a flammable, explosive, orlive payload. Make sure that thelaunch area is free of obstructions such as trees, power lines,and low-flying planes. Also avoidlaunching rockets on windy daysor when clouds will obstruct yourview of the rocket. To avoid firehazards, never launch a rocketfrom ground covered with drygrass or shrubs. Always makesure that nobody is near thelaunch site, especially children.

Launch the rocket using a remote ignition system. About fourminutes of data will be stored,including the time on launchpad. When you recover the rocket, plug in the display, press thepeak button, and the peak al-titude achieved will be displayed.

Next, switch the playback toggle to the closed position. Pressthe zero button (hold it for twosample LED flashes) and release.This will start playback from location zero at Va speed (one sample per second). The flight can beplayed back as many times as desired by pushing zero. The zerobutton may be pushed at anytime to restart.

Conclusion

The collected data may be usedto determine the performance ofa model rocket. Many modelersare flying high-performance composite motors in their "birds" allowing altitudes of thousands offeet to be reached. Use of the altimeter can help optimize rocketdesigns to get maximum altitudefor a given engine size.

Non-rocket uses of the systemmight include kites, hot-air balloons, hang-gliders, skydivers.and mountain climbers. Whatever your application, be careful... and have fun! r-e