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Build this circuit and you can turn your car's burglar alarm on and off by pressing the button on a small keyring transmitter. By JOHN CLARKE The main purpose of this project is to add a remote control facility to the Protector Car Alarm described in February. Basically, it lets you switch your car's burglar alarm on and off from outside the vehicle, simply by pushing the button on a small keyring transmitter. Apart from the obvious added convenience, remote control offers improved security (no need for hidden switches) and allows the elimination of entry and exit delays. All sensors can now be wired to the instant trip input to give instantaneous alarm response to any attempted break in. With this unit, you can customise your burglar alarm to suit your own needs, making it quite complex (as presented) or quite a lot simpler. For example, it gives you a choice of piezo alarm or siren to tell you when the alarm is turned on or off and it will also flash the car's traffic indicators briefly when the alarm is armed or disabled. The circuitry presented here could also be used as the basis for any single channel remote control application. It is simple to set up and very reliable. The accompanying panel lists the main features. As you can see, it's a very versatile circuit although the basic concept is fairly simple. ICl is a Motorola MC145026 trinary encoder. This uses a 9-bit trinary code. Trinary code is like binary code except is uses three logic states instead of two. The three trinary states used by this IC are high, low or open-circuit. Only one transmitter code word (one 9-bit word) is possible, as selected by the connections to the IC's nine address inputs, Al to A9. In keeping with the trinary states just mentioned, each of the nine address inputs can be connected to the + 12V rail, to ground or left unconnected. In our application, because of the particular decoder used in the receiver, the A9 input must be either connected to the + 12V rail or ground. The 9-bit code word is sent as a series of pulses from pin 15 of ICl. The frequency of the pulses is set by the two resistors and the capacitor connected to pins 11, 12 and 13. For our circuit, the frequency is about 2kHz. Transmitter Main Features The transmitter comprises a digital encoder integrated circuit (ICl) and a UHF oscillator operating at 304MHz. Fig.1 shows the details. • • • • • 11 • • • • • SILICON CHIP UHF transmitter and receiver on 304MHz Two versions of the handheld transmitter. Transmitter uses a single IC and one transistor. Single button to activate and disable alarm. 40-metre transmitter range. Trinary digital coding wiMl 13,122 codes for security. Multi-optioned receiver (build it as you want it). Relay switch-on and off for any burglar alarm. Momentary traffic indicator flashing for alarm set. Piezo alarm or siren for audible indication of alarm set. Pushing the Transmit button (S1) causes the IC to deliver the coded word from pin 15. This is used to key the UHF oscillator Ql on and off at a rate of 2kHz. When pin 15 is high ( + 12V), Ql oscillates. Ql is a BFR91A, a surface mounting transistor intended for use in UHF and microwave amplifiers. Inductor L1 and the 2-6pF capacitor The Remote Switch can be teamed with the Protector Alarm, or with any commercial alarm. The small keyring transmitter is in the foreground. form a tuned circuit load for the collector of Ql. Its base is grounded (to AC signals) by a 4 70pF capacitor. Stray capacitance between the emitter and collector of Ql provides positive feedback which causes it to oscillate, at 304MHz. To increase the oscillator's output, the emitter degeneration resistor is bypassed with a 1.5pF capacitor, which is critical in value. The size of this capacitor cannot be too large since it would reduce the positive feedback and thereby stop oscillation. The transmitter is powered from a 12V lighter battery (VR22, EL12, GP23 or equivalent). Actually, the circuit could be made to work at voltages down to 4.5V but the selected battery is the one best suited for the job since it is so compact. The battery is bypassed by a 0. lµF capacitor located near IC1 and by a 0.047µF capacitor near the tuned circuit for Ql. When S1 is closed the current drawn by the circuit is a few milliamps, the exact figure depending on the code word selected at Al to A9. The current through LED 1 is about 7mA. When S1 is open, the current drain is less than 200 nanoamps (0.2 microamps). The transmitter can be built into one of two cases which are small enough to be attached to a key ring. T m: 16 ..I.. 1 LOW 14 IC1 MC145026 47k 15 10k 470pFl .,. 13 12 TE 11 .,. t ENCODING OPTIONS 100k .0022 220k + E TYPE MARKING .,. L1 : 32mm OF 0.71mm TINNED COPPER WIRE FOR SMALL VERSION. LARGER VERSION USES PCB TRACKS. toN LARGE VERSION ONLY UHF REMOTE ALARM SWITCH TRANSMITTER SCD3·1·28B Fig.1: the transmitter uses an MC145026 trinary encoder IC to key UHF oscillator Qt on and off. A1-A9 are connected to give the address code (see text). We'll talk about these later in the section on construction. Receiver The receiver circuit is shown in Fig.2. It consists of four sections: an RF input amplifier and detector (Ql and Q2), a tuned 2kHz amplifier (IC1), decoder IC2, and optional relay driver circuitry. The transmitted signal is picked up by the antenna which is loaded MARCH 1988 27 This view shows the UHF switch receiver installed in the same case as the Protector Car Burglar Alarm described in February. Connections between the two PCBs are via PCB-mounting terminal blocks (see Fig. 12). by inductor L1. The signal is then coupled via a .001µF capacitor to the base of Qt , which is an RF amplifier with a tuned collector load. Signal from the collector of Qt is fed from a voltage divider consisting of a 2.7pF and a 22pF capacitor to self-oscillating detector stage QZ. This operates on the principle that whenever signal is received the circuit oscillates at 304MHz, but when the signal is not received, the circuit is quiescent. The detected signal from QZ is extracted from the 0.001µF capacitor connected to its base. This capacitor bypasses the 304MHz signal but not the ZkHz pulse modulation which is superimposed on the signal fed to the antenna. This ZkHz pulse signal is ACcoupled via a 2.2µF capacitor to ICta, an inverting op amp with a gain of about 470. ICtb is a Schmitt trigger. It squares up the amplified signal from ICla before feeding it to ICZ, the trinary decoder. ICZ is a MC:::145028 decoder which is compatible with the MC145026 used in the transmitter 28 SILICON CHIP circuit. It is set up to respond only to the unique code word sent by the transmitter. This is done by connecting the address inputs Al to A9 in exactly the same way as for IC1 in the transmitter. When IC2 detects a correct code from the transmitter, the output at pin 11 goes high and charges the 2.2µF capacitor at the input of IC3a via the 2.2k0 resistor and diode Dl. It takes about 5ms before the output of Schmitt trigger IC3a goes low. When transmission ceases, the output of ICZ goes low and the 2.2µF capacitor discharges via the 470k0 resistor. This takes about one second after which the output of IC3a goes high again. This delay is to prevent false triggering. Just how the circuit operates from this point on depends on how you build it. For example, you could decide to use it to momentarily close relay RLY1 every time the transmitter button was pushed. To accomplish this, connect link LK1 (following IC3a), leave out link LK2 and omit IC4, IC5, Q4 and all the associated components. If you do this, pins 8 and 9 of IC3 should be connected to pin 7. Every time the transmitter button is pressed, the resulting momentary low output from IC3a causes IC3d to go momentarily high and turn on transistor Q3. This closes relay RLY1 for one second. Alternatively, you can go for a more complex circuit function by omitting link LK1 and installing link LKZ instead. Now, with ICZ detecting the valid code and the resultant pulse delayed and squared up by IC3a, a further pulse inversion takes place in IC3b, before the signal is fed to the clock input of IC4a, a D-type flipflop. This is connected to change states when it receives a clock pulse. The Q-bar output of IC4a connects to the input of IC3d via link LKZ. IC3d drives transistor Q3 when its output is high and this in turn operates RL Yt. Thus, relay RLY1 closes at the first push of the transmitter button and opens with the second push on the button. This function could be used to turn any commercial burglar alarm on and off. Audible options IC5 is a CMOS 555 timer connected as a monostable (ie, con- 4.m . . - - - - -....- - - + - - -....-'JW,-----------..------~>------..---- ANTENNA +8V .01J 4.7M 10k F16 L2 22pF L1 +4 3.3pF 10 10k.,. .,. + 16VWJ .,. .001! .,. L1 : 190mm OF 0.63mm ENAMELLED COPPER WIRE. 15T ON 3.2mm FORMER L2 : 65mm OF 0.71mm TINNED COPPER WIRE. 1.5T ON 5mm FORMER. F16 FERRITE SCREW CORE. 8V + 100 16VWJ .,. 9 INPUT ..----1--+12v I 05 1N4002 LK1 : MOMENTARY RELAY 1 LK2 : ON/OFF RELAY 1 2x1~ioo2 RELAY 1 ...+-M-+-011 RIGHT D1 1N4148 2.2k OUTPUT LK1 06-11>-----..,.09 LK2 .---+----+12v _ _ _..__ _ _ ___,_ _ _ _ _ +8V 02 1N4148 100k PIEZO SIREN VR1 1M 12 47k .022I .001 .,. IC5 TLC555 10 +12V 180k .,. .,. 0.1+ .,. PIEZO TRANSDUCER 13 10 +12V COMMON 470k 11 + 2.2 16VW+ DECODING OPTIONS INDICATORS 214 100k Z01 33V 1W 100 16VW + 1_ 00k _ IC3b CHASsIsn.:1- - - . . __ _ _...._ _ _ _ _- 1 . - - . - .0471 1/E! PLASTIC f1 lmUT 1 s-··.6: i'"'"' m + 8V r - - - - - - - - 0 7 DISABLE TYPE Q1 50 BELOW GNO 3 CK s 4013 IC4a 2.2k ii2 R 7 4 8V 11 16VW 470k SC03·H88 + .011 10 16VW "-4.._._._,......,_.-tiF-Q& OFF Q 12 11 CK .,. .,. UHF REMOTE ALARM SWITCH RECEIVER D4 1N4148 14 IC4b 13 9D R 10 s 8 .,. 03 1N4148 '----li-tll--+---Q 8 ON Fig.2: the receiver circuit can be built so that relay 1 provides momentary or on/off switching. MARCH 1988 29 . •' TRANSMITTER ~TINPLATE 15mm x 5mm SCOJ-1-288-3 TINPLATE /15mm x 5mm ~ '-------4.---' ~ 4 x 1mm x 6mm WIRE STAKES Fig.3: battery clip detail for the small transmitter. Fig.5: parts layout for the small transmitter PCB. The S1 switch contacts are made from tinned copper wire. nected to deliver one pulse when it is triggered). When the output of IC3a goes low (when a transmission occurs), pin 2 of IC5 is pulled low and the output at pin 3 goes high, for about 120 milliseconds. The timing is mainly determined by the 0.22µF capacitor at pins 6 and 7 and the lMO trimpot and lOOkO resistor connected to the + 8V line. The timing is modified by the 100k0 resistor connected to pin 5 from the Q output of IC4a. Pin 5 controls the threshold voltage of the comparators within the IC. When the Q output of IC4a is high, pin 5 is pulled higher than its nominal setting (2/3 Vee) and the PLASTIC SUTTON ~-;::::::::>=====b TINPLATE 16mm x 5mm SCREW ANO NUT 1.5mm OIA. x 3mm Fig.4: the switch for the small transmitter is made from ·a piece of tinplate and a small plastic button. Fig.6: to code the transmitter each A1-A8 input is connected to the high rail, the low rail, or left open circuit. A9 must be connected high or low. timing period becomes longer. When Q of IC4a is low, pin 5 is pulled lower and the timing period becomes shorter. Pin 3 of IC5 drives transistor Q4 to turn on the piezo siren. Because the voltage on pin 5 of IC5 is controlled by the Q output of IC4a, the siren emits a short burst of sound when IC4a is clocked to the off state and a longer burst when IC4a is clocked to the on state. Pin 3 of IC5 is also connected to pin 9 of IC3c. When pin 3 of IC5 is high, IC3c oscillates and drives the piezo transducer. This is a lower cost alternative to the piezo siren driven by Q4. Traffic indicator option Relay RLY2 is also driven by Q4 via diode D6. It has a 2200µF capacitor connected across it and this is charged via D6, Q4 and the 120 resistor. The resistor limits the initial surge current while the 2200µF capacitor is used to keep the relay energised for about a second after Q4 turns off. The contacts of relay RL Y2 are arranged so that they can switch on the vehicle indicators for a short time to provide visual indication of a received signal. Diodes D7 and D8 isolate the left and right indicators. IC4b is clocked by the pin 3 output of IC5 while its D input, pin 9, is connected to the Q-bar output of IC4a via a delay network consisting of the 2.2k0 resistor and O.OlµF capacitor. The delay ensures that IC4b is clocked with data from Qbar of IC4a before it changes state. The Q outputs of IC4b follow the Q outputs of IC4a and are used to provide on and off signalling for the Protector Alarm described in the February 1988 issue. The Q output connects via diode D3 to the on input of the Protector alarm, while the Q-bar output is capacitively coupled to the off input. This provides a short pulse which is sufficient to switch off the alarm circuit. A further output from the. Q-bar output is used to provide the Disable control. This can be used to disable an ultrasonic movement detector which we hope to describe in a future issue. Power for the circuit comes mainly from a 7808 3-terminal regulator. This isolates the sensitive circuitry from the 12V automotive electrical system. A 33V zener diode protects the input of the regulator from any voltage spikes on the 12V line. Construction Fitting the parts in the Jaycar case is bit of a challenge but it can be done if the parts are 'squashed' down on the PCB. The pen points to the two switch contacts which are made from looped tinned copper wire. 30 SILICON CHIP As noted above, the handheld transmitter may be built in one of two cases, one from Dick Smith Electronics and one from Jaycar Electronics. We have designed two transmitter boards to suit the two cases. The larger of the two cases is from Dick Smith Electronics. It measures 31 x 58 x 17mm (DSE Cat No H-2497). The printed board to I 12V J + Fig.7: alternative transmitter PCB for the DSE case. The .0022uF capacitor lies flat across the IC (see photo). Fig.8: Al-A9 address pins for the alternative transmitter. Make sure the transmitter code matches the receiver. The larger of the two transmitters is still compact enough to fit your keyring. The two transmitters look different but their circuits are the same. Coil L1 is part of the PCB pattern for the larger version, while the smaller version uses a wire loop. Power comes from a 12V lighter battery. suit it measures 46 x 33m (SC code 3-1-288-2). The case from Jaycar is smaller, measuring 34 x 43 x 13mm (Jaycar Cat No HB-6072). The board to suit measures 30 x 30mm (SC code 3-1-288-3). While the Jaycar case is notably smaller, it has the disadvantage that a switch and battery clips are not available and will have to be made. Nor can the LED indicator be fitted into it. The Dick Smith case is supplied with battery clips and a commercial switch can be used. Construction of the transmitter in the Dick Smith case should be straightforward. The smaller transmitter is more difficult to construct due to the necessity to make the switch and battery clips. We made the battery clips for ours from pieces of tinplate 15mm long by 5mm wide. They are each soldered to two wire stakes on the PCB. This is shown in Fig.3. The switch is also made using tinplate. It is secured to the lid of the case using a screw and nut. A small plastic button is glued to the tinplate as shown in Fig.4. Contacts for the switch are mounted on the PCB using tinned copper wire loops. These are raised about 4mm above the PCB surface but some adjustment in height may be necessary to provide a satisfactory switch action. The component layout for the smaller transmitter board is shown in Fig.5. The smaller transmitter PCB requires a 5.5mm hole to clear the screw pillar in the lid and a 5mm hole for the transistor. Before assembling components on the PCB, check that the PCB will fit within the case. You may need to file off the corners of the PCB so that it will follow the internal corner radius of the case. Install the IC with pin 1 towards the battery clip side of the PCB. The transistor is mounted on the underside of the PCB. Tin the PCB tracks with solder before soldering the transistor leads in place. The remaining components are not so easily installed. None of the parts can sit more than 6mm above the PCB surface to avoid fouling the lid of the case. To achieve this low profile, the resistors are mounted end on and bent over so that they lie close to the PCB. The smaller capacitors can be mounted upright, however the larger ceramic capacitors should be bent over. The 0.0022µF greencap should be mounted side on. The trimmer capacitor can be mounted in the normal fashion. The 11 inductor is made using 32mm of 0.71mm diameter wire looped and laid flat on the PCB as shown in Fig.5. The larger transmitter version in the Dick Smith case uses the SC03-1-288-2 PCB. Components for this PCB can be installed as shown in Fig.7. The battery clips are pre-shaped and are simply inserted into the PCB and soldered on the underside. Now install the IC and link. Some MARCH 1988 31 Receiver ANTENNA INPUT Fig.9: parts layout for the receiver PCB. For momentary relay switching, install LK1 and omit LK2, IC4, IC5, Q4 and associated parts. Also, connect pins 8 and 9 of IC3 to pin 7. For latched contacts, omit LK1 and install LK2 and all parts. L2 FORMATION DIMENSIONS IN MILLIMETRES Fig.10: L2 is wound using 0.71mm tinned copper wire. resistors are mounted flat on the PCB while others are mounted end on as shown on the overlay. All the capacitors are mounted flush against the PCB except the .0022µF greencap which is bent to lie flat over the top of the IC. The switch is mounted so that the flat side of the switch body is towards the battery terminal end of the PCB. The LED is mounted 11mm above the PCB surface. The transistor is mounted on the underside of the PCB. Tin the PCB tracks with solder before finally soldering the transistor pins in place. Fig.11: connect the A1-A9 receiver inputs to exactly match the transmitter code. A1-A8 can be high, low or open circuit; A9 must be tied high or low. 32 SILICON CHIP The UHF receiver is built on a PCB coded 03-1-288-1 and measuring 132 x 87mm. It can be installed in a plastic utility case measuring 159 x 96 x 51mm or, if you have made the Protector Burglar Alarm, you can build it into the same case. Begin construction of the receiver by installing all the low profile components such as the resistors, links, diodes, and ICs. When installing the links, decide whether relay RLYl is to be wired with momentary or on/off operation and install either link 1 or link 2 accordingly. Fig.9 shows the receiver board with all parts installed. We assume that many readers will build versions with some of the options omitted. If this is the case, examine the layout diagram carefully to determine what parts can be left out. The BFR91 transistors are mounted on the underside of the PCB. Before mounting and soldering each of these transistors, tin the tracks with solder. This makes it easier to solder each transistor into place. L1 is made using a 190mm length of 0.63mm enamelled copper wire wound around a 3.2mm (1/8-inch) drill bit. Wind on 15 turns and strip the insulation from the ends with a sharp knife before soldering it to the PCB. 12 is wound on a 5mm plastic former which is fitted into the PCB so that it is a tight fit. The winding details are shown in Fig.10. Don't forget to screw in the F16 ferrite core. Continue construction by installing the capacitors, relays, remaining transistors, the 3-terminal regulator and the insulated terminal block. Take care with the orientation of the electrolytic capacitors and transistors. The antenna is simply a 300mm length of hookup wire soldered to the antenna input pad on the board (see Fig.9). The receiver can be mounted in its own case using PCB standoffs. A Scotchcal label measuring 90 x 153mm is secured to the front panel. The artwork is shown in Fig.13. Alternatively, the PCB can be PROTECTOR ALARM : 1,2 + 12V TO VEHICLE BATTERY e 3 TO BATTERY BACKUP VIA 3A FUSE-- - - - -- - -- - -•40N - - - 5 OFF 6 POWER GROUND ....- IGNITION COIL e e 7 VEHICLE BATTERY e 8 IGNITION e 9 INSTANT e 10 DELAY a, GND 5A ALARM CONTACTS 4, DASHBOARD FLASHER PIEZO SIREN 1 ,----------------+-12-V14-......,.._----, - ~ -~•m,s PIEZO TRANSDUCER 13 - - : - - PIEZO SIREN 12 ~ INDICATORS R~~~~ : ~ ~ -1------:: COM MON 9 ON 8 DISABLE 7 . . . . _ OFF 6 NORMALLY CLOSED 5 NORMALLY OPEN 4 COMMON 3 UHF REMOTE ALARM SWITCH + 12V 2 GROUND 1 ALTERNATIVE CONNECTION e-!- -..., --- TO "DISABLE" ON e ULTRASONIC DETECTOR e I e : ::::"---Jf----f:2:'.Jr--;::n±-_--=.IJ:::;--....... 1 CHASSIS Fig.12: here's how to wire the UHF remote switch to the Protector car alarm. The piezo transducer can be omitted if you have fitted the piezo siren. mounted in the Protector alarm case as shown in one of the photographs. It is mounted on 15mm standoffs at the terminal end of the PCB and supported using Ushaped brackets at the opposite end. Fig.12 shows how the UHF remote switch is wired to the Protector car alarm. The remaining connections to the Protector alarm are as shown in the February issue. Testing and alignment Both the transmitter and receiver must be coded before they can be tested. Figs.6, 8 and 11 show the Al to A9 code inputs on the copper side of the PCB for two transmitters and the receiver respectively. Note that the receiver code must exactly match the transmitter code, otherwise the unit won't work. Initially, to allow testing, we recommend that only the A9 input of the transmitter and receiver be coded. This input must be bridged to either the high or low rails (it must not be left open circuit). The transmitter frequency must This fully-optioned receiver board features on/off switching for relay 1. The second relay (top right) provides the traffic indicator option. be set to 304MHz by using a frequency meter. Temporarily connect pin 15 of ICl to the positive rail. This will set the oscillator in operation. Now hold the transmitter near the input of the frequency meter and adjust the trimmer capacitor for a reading of 304MHz. In some cases it may be necessary to connect a coil of wire between the inM ARCH 1988 33 r: :-J C L: PIEZO TRANSDUCER PIEZO SIREN RIGHT INDICATORS [ LEFT COMMON ON DISABLE OFF NORMALLY CLOSED NORMALLY OPEN COMMON +12V GROUND UHF REMOTE ALARM SWITCH Fig.13: actual size reproduction of the front panel artwork. I SC03-1·288·2 Flg.14: etching pattern for the larger transmitter PCB. Fig.15: etching pattern for the small transmitter PCB. Fig.16 (right): etching pattern for the receiver 34 SILICON CHIP r 14 13 12 11 10 9 8 7 6 ~ 2 1 PARTS LIST Transmitter 1 transmitter case (Jaycar Cat. HB6072, 34 x 43 x 13mm; or DSE Cat. H-2497, 31 x 58 x 17mm) 1 PCB, code SC03-1-288-3, 30 x 30mm (for Jaycar case); or SC03-1-288-2, 46 x 33mm (for DSE case) 1 PC-mounting pushbutton switch, DSE Cat. S-1 200 (for DSE case) 1 3mm LED (for DSE case) 1 50mm x 5mm tinplate (for Jaycar case) 1 5mm x 4mm plastic button (for Jaycar case) 1 1.5mm dia x 3mm screw plus nut (for Jaycar case) 1 12V lighter battery (VR22, EL 12, GP23 or equivalent) 32mm 0.71mm tinned copper wire (for L 1, Jaycar case) Semiconductors 1 BFR91 NPN lJHF transistor 1 MC145026 trinary encoder Capacitors 1 0.1 µ.F miniature polyester 1 .04 7 µ.F ceramic 1 2200pF metallised polyester (greencap) 1 4 70pF ceramic 1 1.5pF ceramic 1 2-6pF ceramic trimmer Resistors (0.25W, 5%) 1 X 220k0, 1 X 100k0, 1 X put and ground of the frequency meter to obtain a satisfactory reading. Once the frequency has been set, remove the temporary connection to pin 15. Now connect the receiver to a 12V power supply. Apply power and check that the output of the regulator is at + 8V. Next, connect a multimeter set to read DC volts between test point TPl and ground. Apply power and wait 10 seconds for the 2.2µ.F capacitor at the base of Q2 to charge. Adjust the slug in L2 for maximum signal when the transmitter switch is pressed (ie, for maximum reading on the DMM). You may need to progressively move the transmitter away from the receiver 47k0, 1 x 1 OkO, 1 x 1.5k0 (if LED is required), 1 x 1 kO Receiver 1 plastic utility case, 159 x 96 X 51 * 1 Scotchcal front panel, 1 53 x 90mm* 1 printed circuit board, code SC3-1-288-1, 133 x 87 1 1 0-way PC-mounting insulated screw terminal block 1 4-way PC-mounting insulated screw terminal block 2 1 2V SPOT relays* 1 piezo transducer* 1 piezo siren* Semiconductors 1 MC145028 trinary decoder 1 TL062 low power dual op amp 1 4013 dual D flipflop* 1 4093 quad NANO gate 1 7808 3-terminal 8V regulator 1 TLC555 CMOS timer* 2 BFR91 NPN UHF transistors 1 BC337 NPN transistor* 1 B0681 NPN Darlington transistor* 3 1 N4002 1 A diodes* 4 1 N4148, 1 N914 diodes 1 33V 1W zener diode Capacitors 1 2200µ.F 16VW PC electrolytic* 2 100µ,F 16VW PC electrolytic and repeat the adjustment for L2 to obtain the setting for maximum sensitivity. Once adjusted, the receiver should be respond to a transmission by activating relays RLYl and RL Y2 (if fitted). Relay RL Y2 should close on receipt of a transmission and remain closed for about one second. Connecting a piezo transducer between terminals 13 and 14 or a piezo siren between terminals 12 and 14 will then provide the audible indicator for the receiver. A short burst of sound will be heard during the off transmission and a longer burst of sound during the on transmission. Trimpot VRl adjusts the lengths of these tone bursts. 3 1 Oµ.F 16VW PC electrolytic 2 2.2µ.F 16VW PC electrolytic 1 1µ.F 1 6VW PC electrolytic 1 0.22µ.F PC electrolytic 2 0.1 µ.F metallised polyester 1 O. 04 7 µ.F metallised polyester 1 0.022µ.F metallised polyester 1 0.01 µ.F metallised polyester 1 0 .01 µ.F ceramic 1 0 .001 µ.F metallised polyester 4 0 .001 µ.F ceramic 1 22pF ceramic 1 3.3pF ceramic 1 2. 7pF ceramic 1 2.2pF ceramic Inductors and wires L 1 190mm 0 .62mm enamelled copper wire L2 65mm 0 .71mm tinned copper wire, 5mm former DSE cat L-1010, F16 ferrite screw core L3 3 .3µ.H 300mm 1 mm solid core insulated wire (for the antenna) Resistors (0.25W, 5%) 2 x 4.7MO, 3 x 470k0, 1 x 180k0, 1 x 150k0, 6 x 1 OOkO, 2 x 47k0, 1 X 39k0, 1 X 22k0, 1 X 18kQ, 6 x 1 OkQ, 2 x 2.2k0, 1 x 1 kQ, 1 X 4 70Q, 1 X 270Q, 1 X 12n, 1 x 4.7n, 1 x 1Mn miniature vertical trimpot * optional text). components (see Coding The receiver and transmitter can now be coded using selected high, low or open circuit connections to Al to A8. Each Al to AB input can be bridged to the high rail, the low rail or left open circuit. For example, you could bridge Al to the high rail, A2 to the low rail, leave A3 open circuit, bridge A4 high and so on. It's a good idea to write your selected code down on a piece of paper before actually making the necessary connections. Make sure that the receiver and transmitter coding are identical. Finally, drip some molten candle wax into the screw core of L2 to prevent it from moving and thus detuning the receiver. It M ARCH 1988 35