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Compact Ultrasonic Car Burglar Alarm This compact ultrasonic car alarm is based on a proven commercial unit. It's extremely sensitive and will respond instantly to glass breakage, a door opening or movement inside the vehicle. By GREG SWAIN Ultrasonic car burglar alarms have one big advantage when compared to other alarm systems they're dead easy to install. All you have to do is position the two transducers next to the front windscreen pillars, plug the leads into the control unit, and hook up power and a siren. 58 SILICON CHIP There's no need to muck about running leads to door switches as with other burglar alarms. But ultrasonic alarms can have their drawbacks too. Many are prone to false triggering, either because they are temperature sensitive, or are prone to interference, or are sensitive to supply line glit- ches. Given these problems, it's no wonder that some alarms false trigger with monotonous regularity. This unit suffers no such problems. It's based mainly on discrete transistors and has been used for several years in commercial car alarm systems. So it's fair to say that this is one ultrasonic car alarm that has been thoroughly debugged. It's also much more compact than any previously described ultrasonic alarm. The printed circuit board, which holds all the parts, measures just 112 x 43mm and fits neatly into a moulded plastic case. The unit is compact enough to mount under the dashboard of any car. Recently, Oatley Electronics was THE TRANSMITTER TRANSDUCER PRODUCES A TONE WHICH HAS CONSTANT AMPLITUDE AND FREQUENCY +v PARTS LIST TIME TRANSMITTER ((~TSDUF 40kHz 1n1 -,V S T l ~ L M~Ji~~NT ENVIRONMENT +V -V MOVEMENT PRODUCES AMPLITUDE AND PHASE CHANGE IN THE RECEIVED SIGNAL Fig.1: the transmitter transducer produces a steady 40kHz signal which is picked up by the receiver. If movement occurs, the reflected signal undergoes phase and amplitude changes and the alarm triggers. able to purchase a large quantity of surplus parts for these ultrasonic alarms from the manufacturer, including PCBs, cases and transducers. Oatley Electronics has simply added a few on-board components and is offering the complete kit (with transducers) for just $42.95. That's really good value. To make up the kit, all you have to do is install the parts on the PCB and secure the board inside the case. By combining the unit with a UHF remote control switch, you will end up with a really effective burglar alarm for far less money than equivalent commercial units. The two transducers are even supplied prewired and are fitted with protective rubber " boots" and spring clips to allow easy attachment to the windscreen trim. The end of each transducer cable is fitted with a small locking connector and these plug into matching connectors on the PCB. Options You can either use this unit as an add-on ultrasonic sensor for an existing alarm or as a complete selfcontained alarm in its own right. If used to trigger an existing alarm, there's no need to worry about turning the ultrasonic sensor on or off. The main alarm system is simply turned on and off as before using a keyring transmitter or a hidden switch coupled with exit and entry delays. For use as a self-contained alarm, the unit must be combined with a remote control switch. This is because the ultrasonic circuit has no exit or entry delays; it must be turned on and off from outside the vehicle. Either the UHF remote control sold by Oatley Electronics or the SILICON CHIP UHF remote control can be used for this job. We'll have more to say about this later on. Finally, it's also possible to add bonnet and boot protection. Our circuit shows how this can be done using spring loaded pin switches wired to a single control line. Crystal control All ultrasonic movement detectors include an oscillator circuit which drives an ultrasonic transducer element (usually at 40kHz). Many designs use a free-running RC oscillator but this design uses a crystal locked oscillator to eliminate the need for oscillator frequency adjustment during construction. There's only one control on this unit that has to be adjusted and that's the sensitivity control. A crystal oscillator has far better stability than a free-running oscillator. In practice, this means that the unit can be set for far greater sensitivity without false triggering problems. On the other hand, ultrasonic alarms with free-running oscil- 1 PCB (available from Oatley Electronics) 1 plastic case 1 pair of transducers (pre-wired with connectors) 2 PC-mounting connectors 1 miniature SPOT relay 1 40kHz crystal 1 800mm length of 3-way cable 1 plastic cable tie Semiconductors 1 4069 hex inverter IC (IC1) 6 BC548A NPN transistors (01-06) 2 1 N4004 silicon diodes (04, D5) 1 1 N4148 silicon diode (03) 2 OA90 germanium diodes (01, D2) 1 3mm red LED Capacitors 1 4 7 µ,F 16VW electrolytic 1 22µ,F 16VW RBLL electrolytic 1 1 0µ,F 16VW electrolytic 2 4. 7 µ,F 16VW electrolytics 7 0. 1µ,F monolithics 2 .001 µ,F disc ceramics 1 220pF disc ceramic Resistors (0 .25W, 1 2.2MQ 1 5 1MQ 3 3 220k0 1 2 120k0 1 1 47k0 2 1 15k0 1 5 10k0 2 1 1 MO trimpot 5%) 4.7k0 2.2kQ 4 700 3300 2200 1000 470 lators frequently false trigger due to amplitude and phase changes in the oscillator output as the temperature changes. How it works An ultrasonic movement detector works by detecting amplitude variations in a single tone which is beyond the range of human hearing. In this design, the transmitter produces a continuous tone at 40kHz and this signal fills the area inside the car due to reflections from the inside surfaces. Some of this reflected 40kHz signal is picked up by the receiver transducer for processing by the ]UL Y 1989 59 The ultrasonic transducers are supplied pre-wired and fitted with rubber "boots" and spring clips. The unit is small enough to mount under the dashboard of any car. receiver circuit. As shown in Fig.1, a steady tone is picked up by the receiver if there is no movement inside the car. However, if movement does occur, the received signal undergoes amplitude and phase changes and these trigger the alarm circuit. Fig.2 shows the circuit details. There are six transistors in all, together with a single 4069 hex inverter IC. The circuit works like this: oscillator ICl b-ICld provides the 40kHz drive signal to the transmitter transducer which generates the ultrasonic output signal. This signal is picked up by the receiver transducer, amplified by tran- sistors Ql and QZ and then detected by Dl and DZ. If there is no movement, a steady DC signal appears at the output of the detector and no signal is applied to the base of Q3. Conversely, if movement does occur, the detector output fluctuates rapidly and these amplitude variations are amplified by Q3 and Q4. Q4 then drives Q5 which pulls pin 9 of ICla low. Thus, the output (pin 8) of ICla switches high and turns on Q6 to activate the relay and the external siren circuit. Let's now look at the circuit in more detail. The transmitter circuit employs three inverters from the Where to buy the parts Parts for this project are available from Oatley Electronics, 5 Lansdowne Pde (PO Box 89), Oatley, NSW 2223. Telephone (02) 579 4985 . Prices are as follows (mail orders add $2 .50 p&p): Complete kit (does not include siren or pin switches) ... ... ....... .. $42.95 PCB plus on-board parts only ...... ...... .... ........ .... .. ...... .. ... ... .. . $25.90 Pre-wired transducers with connectors (pair) .... ..... .... .. ...... ..... $1 9. 00 Note: copyright of the PCB artwork associated with this project is retained by Oatley Electronics. 60 SILICON CHIP 4069 hex inverter package. ICld is biased in the linear mode by the lMO feedback resistor and its output also biases IClc in the linear mode. ICld and IClc thus form a linear amplifier with an overall phase shift of 360°. A 40kHz crystal is included in the feedback path between the input and the output and so the circuit oscillates at the crystal's resonant frequency [ie, at 40kHz). Note that one side of the crystal is coupled into a voltage divider consisting of two 220k0 resistors. This has been done to prevent excessive drive voltages from being applied to the crystal. ICl b buffers the output from the oscillator (pin 4 of IClc) and drives the 40kHz transmitter. Receiver circuit The reflected signal from the transmitter is picked up by the receiver transducer and applied to the base of transistor Ql via a .00lµF capacitor. Ql is a common emitter amplifier stage with a voltage gain of approximately 100. Its output is coupled via another .00lµF capacitor to the base of QZ Fig.2 (right): reflected signals from the transmitter are picked up by the receiver, amplified by Qt and Q2, and detected by D1 and D2. Q3 and Q4 amplify the detector output and drive timer trigger stage Q5. which operates with a gain of about 40. Q2's output is coupled via a O. lµF capacitor to a "floating" detector stage consisting of diodes Dl and D2 and associated components. The advantage of this type of detector is that it produces very little output due to phase changes or slow changes in the amplitude of the 40kHz signal. This stops the unit from giving false alarms due to air turbulance generated inside the vehicle by temperature changes. Instead, the detector responds only to rapid amplitude changes in the received signal. And that's precisely what happens if there is movement inside the vehicle, or there is glass breakage etc. The output from the detector is coupled via sensitivity control VRl, a 4. 7µF capacitor and a low-pass filter stage (47k0 and O.lµF) to two more common emitter amplifier stages consisting of Q3 and Q4. These two stages operate with gains of 40 and 100 respectively. The low-pass filter stage provides further insurance against false triggering by rolling off the detector's output for frequencies above 30Hz. The amplified detector output appears at Q4's collector and is coupled to timer trigger stage Q5 via a 4.7µF capacitor. When Q5's base voltage exceeds 0.6V, its collector voltage goes low and quickly charges the 22µF timing capacitor via the associated 2.2k0 resistor. ICla's pin 8 output then switches high and turns on Q6 and the relay to sound the siren (or to trigger another alarm circuit). When the relay is energised, its normally closed contacts open and Q4's 470 emitter resistor loses its ground connection. Q4 and Q5 now turn off and the 22µF timing capacitor begins discharging via the parallel 2.2MO resistor. Thus, the 22µF capacitor and 2.2MO resistor determine the time taken for the circuit to reset. After about 40 seconds, pin 8 of CF-:. ~ a: <( ...J ~ 0 1- u w ljj Cl 1- zw ~ w > 0 ~ (.) z > + 0 "'a: <( _H•· Y ~ = + Wlv......-4........_H•· = ~ ::::> "" "" ~f-,•• ~ :,; = "' = I• ~ = -" + !;;H•· .,_-w,.,---1,. = "" JULY 1989 61 This larger-than-life size photo shows how the parts are mounted on the PCB. Note that all the transistors face the same way except for Q5 which faces in the opposite direction. The two connectors at left accept the transducer plugs. ICla switches low again, Q6 and the relay switch off, and the siren · stops.At the same time, the ground connection for Q4's emitter resistor is restored and so the circuit is reset, ready to detect any further movement. LED 1 is there to provide visual indication that the circuit is working. It lights whenever Q6 turns on and activates the relay. D4 protects Q6 from spike voltages when the relay turns off. Protection for the bonnet and boot is- provided by the normally open spring loaded pin switches. If either of these closes, Q3's collector is momentarily pulled to ground via a O.lJ.lF capacitor and this instantly triggers the alarm. The alarm is disarmed whenever the DISABLE input is held high. When this happens, pin 9 of ICla is pulled high via D3 and the 4700 resistor. Thus, pin 8 of ICla will be low and Q6 and the relay will be off. In practice, the DISABLE input is simply linked to the DISABLE ouput of the SILICON CHIP UHF Remote Alarm Switch Receiver (described in March 1988) or to the emitter of 62 SILICON CHIP transistor Q5 in the Oatley Electronics UHF remote control receiver. Power for the circuit is derived directly from the car's battery via diode D5. D5 is there to protect the circuit against possible reverse polarity connection to the battery. Construction The copper pads on the PCB are all quite small so you will need a soldering iron with a fine tip to successfully build this project. Apart from that, construction is straightforward but watch out for solder bridges between adjacent pads on the board. Fig.3 shows how all the parts are mounted on the PCB. Install the two wire links first, followed by the resistors and the diodes. Be sure to use the correct diode at each loca tion. You must use the OA90s (the ones in the clear bodies) for Dl and D2, while D3 and D4 are the 1N4004s. The monolithic, ceramic and electrolytic capacitors can all be installed next. Push all the parts down onto the PCB as far as they will go before soldering the leads 0 and make sure that the electros are all installed the right way around. Next, you can install the six transistors, the IC and the crystal. Note that all the transistors face in the same direction except for Q5, which faces the opposite way. Pin 1 of the IC is adjacent to a small dot or notch in the plastic body. It doesn't matter which way around you install the crystal. Before mounting this part, bend its leads at right angles where they emerge from the case. This will allow the crystal to lie flat against the PCB as shown in one of the photos. The PCB assembly can now be completed by installing the two transducer connectors, the trimpot, the relay and the LED. Take care with the orientation of the LED its anode lead is the longer of the two. Don't worry about the off-board components associated with the DISABLE input and the pin switches for the time being. These parts are "add-ons" to the original circuit and have no effect on the basic operation. They can be installed after the circuit has been tested. Fig.3: here's how to wire the unit as a self-contained alarm. If you intend using the unit to trigger an existing alarm, the 2.2MO timing resistor should he reduced to lOkO and the horn siren delected. OPTIONAL BONNET ANO BOOT PIN SWITCHES .,. RESISTORS □ □ □ □ □ □ □ □ □ □ □ □ □ □ No. 1 5 3 2 1 1 5 1 3 1 1 2 1 2 Value 2 .2MO 1MO 220k0 120k0 47k0 15k0 1 OkO 4 .7k0 2 .2k0 4700 3300 2200 1000 47n Now go back over your work and carefully inspect the PCB assembly. Check that all the parts are in their correct locations and are correctly oriented. You should also check the underside of the board for solder bridges and missed solder joints. When you are satisfied that everything is correct, the 3-wire cable can be wired to the PCB. Connect the black lead to the earth track (between the two connectors), the red lead to the + 12V rail, and the yellow lead to the output terminal. Testing To test the unit, set the sensitivity 4-Band Code red red green gold brown black green gold red red yellow gold brown red yellow gold yellow violet orange gold brown green orange gold brown black orange gold yellow violet red gold red red red gold yellow violet brown gold orange orange brown gold red red brown gold brown black brown gold yellow violet black gold 5-Band Code red red black yellow brown brown black black yellow brown red red black orange brown brown red black orange brown yellow violet black red brown brown green black red brown brown black black red brown yellow violet black brown brown red red black brown brown yellow violet black black brown orange orange black black brown red red black black brown brown black black black brown yellow violet black gold brown control to maximum (fully clockwise) and solder a 10k0 resistor across the 2.2MO timing resistor on the copper side of the board. This will reduce the timing period to about 0.2 seconds so that you don't have to wait for the unit to reset each time it is triggered during testing. You don't have to hook up the external siren at this stage - the LED and the relay will provide sufficient indication that the unit is working. Now plug the transmitter (SQ40T} and receiver [SQ-40R} transducers into the on-board connectors [careful: don't mix them up) and connect up a 12V power supp- CAPACITORS □ □ □ No. 7 2 1 Value 0 .1µ,F .00 1µ,F 2 20 pF IEC EIA 100n 104M 1n 102K 220p 221K ly. The unit should now be operational; ie, the LED should light and the relay should operate whenever movement occurs. In practice, you should find that the unit is quite sensitive. With the transducers set up about 30cm apart on a workbench, we found that our unit would trigger reliably JULY 1989 63 Above: the ultrasonic transducers simply plug into connectors at one end of the case. The receiver socket is the one on the left. Left: don't confuse the two transducers. The transmitter is labelled SQ-40T while the receiver is labelled SQ- 40R. at distances up to 7 metres. So, in the confines of a car, the unit will have tons of sensitivity. What! - it doesn't work? If the unit fails to operate as expected, look for incorrect component placement on the PCB and for shorts on the copper side. Are all the resistor values correct? Are the electrolytic capacitors correctly oriented? Is Q5 the right way around? If these checks fail to reveal anything, you can quickly determine which half of the circuit is not operating by momentarily shorting the collector of Q3 to ground (unplug the transducers first). If the LED now lights and the relay operates, then the fault lies before Q3 or in the transmitter. Conversely, if nothing happens, then the fault lies after Q3. The relay driver circuit can be easily checked by momentarily shorting pin 9 of ICla to ground. Similarly, you can check the circuit operation from Q5 on by momentarily shorting the 4. 7µF capacitor. A few voltage checks will also quickly reveal a faulty stage. Again, these checks are best made with the transducers unplugged. When power is applied, Ql, QZ, Q3 and Q4 should all have base-emitter voltages of about 0.65V. On the prototype, the collector voltages were + 4.6V for Ql, + 4.0V for QZ, + 4.6V for Q3 and + 4.0V for Q4. These collector voltages should be taken as a guide only, since they The transducers are best mounted adjacent to the front windscreen pillars. They can be secured by forcing the spring clips under the trim. 64 SILICON CHIP will vary according to the beta of the transistor. Installation Fig.3 shows how the unit is wired as a self-contained alarm. The offboard components can simply be soldered to the appropriate points on the top of the PCB and wrapped in insulation tape to prevent shorts. Alternatively, you can drill extra holes in the PCB and mount the parts on the board itself. The necessary wiring connections can then be made by running insulated links on the copper side of the board. A dab of Araldite can be used to secure the parts to the top of the PCB. The PCB is secured inside the case using a single self-tapping screw. Once the assembly has been completed, it can be mounted in the car under the dashboard. In many cars, you can gain access behind the dash panel by removing the loudspeaker grille and the loudspeaker. The two transducers are best installed on top of the dash adjacent to the windscreen pillars. Plug the transducer leads into the control unit, then connect up the siren and the supply leads. The + 12V supply can be derived from the fuse panel. Finally, if you intend using the unit as a sensor to trigger an existing alarm, the 2.2MO timing resistor should be reduced to lOkO. The 12V siren and the off-board components are deleted and the yellow output lead is simply connected to one of the alarm inputs.~