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Protect your car with this Engine Immobiliser This circuit will immobilise your car if a thief tries to start it. Fit it to your car as cheap insurance. If a thief tries to steal your car, the engine will repeatedly stall and he will move on to easier pickings. By JOHN CLARKE There are many ways to prevent someone pinching your pride and joy. Poison gas, electrocution and automatic garrotting are some options that have been suggested by victims of car theft but sadly, these are illegal. Disabling the ignition is one of the better methods, because it prevents the thief from starting the car and driving off –unless he is keen to take your particular vehicle, he won’t want to bother finding out why the engine will not start. While simply disabling the ignition is effective, the fact that the thief may realise that the ignition has been disabled still places the car at risk. If the thief is inclined to undo the relevant wiring, the car can be started and driven away. 8  Silicon Chip On the other hand, if the thief has hot-wired your car and leaves the jumper in place, and if the coil has been permanently shorted by a hidden switch, there is another big risk –the coil could burn out. A better method is to have the ignition disabled on an intermittent basis. This is where our Engine Immobiliser comes in. Initially, the Engine Immobiliser allows the engine to be started but stops it after about 3.5 seconds. The car can then be restarted, only to stop again. After several more tries, the thief is likely to decide that the car has an intermittent problem and leave it. In the event that the thief persists, the job will get no easier. If he tries to pump the gas pedal, he is likely to flood the engine which will compound the problem. Note that there are many possible faults which will cause this sort of engine misbehaviour. They can range from dirt in the fuel causing blockages to an intermittent ignition which is exactly what it is. If the thief decides to lift the bonnet to investigate further, it is important that the single disabling wire be well hidden. Naturally, the switch to turn the Immobiliser on and off must be well concealed or camouflaged to look like one of the accessory switches, otherwise this subterfuge will be for nothing. Disabling the ignition The principle of this Engine Immobiliser is quite simple. In effect, a switch is placed in parallel with the car’s points or the ignition switching transistor, as shown in Fig.1 and Fig.2. Each time the Engine Immobiliser switch is on, it effectively shorts out the points or the switching transistor and prevents the coil from producing any sparks. By shorting out the points or ignition transistor and diverting the coil current for just a brief period, no Warning!! Don’t be caught out yourself and have the car stall just as you pull out into traffic. Always check that the Immobiliser switch is off before you start the car. For safety, it is wise to wait a few seconds before moving off, just to be sure that the Engine Immobiliser is not in effect. damage results to the coil as it possibly could if the ignition was disabled permanently. Now have a look at the circuit for the Engine Immobiliser in Fig.3. This circuit uses a high voltage Darlington transistor (Q1) which is connected in parallel with the points or the ignition transistor. IC1, a 555 timer, is connected to operate as an astable oscillator. It is powered from the ignition circuit of the vehicle via enable switch S1. Initially, when power is first applied, pin 3 of IC1 goes high. This holds transistor Q2 off and so Q1’s base is not driven. Thus the ignition system operates normally and the engine will start. Four 75V 1W zener diodes (ZD2ZD5) protect Q1 from high voltage transients generated each time the ignition coil fires. The 10µF capacitor at pins 2 and 6 of IC1 then begins charging via the 100kΩ and 220kΩ resistors. When the capacitor’s voltage reaches about +8V, the output at pin 3 goes low. This occurs some 3.5 seconds after switch-on. Pin 3 turns Q2 on via base current through the 1kΩ resistor and this turns on Q1 via its 82Ω base resistor. With Q1 on, any opening of the points or ignition transistor will not fire the coil. At the same time that the pin 3 output goes low, pin 7 also goes low to discharge the 10µF capacitor at pin 2 via the 100kΩ resistor. When the capacitor voltage drops to about +4V, pin 3 will go high and pin 7 will go open circuit to allow the capacitor to charge again via the 220kΩ and 100kΩ resistors. Since the 10µF capacitor now only has to charge from +4V to +8V, it only takes about 2.2 seconds before it begins discharging again. Hence, Q1 will be off for 2.2 seconds and on for about 0.7 seconds. So the car can be repeatedly started and will Fig.1: when fitted to a car with conventional ignition, the Immobiliser effectively shorts out the points and thereby stops the coil from producing spark voltage. Fig.2: when fitted to a car with electronic ignition, the Engine Immobiliser shorts out the main switching transistor. This does no damage because the coil current is intermittently shunted through the Immobiliser. ENABLE S1 4. 7W 470  220k 7 100k 4 8 3 1k Q2 BC327 E B IC1 555 6 2 C 1 100 16VW Q1 MJ10012 C 82 5W B 10 16VW E B E C VIEWED FROM BELOW C E B ENGINE IMMOBILISER +12V FROM IGNITION ZD1 16V 5W TO POINTS OR IGNITION TRANSISTOR ZD2 75V 5W ZD3 75V 5W ZD4 75V 5W ZD5 75V 5W Fig.3: the Engine Immobiliser is basically a 555 oscillator with a short duty cycle. It turns on high voltage transistor Q1 every 2.2 seconds to disable the car’s ignition system. just as surely stall before it can begin to move off. The process repeats itself until the ignition is turned off or switch S1 is turned off, to allow the car to run normally. IC1 is protected from transients December 1995  9 Fig.4: use this diagram when you install the parts on the PC board. by zener diode ZD1 which limits the supply voltage to +16V. A 4.7Ω resistor limits the zener current while the 100µF capacitor across the supply provides filtering of noise. Construction The Engine Immobiliser is made on a small PC board coded 05310951 and measuring just 47 x 61mm. The board is designed to clip into a plastic case measuring 82 x 54 x 31mm. Alternatively, the case could be dispensed with and the board protected by a length of large heatshrink tubing or wrapped in gaffer tape and mounted under the dashboard. Begin construction by inserting the three PC stakes for the external wiring Fig.5: this is the full-size etching pattern for the PC board. connections, as shown on the wiring diagram of Fig.4. This done, insert and solder the low profile components such as IC1, various zener diodes and resistors. Note that the zener diodes are mounted with a loop in the leads as shown in the photographs. This is to provide stress relief for the component. Make sure that the diodes are installed the right way around, as shown in Fig.4, otherwise the circuit may not work. Use the accompanying resistor colour code table to help you in identifying the correct resistor value for each position. The 5W resistor can be mounted against the PC board since it will not run hot. Now solder in the capacitors, taking care that the electrolytic capacitors are installed the right way around. Transistor Q2 is inserted and pushed down firmly so that its body is about 4mm above the PC board. Q1, the high voltage Darlington transistor, is mounted directly onto the PC board. It is secured with 3mm screws and nuts which make the collector to PC board track connection. Solder the nut nearest ZD2 to the copper pad to ensure a permanent connection and use a star washer under the screw head. Testing The circuit board can be initially tested using a 12V battery or DC power supply and a multimeter. Connect power to the board between the GND and “ignition via S1” terminals. Now set your multimeter to check that +12V is present at pins 4 & 8, then measure the voltage at pin 3. It should switch high (ie, +12V) for about two seconds and low (close to 0V) for about 0.7 seconds. If that checks out, turn off the power and connect a resistor between the collector of Q1 and positive supply (any value from 1kΩ to 10kΩ will do). Now re-apply power and check that the collector voltage goes high for approximately two seconds and low for 0.7s. If it checks out correctly, the board is ready for installation. Installation The zener diodes are mounted with a loop in one of their leads as shown here. This is to provide stress relief for these components. Make sure that all polarised parts are correctly oriented. 10  Silicon Chip As previously mentioned, the Engine Immobiliser must have all its wiring and the unit itself well concealed. We recommend that the unit be installed under the dashboard. The only wire passing through the firewall Above: installed in a plastic case or sheathed in heatshrink plastic, the unit should be concealed underneath your car’s dashboard. Left: solder the nut near ZD2 that’s used to secure Q1 to ensure a good connection to the copper track of the PC board. will be the connection to the ignition coil’s negative terminal. Note that the collector of Q1 and its associated zener diodes can have up to 300V on them when the coil fires. Consequently, they must be well isolated from contact with any under-dash wiring or metalwork. Mounting the board in a plastic case or sheathing it with heatshrink sleeving will do the job. The GND connection can be made directly to a nearby chassis point already used for existing wiring. Use a crimp eyelet for this termination. The enable switch S1 must be mounted in a concealed position where it can be easily reached from the driver’s seat but its purpose should not be obvious to anyone but yourself. Be sure also to install this switch in a location where it cannot be accidentally bumped. You might also have two such switches in series so that they have to be in the right setting before the car can be started. Now connect the “ignition via S1” terminal on the PC board to the wiper of S1. The contact terminal of the switch is wired to the fused side of the ignition switch. Use a “quick connect” spade connector to make the connection into the ignition wire or use whatever matches the harness connections in the car. The wire from Q1’s collector to the coil negative terminal should pass through the firewall via an existing grommet. Where the wire connects to the coil, make sure that it is well disguised so that it is not obvious that there is an extra wire installed. If possible, conceal it within the existing harness plastic sheathing. Does it immobilise? Now for the big test: enable the Engine Immobiliser by switching S1 to the on position and start your car. It should stall within about three seconds after you first turn the key. Try again and the engine should stall again. If it doesn’t stop the engine – you haven’t wired it in correctly. Once you have it operating correctly, switch off S1, start the car again and take it for a run. This is to check that the Engine Immobiliser does not affect normal operation in any way. Now provided you remember to switch on the Immobiliser each time you leave your car, you can enjoy extra peace of mind knowing that no-one SC can take it for a joy-ride. PARTS LIST 1 PC board, code 05310951, 47 x 61mm 1 SPST switch (S1) 3 PC stakes 2 3mm screws, nuts and star washers Semiconductors 1 555 timer (IC1) 1 MJ10012 500V NPN Darlington (Q1) 1 BC327 PNP transistor (Q2) 1 16V 5W zener diode (ZD1) 4 75V 5W zener diodes (ZD2ZD5) Capacitors 1 100µF 16VW PC electrolytic 1 10µF 16VW PC electrolytic Resistors (0.25W 1%) 1 220kΩ 1 470Ω 1 100kΩ 1 82Ω 5W 1 1kΩ 1 4.7Ω Miscellaneous Automotive hook-up wire, eyelet lugs, self tapping screws, plastic case 82 x 54 x 31mm or heatshrink tubing. RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ No. 1 1 1 1 1 Value 220kΩ 100kΩ 1kΩ 470Ω 4.7Ω 4-Band Code (1%) red red yellow brown brown black yellow brown brown black red brown yellow violet brown brown yellow violet gold brown 5-Band Code (1%) red red black orange brown brown black black orange brown brown black black brown brown yellow violet black black brown yellow violet black silver brown December 1995  11