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Engine ImmobiliserMk.2 Protect your car from theft with the . . . This basic engine Immobiliser kills the ignition if a thief tries to steal your car. Fit it to your car as cheap in­surance and peace of mind. If a thief tries to start your car, the engine will repeatedly stall and he will move on to an easier target. By JOHN CLARKE While many modern cars include a comprehensive anti-theft system with ignition disable, central locking and rolling code entry, older vehicles or the less expensive models do not have this protection. The lack of engine immobilisation renders the vehicle more susceptible to theft, particularly 24  Silicon Chip for older style vehicles, some of which can be entered, started and driven away in just a few seconds. You can improve the odds against your vehicle being stolen simply by adding some form of engine immobilisation. Whether it is a hidden switch which breaks the points signal from the dis­tributor, or a more fancy method, the inclusion makes it more difficult for a thief to start the engine. But if the ignition system can be “hot wired” to effectively bypass the immobilisa­tion wiring then it will be worse than useless. This Engine Immobiliser shorts out the switching transistor or points which control the ignition coil. It does not produce a permanent short because it is switched on and off at a slow rate. The engine can be started with the Immobiliser in action but it will only run for about two seconds and then switch off. The engine can then be restarted only to stall again. If the thief persists, the engine will continue to start, only to stall again and after several tries he is likely to decide that the car is not worth the trouble. On the other hand, if the thief decides to lift the bonnet to investigate further, it is important that the wire from the Immobiliser to the ignition coil is 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, other­ wise the whole subterfuge will be for nothing. Killing the ignition In effect, a switch is placed in parallel with the car’s points or the ignition switching transistor, as shown in Fig.1 & Fig.2. Each time the Engine Immobiliser switches 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 switching transistor and diverting the coil current for just a brief period, no damage can result to the coil. But the ignition coil could be easily burnt out if the coil current was continuously diverted, as it would be if the ignition was permanently disabled by a simple switch. Now have a look at the circuit of the Engine Immobiliser in Fig.3. It uses a high voltage Darlington transistor (Q1) which is connected in parallel with the points or the ignition transistor. Q1 can switch the coil current of several amps and can withstand the high voltages normally developed when the ignition system is functioning normally. This circuit is quite similar to the Fig.1: when fitted to a car with conventional ignition, the Immo­biliser effectively shunts the points and stops the coil from producing spark voltage. Fig.2: when fitted to a car with electronic ignition or an engine management system, the Immobiliser shunts the main switching transistor. This does no damage because the coil current is inter­mittently diverted through the Immobiliser. original Engine Immo­biliser which we featured in the December 1995 issue of SILICON CHIP but there are some important differences which we will mention later in this article. IC1, a 555 timer, is connected to operate as an astable oscillator. It is powered from the ignition circuit of Fig.3: the circuit consists of a 555 timer which cycles the transistors on and off to periodically shunt the ignition and hence stall the engine. December 1998  25 started and will just as surely stall each time. One important feature, which may not be immediately obvi­ous, is that the Immobiliser does not do any damage to the car’s ignition system if the thief leaves the car stalled and hot-wired. The Immobiliser will continue its cycle of 0.7s on and 2.3s off indefinitely but no damage should result apart from the possibility of the battery becoming discharged. One other minor point is that when power is first applied to the Immobiliser circuit, as when the ignition is first switched on, pin 3 of IC1 will be high and so Q1 will be on, pulling the negative side of the coil low and thus preventing any sparks from being delivered for about one second. However, most cars need to be cranked for at least a second to start them so there is really no noticeable effect on starting the car. Power for the Immobiliser comes from the ignition switch and the enable switch S1. It is fed via diode D2 which protects transistors Q2 & Q3 against reverse connection of the supply while the associated 0.1µF capacitor decouples the supply from hash. IC1 is protected from voltage transients with the 16V zener diode ZD1, together with the series 10Ω resistor and 100µF decou­pling capacitor. Fig.4: the component overlay for the PC board. Note that the zener diodes must be installed the right way around otherwise the circuit won’t work. If the 3W zeners are installed the wrong way around they could be burnt out by the coil current when the circuit is connected up. the vol­tage across the capacitor rises above +8V (ie, 2/3 of the posi­ tive supply), pin 3 of IC1 goes low. The 10µF capacitor is then discharged to about +4V via the 330kΩ resistor connected between pins 6 & 7 and pin 3 goes high again. The cycle then continues with pin 3 being switched high for about 0.7 seconds and low for 2.3 seconds. Each time pin 3 of IC1 is high, Q3, Q2 & Q1 are switched on This photo shows the keypad version of the Engine and so the ignition Immobiliser, to be published next month. The keypad coil is prevented from circuit board mounts above the Engine Immobiliser producing its normal board in a standard plastic case. primary voltage and the engine will be the vehi­cle via the enable switch, S1. stalled. This 0.7s on-time for Q2 is Initially, when power is first applied, quite sufficient to stall the engine and pin 3 of IC1 goes high. The 10µF cameans that there is no chance of any pacitor at pin 2 is then charged via the damage to the ignition system. 100kΩ resistor and diode D1. When So the engine can be repeatedly Construction The Engine Immobiliser circuit is accommodated on a PC board measuring 106 x 60mm and coded 05412981. The component overlay for the board is shown in Fig.4. Before discussing the construction details, we need to mention a number of differences between this version of the circuit and that originally published in December 1995. The first and most obvious difference is that this Mk.2 version uses the MJH10012 which is the plastic version of the Table 1: Resistor Colour Codes  No.   1   1   1   1   1   1 26  Silicon Chip Value 330kΩ 100kΩ 4.7kΩ 1kΩ 82Ω 5W 10Ω 4-Band Code (1%) orange orange yellow brown brown black yellow brown yellow violet red brown brown black red brown not applicable brown black black brown 5-Band Code (1%) orange orange black orange brown brown black black orange brown yellow violet black brown brown brown black black brown brown not applicable brown black black gold brown Parts List Fig.5: this is the actual size artwork for the PC board. Check your board carefully before installing any of the parts. MJ10012 TO-3 power transistor. An alternative transistor which may be supplied in some kits is the BU941P (manufactured by 57 Microelectronics). While the plastic Darlington high voltage transistor should be cheaper it does require a small heatsink. The second point of difference is that there is provision on the board for another transistor and this will be used in a keypad-operated Engine Immobiliser to be published next month. The version being published this month has the virtue of simplic­ ity; next month’s version offers more bells and whistles and the security of a keypad to disable it. Now that we’ve got those points out of the way, we can discuss assembly of the board. You can begin construction by checking the PC board for shorts between tracks, breaks in the pattern or undrilled holes. You will need to fit PC stakes at the external wiring points (four) and then insert the links using the tinned copper wire. The resistors can be installed next and you can use the colour codes in Table 1 as a guide to selecting each value. Alternatively, you can use a digital multimeter to measure each resistor before it is soldered in. The diodes can go in next, taking care with the polarity of each. Make sure that you use the 1N914 or 1N4148 type for D1 and 1N4004 for D2. The 16V zener diode ZD1 is quite small and may be marked 1N4745 while the four 75V 3W zeners (which may be marked 1N5374) are quite a lot larger. Transistors Q2 & Q3 are positioned as shown but make sure you don’t get them swapped around; Q2 is a BC327 while Q3 is a BC337. Transistor Q1 is mounted on a small heat­sink and secured with an M3 screw and nut to the PC board. Next, insert the 555 IC and the three capacitors, making sure that the IC and the electrolytic capacitors are installed the right way around. The 0.1µF capacitor may be marked as 100n or 104, being the IEC and EIA codes, respectively. Testing To test the circuit, connect it to a 12V DC supply or bat­tery. There is no need to connect a coil to the collector of Q1. Connect your multimeter, set to measure 12V DC, to check the voltage at pin 3 of IC1. You can do this most conveniently by connecting to the 4.7kΩ base resistor for Q3. Now apply power and check that pin 3 goes high immediately and then drops low after about a second. It should then stay low for 2.3 seconds or thereabouts, then go high for 0.7s and so on. You can then check the sequence at the collector of Q3 and the col­lector of Q2. Q3 will invert the voltage from pin of IC1 and Q2 will invert it back again. Finally, you can verify that the high voltage transistor Q1 comes on by measuring the resistance between its emitter and collector. The transistor will be on when the resistance is low and off when its resistance is high. If the circuit operates properly you are now ready to install it into your vehicle. The board can be housed in several ways. It can be mounted in a standard plastic case measuring 130 x 67 x 43mm or it could be sheathed in heatshrink tubing. 1 plastic case, 130 x 67 x 43mm 1 PC board, code 05412981, 106 x 60mm 4 PC stakes 1 mini heatsink, 19 x 19 x 9.5mm 1 M3 x 9mm screw 1 M3 nut 1 1m length of heavy duty black automotive hookup wire 1 1m length of heavy duty red automotive hookup wire 1 1m length of light duty red automotive hookup wire 1 1m length of heavy duty yellow automotive hookup wire 1 150mm length of hookup wire Semiconductors 1 555 timer (IC1) 1 MJH10012, BU941P power Darlington transistor (Q1) 1 BC327 PNP transistor (Q2) 1 BC337 NPN transistors (Q3) 1 16V 1W zener diode (ZD1) 4 75V 3W zener diodes (ZD2-5) 1 IN4148, 1N914 diode (D1) 1 1N4004 1A diode (D2) Capacitors 1 100µF 16VW PC electrolytic 1 10µF 16VW PC electrolytic 1 0.1µF MKT polyester Resistors (0.25W, 1%) 1 330kΩ 1 1kΩ 1 100kΩ 1 82Ω 5W 1 4.7kΩ 1 10Ω Miscellaneous Automotive connectors, solder. Find a suitable position under the dashboard to mount the unit and then locate the fused side of the ignition circuit and the fused side of the battery supply. The wiring to these points should be made using automotive connectors. Also you will need a chassis point to connect the ground supply of the circuit to the battery negative terminal. This can be an existing screw in the bodywork or a separate self-tapping screw which secures the eyelet terminal for the ground lead in place. The connection to the ignition coil should be made with an eyelet terminal. Try to conceal this wire as SC much as possible. December 1998  27