In older cars with carburetors, the spark plugs are lit by a system called a contact breaker (CB) point. Using transistorized switching, you can change the CB-point-cum- condenser type of ignition system in your car to an electronic one. Figure 1 shows the electronic ignition coil's circuit, and Figure 2 shows how its wires connect to each other. Electronic ignition coil Fig. 1: Circuit diagram of electronic ignition for old cars In Fig. 1, when the CB point is open, both transistors T1 and T2 are turned off, and the ignition coil doesn't get any power. When the CB point is closed, the T1 and T2 transistors conduct, and a very high current flows through the ignition coil. As a result,

the ignition coil builds up a very high voltage that drives the spark plug. A red LED (LED1) lights up to show that the CB point is moving back and forth. The current going through the ignition coil is limited by the 2.2-ohm, 10W ballast resistor R6. Back emf makes the green LED (LED2) in the output section light up, so it needs to be wired with reverse polarity, as shown in Fig. 1. Power transistor C4424 needs a good heat sink (T2). In Fig. 2, the ignition coil, a condenser, and the connection from the EHT to the spark plugs and distributor are shown. For electronic ignition, you don't need the condenser. One of the CB point's pins is already connected to the ground, and the other pin, which goes to the ignition coil, needs to be disconnected. This pin is marked with a "X" on the diagram, and it tells the electronic ignition what input to use. Fig. 2: Wiring diagram of ignition coil Note: Don't take out the CB-point screw because you don't need to change the switch gap, which will change how the engine is tuned. You just have to take the capacitor off the distributor and cut the wire that goes to the negative point on the ignition coil. During the wiring process, you must be careful not to touch the pins on the ignition coil. At point "B," where the wiring of the car battery is already set up, the circuit gets its power (positive). Make sure you don't short out the wires because that could start a fire.

Circuit operation Ground (negative) can be taken from any screw on the car's chassis for the ignition system. Put the circuit together on a general-use PCB and put it in a metal box with the right insulation. You can also use a switch box made of plastic. But care must be taken to keep the transistor C4424's heat sink from touching the box because it will be very hot. As we've already talked about, the distributor has a condenser. You might need to open the two clips to take off the distributor cap. Then cut the condenser wire on one side. Take your time. You can take the condenser out of the distributor if you want to. After covering the other end of the condenser with insulation tape, connect one end to ground and leave the other end open. In case the electronic ignition fails, you can just switch back to the old system to start the car. Don't get rid of the condenser.

Don't short out any wire that goes to ground. The car battery can put out up to 100A of current, which is dangerous. So, you have to be careful when you work with the wiring in the ignition circuit. For the circuit to work well, use an ignition coil from Autolec/Lucas, spark plugs from Champion, and any type of ignition cable that doesn't have a resistor (when you measure the resistance of the ignition cable, it should be nearly zero). Don't use any other transistor to replace C4424, because only C4424 can handle the back-emf from the ignition coil. You don't need to replace the spark plugs. Also, never take the distributor or its alignment nut out of the engine. When the old system is replaced with electronic ignition, the timing of the spark stays the same.