This is a very simple automatic immobilizer circuit for a car or motorbike.
It is made from a 30A car relay 1 diode 1 resistor and 1 flashing led and some wire.
You should be able to buy these items from maplin or somewhere similar.
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Step 1: How the Circuit Works
When the relay is in the off state 12v will go through the relay and power the LED.
Until there is a voltage applied to terminal 86 from a hidden switch which will energize the coil in the relay causing the relay to switch the power from terminal 87 to terminal 87A which will feed the coil in the ignition circuit. The diode is in place to stop the power from the hidden switch supplying power directly to the ignition circuit.
(Note the relay you purchase my be labeled differently)
Step 2: Wiring the Circuit in to the Vehicle
You will need to cut the wire supplying the vehicle’s coil or an ignition wire for the electronic ignition. You will have to check the wiring of your vehicle for this information. On my motorbike I just used the engine kill switch wire.
Then you will have to connect a 12 volt supply to your relay from the vehicles battery. It’s a good idea to fuse this wire.
Then just find a good place to mount the led and hidden switch.
Step 3: Testing
Now when you try to start you vehicle nothing should happen.
Press the hidden switch and the vehicle should start.
If not go back and recheck you wiring and connection.