The circuit was designed for the situation where a hijacker forces the driver from the vehicle. If a door is opened while the ignition is switched on – the circuit will trip. After a few minutes delay – when the thief is at a safe distance – the alarm will sound and the engine will fail.
Before fitting this or any other engine cut-out to your vehicle – carefully consider both the safety implications of its possible failure – and the legal consequences of installing a device that could cause an accident. If you decide to proceed – you will need to use the highest standards of materials and workmanship.
You’re going to trip this alarm unintentionally. When you do – the LED will light and the Buzzer will give a short beep. The length of the beep is determined by C3. Its purpose is to alert you to the need to push the reset button. When you push the button – the LED will switch-off. Its purpose is to reassure you that the alarm has in fact reset.
If the reset button is not pressed then – about 3 minutes later – both the Siren and the Buzzer will sound continuously. The length of the delay is set by R7 & C4. For extra effect – fit a second siren inside the vehicle. With enough noise going on – you may feel that it’s unnecessary to fit the engine cut-out. In which case – you can leave out D8, D9, R11, R12, R13, C6, Q3, Q4 & Ry2.
Even if you missed the early warning provided by the Buzzer – there is still time to reset the alarm before Ry2 de-energizes – and the engine fails. This additional delay – currently about 1 minute – is set by C6 and R13.
To reset the circuit you must – EITHER turn off the ignition – OR close all of the doors – before you press the reset button. While BOTH the ignition is on – AND a door remains open – the circuit will NOT reset.
The reset button carries virtually no current – so any small normally-open switch will do. Eric Vandel from Canada suggests using a reed-switch hidden behind (say) the dash – and operated by a magnet. I think this is an excellent idea. As Eric said in his email: – “… that should keep any thief guessing for a while.”
source : zen’s circuit collections