Hijack Alarm


The first 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.

Circuit diagram

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.

Circuit Notes

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."

The Flow Chart is another of Eric's suggestions. It will help you to visualize how the alarm is operated. It also explains the sequence of events that lead to siren activation - and subsequent engine failure.

Veroboard Layout:

How you bring your vehicle to a standstill is up to you. It should happen when Ry2 de-energizes. The contacts of Ry2 are too small to do the job themselves. So use them to switch the coil of a larger relay. Remember that the relay must be suitable for the current it's required to carry. Choose one specifically designed for automobiles - it will be protected against the elements - and will give the best long-term reliability. You don't want it to let you down on a cold wet night - or worse still - in fast moving traffic!!! Remember also that you must fit a 1N4001 diode across YOUR relay's coil - to prevent damage to the Cmos IC.

YOUR relay should drop-out when Ry2 de-energizes. Wire YOUR relay so that when it drops-out the engine will stop. Because turning-off the ignition will cause both Ry2 and YOUR relay to de-energize - the standby current will be low - and the engine will be disabled while the vehicle is parked.

The circuit board must be protected from the elements. Dampness or condensation will cause malfunction. Fit a 1-amp in-line fuse AS CLOSE AS POSSIBLE to your power source. This is VERY IMPORTANT. The fuse is there to protect the wiring - not the components on the circuit board. Please note that I am UNABLE to help any further with either the choice of a suitable relay - or with advice on installation.

Both the Siren and the Buzzer will go on sounding until the alarm is reset. The circuit is designed to use an electronic Siren drawing up to about 500mA. It's not usually a good idea to use the vehicle's own Horn because it can be easily located and disconnected. However, if you choose to use the Horn, remember that Ry1 is too small to carry the necessary current. Connect the coil of a suitably rated relay to the "Siren" output. This can then be used to sound the Horn.

The Support Material for the First Hijack Alarm includes a parts list - a step-by-step guide to construction - a detailed circuit description - and more.

Enhanced HiJack Alarm

The second alarm is a variation on the first. My original idea was to have the circuit sit quietly in the background - and only require the driver to intervene if the alarm were tripped accidentally. However, I'm obliged to Jeff Chia from Canada who suggested the following enhancement.

By making the alarm trip automatically the moment the ignition is switched on - it will protect the vehicle in many more situations. For example at the Service Station - while you're filling the tank - checking the tyre pressure - paying at the cash desk etc. In fact it works whenever you leave the vehicle unattended with its ignition switched off - even overnight in your driveway.

Jeff's suggestion has made me reconsider the value of the initial design approach - and I think that having to press the reset button every time you turn on the ignition - is a small price to pay for the added protection.

If you've already built the original version - and want to add the modification - remove the 12-volt input terminals. That should provide enough space for the additional components.

The Flow Chart for the Enhanced Alarm will help you to visualize how it's operated. It also explains the sequence of events that lead to siren activation - and subsequent engine failure.

The Support Material for the Enhanced Hijack Alarm includes a parts list - a step-by-step guide to construction - a detailed circuit description - and more.

Return to Alarm Circuits

Content sourced from Zen Schematics

Circuit: Ron J