Do not let thieves steal your car. Secure your car when you leave your favorite car. This circuit is designed to secure your car, of course with little cost if you build it yourself. Much cheaper than buying a car alarm device that is already on the market.
The alarm is “set” by opening Sw1. It can be any small 1-amp single-pole change-over switch – but for added security you could use a key-switch. Once Sw1 is opened you have about 10 to 15 seconds to get out of the vehicle and close the door behind you. When you return and open the door the buzzer will sound. You have 10 to 15 seconds to move Sw1 to the “off” position. If you fail to do so, the siren will sound. The output to the siren is intermittent – it switches on and off. The speed at which it switches on and off is set by C6 and R10. While any trigger-switch remains closed, the siren will continue to sound. About 2 to 3 minutes after all of the switches have been opened, the circuit will reset.
Detailed explanation about this car alarm circuit, visit this page: http://www.zen22142.zen.co.uk/Circuits/Alarm/car_immob.htm