This alarm circuit is based on two 555 timers. The alarm will sound your car horn if anyone opens the car door while the circuit is armed. The timers will allow you to leave the car without sounding the horn. To turn the circuit on S1 must be closed. To set the alarm, open S2 ( it is normally closed ) this will give you about 5 seconds to get out and close the door. The exit delay time is set by R1 and C1. If anyone opens the doors for more then two seconds the horn will sound until power is removed from the circuit. The 2 second time is set by R2 and C2. If you open the door, you must deactivate the alarm by closing S2. This very basic circuit could be used for a home also.
From : http://home.maine.rr.com/randylinscott/jan20.htm
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