Even the best car alarm is useless if you forget to set it upon leaving your car, whence this circuit. The relay has a make and a break contact: the former is necessary to delay the switching in of the alarm after you have got out of your car, and the latter serves to switch on the car alarm proper. Immediately on re-entering your car, you must press the hidden switch, Si. This causes silicon-controlled rectifier Thi to conduct so that the relay is energized. At the same time, the green LED lights to indicate that the alarm is switched off.
Circuit diagram :
As soon as the ignition is switched off, T, is off, T2 is on, and the buzzer sounds. At the same time, monostable IC1 is triggered, which causes T3 to conduct and the red LED to light. The silicon- controlled rectifier is then off, and D4 is reverse biased, but the relay remains energized via its make contact for a short time, preset by Pi As soon as this time has lapsed, the relay returns to its quiescent state, and the alarm is set via the break contact. The delay time can be set to a maximum of about 1 minute.