automatic car alarm

elektor july/august 1985 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 con- tact: the former is necessary to delay the switching in of the alarm after you have got out of your car, and the latter H Gulitz re 1b C1---4161"4--Cf Immediately on re-entering your car, you must press the hidden switch, S1. This causes silicon-controlled rec- tifier Th1to conduct so that the relay is energized. At the same time, the green LED lights to indicate that the alarm is switched off. As soon as the ignition is switched off,T1isoff,T2ison,andthebuzzer sounds. At the same time, monostable IC1is triggered, which causes T3to conduct and the red LED to light. The silicon-controlled rectifier is then off, and D4 is reverse biased, but the relay remains ener- gized via its make contact for a short time, preset by P1. As soon as this time has lapsed, the relay returns to its 47p 16V 84 IC1 555 automatic car alarm 1...