Self-resetting intruder alarm

摘要

This device is designed to detect intruders.When a heam of light focused on the light-dependent resistor(LDR)is interrupted,this alarm produces loud noise for about ten seconds and then automatically resets.You may find this device useful in circumstances where the intruder need not be a thief;it may be a family member or an’expected’person.This circuit is based on a 555 timer IC,used as a multivibrator,and a transistor-based audio oscillator.When the light beam is being focused on the LDR,its resistance remains low and hence trigger pin 2 of IC1 is held high,so the output at pin 3 remains low.