Super Sensitive Intruder Alarm Circuit

admin May 15, 2010 4 Comments

Circuit Description

The circuit diagram of super sensitive intruder alarm is given here. The main parts of this circuit are two ICs IC1 (uA 741), IC2 (NE 555) and transistors Q1 (BC 157), Q2 (SL 100). The shadow of an intruder passing few meters away the circuit is enough to trigger the alarm. In this circuit, the operational amplifier IC uA 741 acts as comparator. The set point of this comparator is determined by the resistors R6 (10K) and R7 (10K). The input to the non inverting terminal of IC2 is the divided voltage between potentiometer R9 (47K) and LDR R8. At standby mode, the voltages at both the inputs of IC2 are made equal by adjusting potentiometer R9. Then the output of the comparator becomes high and that turns off transistor Q1. That means the voltage at trigger pin of IC1 will be positive and there will be no alarm. When there is an intruder near the LDR its resistance increases and the voltages at the inputs of comparator will be different and the out put of IC2 will be low. This makes Q1 on and negative going pulse to trigger the IC1 which is made as monostable multivibrator. Transistor Q2 amplifies the output of IC1 and the alarm is produced from that output.

Circuit Diagram

Super Sensitive Intruder Alarm Circuit

Super Sensitive Intruder Alarm Circuit


  • Use a good quality PCB to assemble this circuit.
  • Power up the circuit and adjust R9 so that LED D1 goes off while setting up the alarm.
  • The sensitivity of LDR can be improved by employing a dark house.
  • Aiza Jacob
    August 31, 2010

    thank’s for the circuit, it was great!

  • October 20, 2010

    Thank you for the project.My project worked properly but there were some problems with it. The alarm remains on only till the shadow falls on it, i need it to be on for few seconds after the shadows falls. how can i do it? it’s if no use if the alarm remains only till the LDR gets no light.
    For example, If this circuit is placed on the entrance of the door & if the burglar enters from the door, the alarm may ring only for a second, and nobody is able to identify that the burglar had entered the room by the door.

    What’s the use of 47K POT? what can we do with it? And the LED does not on.

  • May 7, 2011

    Thanks for the circuit. I need to find out why the alarm is locked on after an intruder is detected.

    January 1, 2017

    sir how can i ring alarm for long time

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