Sensitive Intruder Alarm Circuit

Intruder Alarm Circuit


Here is the circuit diagram of an ultra sensitive intruder alarm. The shadow of an intruder passing few meters nearby the circuit is enough to trigger the alarm.

Here IC2 uA 741 is wired as a sensitive comparator ,whose set point is set by R6 &R7. The voltage divide by LDR and R9 is given at non inverting pin of IC2. At standby mode these two voltages are set equal by adjusting R9. Now the output (pin6) of comparator will be high.Transistor Q1 will be off. The voltage at trigger pin of IC1 will be positive and there will be no alarm. When there is an intruder near the LDR the shadow causes its resistance to increase. Now the voltages at the inputs of comparator will be different and the out put of IC2 will be low. This makes Q1 on. This makes a negative going pulse to trigger the IC1 which is wired as a monostable multivibrator.The out put of IC1 will be amplified by Q2 (SL 100) to produce alarm.

Intruder Alarm Circuit Diagram with Parts List

Intruder Alarm

Intruder Alarm Circuit Diagram


  • To setup the alarm ,power up the circuit and adjust R9 so that LED D1 goes off.
  • The LDR can be housed in a dark tube to increase sensitivity.
  • The sensitivity is very important here. If you cannot adjust the required sensitivity properly, use one LOW resistance (~1K ) POT in series with R9 for fine adjustment.

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