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L55, Super Sensitive Intruder Alarm

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 UA741 is wired as a sensitive comparator, whose set point is set by R6 &R7.The voltage divided 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. And Q1 will be on, it will give the trigger on IC1 and then the output of the pin 3 will go on Q2 and the buzzer will operate.


  • IC1 = NE555, IC2 = UA741

  • Q1 = BC157, Q2 = SL100

  • R1, R2, R5 = 1K, R3 = 470ohm,R4 = 1M,

  • R6 = 10K, R7 = 10K, VR9 = 47K

  • C1 = 0.01uf, C2 = 10uf 25v

  • R8 = LDR (Light depending resistor)

  • D1 = LED (Red)

  • K1 = Piezo Buzzer

  • 9v Battery

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