Super sensitive intruder Alarm

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Super sensitive intruder Alarm

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Super sensitive intruder Alarm


 This circuit is used to alert the user when a intruder enters the room. when a obstacle interrupts the IR sensor, it generates the interrupt signal. This interrupt signal is given to the speaker to alert the user.


IR transmitter always emits the IR (Infrared) rays which have to be received by IR receiver. When there is an obstacle in front of IR sensor, the transmitted IR rays are reflected back to the IR receiver. The output of Op-Amp is high when IR receiver receives the reflected IR rays. This output of operational amplifier is connected to the RESET pin of 555 timer.

The output of 555 timer is low when input voltage applied at pin 4 of 555 timer is low.

When input voltage at pin 4 is high, then 555 timer produces the frequency which can be adjusted by the different value combinations of resistors R1, R2 and capacitor C3.

  • Obstacle Present → IR Receiver Receives IR Rays → Op-amp Output High → 555 RST Pin High → Speaker Produces the Sound.
  • No Obstacle → IR Receiver does not Receive the Rays → Op-amp Output Low → RST Pin Low → No Sound

The output of NE555 is filtered by the 1uF capacitor and fed to the speaker.

Many IR sensors are arranged in house at different locations. The outputs of all these sensors are given to RESET pin of common 555 timer. When any sensor detects the intruder, the reset pin of 555 timer becomes high and speaker will produce the sound.

Circuit design

555 Timer: Here 555 timer acts as a free running oscillator. It will generate the frequency when high pulse is applied at RESET pin. The generated frequency of 555 timer varied by varying resistor values R4, R5 or by varying the capacitor value C3.

Here Pin 2 and pin 6 of 555 timer are shorted to allow the triggering after every timing cycle. In this circuit, capacitor C3 charges through the resistors R4, R5 and discharges through the resistor R5.


This circuit is used in homes and offices for security purpose.

Additional information

Weight 1.000000 kg


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