Laser Security System

Laser Security System

Here is a simple and cheap Laser Based Security System, which costs below Rs. 100. It is a simple circuit using LM358 OP-AMP and LM555 Timer. This circuit is triggered when the Laser beam falls on the LDR interrupts, which sounds the buzzer connected to it. This can be used to protect your valuable items, money, room etc.. By using different mirror arrangements you can form different security systems. This can also be used to protect more than one objects by suitably arranging mirrors.  One possible arrangement is shown below.

Mirror Laser LDR Arrangement
Mirror Laser LDR Arrangement

In this arrangement Mirrors B,C,D are arranged such that the reflected beam is making 90° to incident beam and the Mirror A is placed such that the beam form Laser falls on the  Mirror B and the beam from Mirror D falls on the LDR.

Circuit Diagram

Laser Security System Circuit Diagram
Laser Security System Circuit Diagram


In this circuit the IC LM358 ic is working as a comparator and IC LM555 is wired as a bistable multivibrator. The transistor BC548 is wired as a switch to provide the negative trigger at pin 2 of LM555. The 50K pot is used to adjust the sensitivity of the circuit.

When any one interrupts the laser beam, the LDR will go to darkness which increases the resistance across LDR, which in turn increases the voltage across it. When the voltage at Non-Inverting terminal (PIN 3) will greater than Inverting terminal (PIN 2) the output of the comparator goes high. Thus the output of transistor goes low and triggers the 555, which sounds the buzzer.



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  • how much time will the buzzer buzz if someone interrupt the laser,and if the circuit will run if the 47k is replaced by 2.2k resistor with 9volts supply plz answer me?

  • how much time will the buzzer buzz?? and if the 47k resistor is changed by 2.2k will the circuit run!? plz answer me

  • Sir..
    Mein ADC k through leds control krna chahta hn…I am using mikro c
    And pic16f877a
    Plz help mee

  • I am Doing it with 3v Battery , whenEver i connect battery it starts buzzing continuously , y it is happening?? and one thing is more, i have used 2.2k resistor instead of 47k b/w Op-amp and Npn transistor ….???
    my Project is on this coming Monday plz help me out..

  • You can also use without opamp, but the LDR resistance will vary a lot. So if you need to adjust the sensitivity, it is better to use a comparator.

  • What’s wrong if OP Amp is not used I didnt used OP Amp and took trigger input from emitter

  • Imagine somebody passes through the laser. He/She will cut the laser only for some time, after that the laser re-establishes its line.

    So if we are not using 555 timer, it will sound only for the moment laser is cutting and will turn off when it re-establishes.

  • Sir, where should i connect to if im making the laser with the circuit? if this circuit is 12v, and the laser only using 3v-5v.

  • will the buzzer buzz? because i don’t see any capacitors in 555 circuit

  • how the power supply is given to this circuit without power how it works please reply to my comment

  • The 555 is not needed. Simply take the output of the transistor back to the inverting input of the op-amp via a diode and the circuit will latch ON.

  • its very small…in a faction of a second…to decrease this time delay u should set laser beam at lowest intensity,and change the sensitivity of circuit can be set by 50K pot.

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