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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  • Prakharshrim

    electronics student please pay attention

  • What is the time delay, for the alarm to start after the light beam get interrupted??

  • Abhay Bajpai

    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.

  • Trivedi 1412

     chaukass hai re ……………….

  • Anurag

    sahi h yaar…..lge rho….

  • Ankurkillerboy

    awesome …bro!! keep it up..

  • Good work…..

  • Sankhyamishra

    mst hai yrrrrrr……..

  • Colin55

    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.

  • John

    will this work with 9 volts input? what do i need to change if using 9 volts input?

  • Yes, it will work.. no change required. .

  • vignesh

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

  • vignesh

    how the power supply is give to that circuit

  • vignesh

    how the power supply is give to that circuit without the power hw its work

  • vignesh

    how the power supply is give to that circuit

  • It don’t get your question. Do you need a backup battery to power the above circuit ??

  • Davit Kordzaia

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

  • 555 is wired as BISTABE MULTIVIBRATOR so capacitor not required.

  • Luqman Hakim

    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.

  • Shinoj Augustine

    Will it work with 3V battery( or what is the minimum power to make working this circuit)

  • You can provide separate power supply for Laser.

  • It might not work with 3V battery. You try replacing the opamp with a 3V one.

  • oswald

    i would like to know how does it work without using 555timer

  • 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.

  • Hamza Ghani

    its not working for me plz help..

  • Hamza Ghani

    Whats wrong in this one

  • Divide the circuit in to 3 parts and test individually.
    1. Comparator
    2. Transistor Switch
    3. 555

  • Hamza Ghani

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

  • 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.

  • sarthak roy

    can you explain me this circuit of laser light alarm

  • Muhammad Salman

    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..

  • Adjust the sensitivity using the variable resistor.

  • rhenryan

    how long is the maximum length between the laser and the LDR?

  • Adam Ubaid

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

  • rutagengwa eyal

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

  • rutagengwa eyal

    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?

  • rutagengwa eyal

    i have copied your circuit but it is not running what might be the problem