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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Comments (37)

  • Prakharshrim

    electronics student please pay attention

    September 16, 2012 at 6:06 pm
  • Harisankar Ps

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

    September 16, 2012 at 6:21 pm
  • 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.

    September 16, 2012 at 10:01 pm
  • Trivedi 1412

     chaukass hai re ……………….

    September 17, 2012 at 3:26 pm
  • Anurag

    sahi h yaar…..lge rho….

    September 18, 2012 at 4:59 pm
  • Ankurkillerboy

    awesome …bro!! keep it up..

    September 21, 2012 at 2:30 pm
  • Mohit Mishra

    Good work…..

    September 21, 2012 at 7:12 pm
  • Sankhyamishra

    mst hai yrrrrrr……..

    September 23, 2012 at 10:01 pm
  • 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.

    March 7, 2013 at 9:36 am
  • John

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

    August 12, 2014 at 6:31 pm
  • Ligo George

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

    August 14, 2014 at 6:30 pm
  • vignesh

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

    September 30, 2014 at 7:50 pm
  • vignesh

    how the power supply is give to that circuit

    September 30, 2014 at 7:51 pm
  • vignesh

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

    September 30, 2014 at 7:52 pm
  • vignesh

    how the power supply is give to that circuit

    September 30, 2014 at 7:55 pm
  • Ligo George

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

    October 6, 2014 at 10:01 pm
  • Davit Kordzaia

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

    June 10, 2015 at 6:44 pm
  • Ligo George

    555 is wired as BISTABE MULTIVIBRATOR so capacitor not required.

    June 16, 2015 at 9:39 pm
  • 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.

    August 27, 2015 at 5:04 pm
  • Shinoj Augustine

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

    September 26, 2015 at 8:54 am
  • Ligo George

    You can provide separate power supply for Laser.

    October 3, 2015 at 3:06 pm
  • Ligo George

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

    October 3, 2015 at 5:44 pm
  • oswald

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

    November 1, 2015 at 9:52 pm
  • Ligo George

    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.

    November 21, 2015 at 1:59 pm
  • Hamza Ghani

    its not working for me plz help..

    December 13, 2015 at 5:38 pm
  • Hamza Ghani

    Whats wrong in this one

    December 13, 2015 at 5:45 pm
  • Ligo George

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

    December 19, 2015 at 3:56 pm
  • Hamza Ghani

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

    December 19, 2015 at 9:57 pm
  • Ligo George

    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.

    December 30, 2015 at 8:30 am
  • sarthak roy

    can you explain me this circuit of laser light alarm

    May 23, 2016 at 1:11 pm
  • 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..

    December 17, 2016 at 11:22 am
  • Ligo George

    Adjust the sensitivity using the variable resistor.

    December 17, 2016 at 3:14 pm
  • rhenryan

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

    February 1, 2017 at 12:01 pm
  • Adam Ubaid

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

    June 12, 2017 at 3:14 pm
  • 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

    September 7, 2017 at 1:19 pm
  • 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?

    September 7, 2017 at 2:08 pm
  • rutagengwa eyal

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

    September 8, 2017 at 2:12 pm

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