Laser Security Alarm Circuit


BySaddam 104

Laser Security Alarm Circuit using IC 555 and LM358

Security is main concern for various buildings, houses and offices. There are a variety of security alarms available in market which uses various types of technology for intruder detection like infrared sensors, motion sensors, ultrasonic sensors, laser sensors, etc. Previously we have also built some security alarm circuits like this PIR sensor based motion detector and burglar alarm circuit. In this circuit tutorial we are going to build a laser security alarm system which uses a laser light and a laser light detector circuit. It gets activated when someone crosses it. 


Circuit Components

  • IC LM358
  • 555 Timer IC
  • Laser light
  • 150 Ohm, 10K Resistor
  • 10 K POT
  • 220uF capacitor
  • LDR
  • Breadboard
  • 9 Volt Battery and Connector
  • LED


Laser Security System Circuit Diagram and Explanation

Laser Security Alarm Circuit Diagram

In this laser security alarm circuit we have used LM358 Dual Comparator IC for comparing voltages coming from LDR. Comparator is configured as Non-inverting mode and one 10K potentiometer is connected at its non-inverting terminal. An LDR is used for detecting light or laser light with respect to ground through a 10K resistor. And the midpoint of LDR and resistor is directly connected to inverting terminal of comparator.  A red LED is connected at output pin of comparator for indicating intruder detection. A mono-stable multi-vibrator is also used for activating buzzer and LED for a time period.  And a 9 volt battery is used for powering the circuit. 


Working  of Laser Security Alarm Circuit

In this circuit we have set reference voltages of comparators by using potentiometer, we can say this sensitivity of the circuit. Comparator is configured in non-inverting mode. In this system we have placed laser light and LDR facing each other, so laser light continuously falls on LDR. Due to this a potential difference generated across the non-inverting pin of comparator, then comparator compare this potential difference with reference voltage and generate a digital output as HIGH. Before this we have configured 555 timer in monostable mode so we it required a LOW trigger pulse at its trigger pin to activate buzzer and LED. So we applied output of comparator at trigger pin of 555 timer. Even comparator’s output is HIGH when laser lights falls on LDR so at this time buzzer and LED are deactivated. When someone crosses the laser light due to this LDR lost the laser light and generates a different potential difference across the same comparator terminal. Then comparator generates an output as LOW. Due to this LOW signal 555 timer gets a LOW trigger pulse and activates buzzer and LED for a time periods that is defined by R1 and C1 at 555 timer circuit.   

Block Diagram for Laser Security Light

Main component of this circuit is LDR which detects dark and light. LDR is a light dependent resistor which changes his resistance according to the light. When light falls on the LDR surface, it reduces its resistance and when no light resistance of LDR become Maximum. Understand more about working of LDR in this Dark Detector Circuit.


The 555 timer time calculation formula in mono-stable mode is:

Time period T is given by :

T = 1.1 R1 * C1

Where T is time in seconds, R1 is resistance in ohm and C1 is capacitor in farads

To demonstrate this project we have used a small toy laser light.


Get Our Weekly Newsletter!

Subscribe below to receive most popular news, articles and DIY projects from Circuit Digest


  • Nehal's picture
    Dec 15, 2017

    i want to know why this is a dark sensor ,please ?

  • Aswinth Raj's picture
    Aswinth Raj
    Apr 06, 2018

    The sensor used here is an LDR, if light falls on it, the resistance of it will decrease hence it can detect light 

  • Rutwad's picture
    Jan 01, 2018

    To what maximum distance could the laser and ldr be set apart?

  • Josh's picture
    Jan 05, 2018

    What influenced the choice of your components.

  • Laxmi Shivani's picture
    Laxmi Shivani
    Feb 12, 2018

    Can I get this laser security alarm kit and where can I get it?

  • Snehal Mane's picture
    Snehal Mane
    Apr 04, 2018

    Why we can not use battary without 7805

  • dran's picture
    May 07, 2018

    i tried making this project but it doesn't work .. the voltage that goes to the buzzer terminals are both negative... how can i fix this sir?

  • Aswinth Raj's picture
    Aswinth Raj
    May 09, 2018

    How are you telling that the voltage is negative to the buzzer? Check your connection please 

  • Amit's picture
    Jul 30, 2018

    How to increase the time of buzzer for 1 min

  • Rosario's picture
    Nov 28, 2019

    Hi.Is it possible to replace the buzzer with a relay? Thanks

  • Nikhitha's picture
    Sep 19, 2020

    i did the whole circuit in proteus,i'm not getting the sound and the led's are  not glowing.please help

  • Nikhitha's picture
    Sep 19, 2020

    i did the whole circuit in proteus,i'm not getting the sound and the led's are  not glowing.please help

  • Nikhitha's picture
    Sep 19, 2020

    i did the whole circuit in proteus,i'm not getting the sound and the led's are  not glowing.please help


Log in or register to post Comment