Laser Security Alarm Circuit

Published  December 18, 2015   106
S Saddam
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.


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Submitted by sunny rao on Fri, 02/12/2016 - 19:02


Sir,the circuit is not working properly ,the led yellow led is constantly glowing without irrespective of the ldr resistance

Submitted by Smitha on Tue, 03/01/2016 - 09:18


In the circuit you have used 5 resistors but in the demonstration video you have used four resistors...which resistor have you removed???

Submitted by Praveen on Thu, 04/07/2016 - 09:46


Sir, why the IC555's 5th pin has opened.. Where it will be connect..??!

Submitted by John on Fri, 04/22/2016 - 23:24


Where did you get that laser light from and how much did it cost.
Also your breadboarded circuit does not look similar to the schematic. So if i do this circuit using the schematic, will it work?
Also what is that component on the top in the schematic

Submitted by Sadaf yousef on Thu, 04/28/2016 - 21:04


Hi i made a laser security system with resistors 1k and 6.8k and fet transistor with buzzer and photocell and led
When the laser beam hit the photocell buzzer doesnt stop why is it so

Submitted by mogtaba on Thu, 05/26/2016 - 20:26


What is the function of resistance R2?

Submitted by Asad on Mon, 06/06/2016 - 15:39


I just wanna know about the capacitor. is it polar or non polar.. if it's polar.. what is the voltage rating?

Submitted by balaji on Fri, 07/08/2016 - 20:52



Need to know the 555 circuit , which i want to use for disable the Alarm after 10 mins once it get trigred by my secruity circuit.


Submitted by challe on Wed, 07/13/2016 - 18:37


i need a simple school project on door and windows security alarms

Submitted by zulfadzly on Thu, 07/21/2016 - 20:38


hello sir, is it possible to replace the 7805 with 7812?

Submitted by Ugur ECE on Sun, 10/23/2016 - 02:54


I guess my LDR is so sensitive that when I connected the battery my all circuit elements start running (Lights are turned on and buzzer is sounding) What should I do to fix it?

Submitted by Maddy on Thu, 10/27/2016 - 22:22

In reply to by Ugur ECE


You can adjust the sensitivity by rotating variable resistor RV1 and so that it wont trigger in normal daylight.

Submitted by AditiBhatt on Fri, 11/04/2016 - 16:04


Sir, my circuit is working in an opposite matter. when light falls on the ldr, led glows and in dark it does not glow. how to correct it??

Submitted by Swapnil on Tue, 12/06/2016 - 11:04


Sir, if I remove 7805 from the above circuit, do i need to replace the 9V power supply by a 5V power supply ??

Submitted by manar on Fri, 12/23/2016 - 16:18


how i can cross the lazier in my circuit...which is drawing in Proteus program???
how i can cross it in my test it?

Submitted by favas khan on Tue, 02/07/2017 - 14:53


I like the circuit. I am planning to do it, but before that i need your help on some questions
1: i am planning to connect about 5 or 6 LDR s. How to connect( serail or parallel) it to get best result ? ( does it work)
2:how to change r1 and c1( imcrese or decrease) for long buzzering ?
3 how to add a manual switch for turning off buzzer instead of turning off itself?
pls give me a best answer instead of links. I read a lot of articles still i didnot understand it.

Submitted by Subroto naik on Wed, 02/08/2017 - 17:18


I made the circuit but its not working properly when a put the laser on LDR light turns on and when I remove it it goes off tell me any solution

Submitted by Shreepada Mishra on Wed, 03/15/2017 - 08:13


Sir do we have to make the 555 timer in monostable mode before connectingit in the circuit

Submitted by Asaeli Baleimaleya on Thu, 06/29/2017 - 03:35


sir how can this be connected together with an arduino and a keypad....

Its far easier to build it using Arduino, you can connect alarm with any general purpose pin of Arduino and program it to trigger when LM358 is high. Search using search box above for these kind of project. 

Submitted by Mahima on Sat, 09/23/2017 - 15:34


Sir, I am working on a project to sense the point of laser intrusion and send the alert message about the intrusion and location of intrusion.I am a beginner.Can u please tell how this can be implemented?

 sense the point of laser intrusion?

I think that can be done by using multiple LDRs and read every one to know which LDR has been affected by the laser intensively. Since the position of each LDR is already known we can determine the positoin using the same. Just an idea hope it helped.


A pot will have three pins. Connect both the exterme pins to +5V and ground. Now connect the center pin to place where you have to set reference voltage. You can measure the current voltage by placing your multimeter lead on the Ground and the center pin of your POT

Submitted by LAZu on Wed, 11/22/2017 - 04:04


Am getting an error using proteus to do the simulation of the project. Errors include
Transient GMIN stepping at time =5.145e-005
Tran: timestep too small= 1.25e-019.

Please I need help am doing it as my final project