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.


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  • kumar's picture
    Nov 03, 2015

    sir plz canu explain how to connect it on breadboard by stepwise by tomarrow

  • Norwell's picture
    Dec 15, 2015

    Thanks for your info and circuit diagram. Can you make a diagram of laser alarm with timer and manual reset?

  • uma 's picture
    Jan 27, 2016

    Y can't using. Ic741 it is also a comparator

  • Ajay's picture
    Feb 12, 2016

    You can use LM741, but LM358 is newer and advance than 741, compare the datasheets of both the ICs.

  • sunny rao's picture
    sunny rao
    Feb 12, 2016

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

  • Abhishek's picture
    Feb 23, 2016

    Adjust the Variable Resistor RV1 and check circuit connections.

  • rohan's picture
    Feb 19, 2016

    can you let the buzzer run a little longer that

  • vibhavith's picture
    Jul 23, 2018

    should i reduce R1 and C1 or increase their values to run the buzzer for longer time

  • nathaniel's picture
    Feb 21, 2016

    i need clap on and clap off switch

  • raji's picture
    Mar 01, 2016

    how much time does LDR require to get adjusted to laser light beam??

  • Smitha's picture
    Mar 01, 2016

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

  • priya's picture
    Mar 13, 2016

    Why use 150ohm resistance

  • tait's picture
    Mar 20, 2016

    WHAT IS THIS 7805.

  • Abhishek's picture
    Mar 28, 2016

    It is a Voltage Regulator IC to provide the constant and smooth supply of 5v voltage.

  • Praveen's picture
    Apr 07, 2016

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

  • anusha's picture
    Apr 10, 2016

    what is the intensity of the laser light source used??

  • John's picture
    Apr 22, 2016

    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

  • devid's picture
    Apr 27, 2016

    laser usually costs about 1.5$ on ebay, all spot lasers will be ok !
    i did like on this schematic, leds show that everything is working but buzzer doesnt gev any sound....
    its voltage regulator , it converts 9v to 5 v.....

  • saddam khan's picture
    saddam khan
    Apr 27, 2016

    you may also use toy laser light and ldr.

    yes we used 7805 to convert to 5 volt nad 7809 for converting 9 volt.

  • Sadaf yousef's picture
    Sadaf yousef
    Apr 28, 2016

    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

  • Maddy's picture
    May 17, 2016

    Please use the components of mentioned values, different resistor values change the High time of Monostable mode of 555.

  • Meji's picture
    May 11, 2016

    Is the 7805 compulsory? What happens if I don't use it

  • Maddy's picture
    May 17, 2016

    7805 is used for 5v regulated power supply. For this particular circuit, you can omit 7805.

  • mogtaba's picture
    May 26, 2016

    What is the function of resistance R2?

  • shivansh rastogi's picture
    shivansh rastogi
    Jun 01, 2016

    what is RV1 and 10k written under it

  • Asad's picture
    Jun 06, 2016

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

  • Maddy's picture
    Jun 11, 2016

    Its polarized capacitor with Voltage rating 16v, you can use any 220uF normal cap.

  • balaji's picture
    Jul 08, 2016


    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.


  • challe's picture
    Jul 13, 2016

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

  • zulfadzly's picture
    Jul 21, 2016

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

  • Abhishek's picture
    Aug 13, 2016

    You can simply remove the 7805 and directly connect the battery in this project.

  • sowmya's picture
    Aug 21, 2016

    Irrespective of laser light the sound is coming from the buzzer...what to do?

  • saurabh adsule's picture
    saurabh adsule
    Aug 29, 2016

    it can be used also in pcb and which plz tell

  • Mohammed's picture
    Sep 10, 2016

    The sound comes as soon as the battery is connected irrespective of the ldr. It goes off after some time but never sounds again until battery reconnected.

  • Maddy's picture
    Sep 21, 2016

    Adjust the RV1 properly. Circuit is simple and should work easily.

  • Hrithik's picture
    Sep 24, 2016

    I made it...all soldered correctly...buzzer and all ligHTS r working simultaneously

  • chirag's picture
    Sep 30, 2016

    thanks the circuit worked !!

  • student's picture
    Oct 02, 2016

    sir can you give us the pcb layout for this?

  • danish's picture
    Oct 14, 2016

    sir if we remove LM358 ic ?

  • Ugur ECE's picture
    Ugur ECE
    Oct 23, 2016

    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?

  • Maddy's picture
    Oct 27, 2016

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

  • Abu Sidhiq's picture
    Abu Sidhiq
    Oct 23, 2016

    Sir,When i give the connection to the Pot...The LED does not glow

  • AditiBhatt's picture
    Nov 04, 2016

    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??

  • Abhishek's picture
    Dec 02, 2016

    Check the connection to LM358 IC's inverting and non inverting terminals.

  • ahmed enaami's picture
    ahmed enaami
    Nov 05, 2016

    Are there changes in the 555

  • Swapnil's picture
    Dec 06, 2016

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

  • Maddy's picture
    Jan 28, 2017

    You can use 9v battery directly without 7805.

  • pavan's picture
    Dec 10, 2016

    Sir plz tell me about how to connect the components in breadboard

  • manar's picture
    Dec 23, 2016

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

  • Maddy's picture
    Jan 28, 2017

    Use Proteus for drawing circuit and simulation.

  • aishu's picture
    Feb 06, 2017

    Sir i done this project successfully. But i want to know the application and uses of this project

  • Muvin's picture
    Feb 16, 2017

    i am not being able to success in this project sir buzzer and yellow led is continuously activate no matter light is in ldr or not

  • favas khan 's picture
    favas khan
    Feb 07, 2017

    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.

  • Subroto naik's picture
    Subroto naik
    Feb 08, 2017

    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

  • Maddy's picture
    Mar 04, 2017

    Adjust the RV1 (potentiometer) and check the connection with LM358.

  • Aniket's picture
    Mar 03, 2017

    Sir plzz tell use 10k potantiometer just like preset...can i use 10 k potentiometr..sir tell me

  • Shreepada Mishra's picture
    Shreepada Mishra
    Mar 15, 2017

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

  • abenthung's picture
    May 02, 2017

    sir can I get an introduction,abstract,conclusion on the the project "laser security alarm using ic555 and lm358"

  • Exequiel's picture
    May 10, 2017

    What is the value of voltage of the capacitor? Is that ceramic or electrolytic?

  • abenthung's picture
    May 13, 2017

    can I get a perfect working operation and methodology on "laser security alarm using ic 555 and lm358".

  • sisemimwongo's picture
    Jun 15, 2017

    awesome i like it

  • Asaeli Baleimaleya's picture
    Asaeli Baleimaleya
    Jun 29, 2017

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

  • Abhishek's picture
    Jul 05, 2017

    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. 

  • Zillur Rahman's picture
    Zillur Rahman
    Jul 03, 2017

    Sir,,I am new for Micro- controller project program.plz,,help me.I wanted made of micro- controller project

  • Kate's picture
    Jul 25, 2017

    What is that 10k parallel to R4?? Please reply.

  • Revanth A's picture
    Revanth A
    Aug 23, 2017

    Can i increase the buzzer time in the circuit.

  • O'kkali Calme's picture
    O'kkali Calme
    Aug 26, 2017

    great minds with great ideas like the project

  • Mahima's picture
    Sep 23, 2017

    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?

  • AISHA's picture
    Sep 24, 2017

     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.


  • Lynn's picture
    Sep 26, 2017

    sir can you tell me what is the function of each components? thaaaanks

  • Zia's picture
    Oct 08, 2017

    Great project , can you provide Proteus file for this simulation ?
    Thanks in advance.

  • angelo 's picture
    Oct 13, 2017

    Wer did you put the other one resistor? In diagram it has 5 resistor and when you do it on breadboard it just 4 resistor.. Pls answer

  • satish's picture
    Oct 26, 2017

    how to set the reference voltage in pot

  • Aswinth Raj's picture
    Aswinth Raj
    Oct 26, 2017

    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

  • Faiq's picture
    Oct 30, 2017

    Sir, what is the meaning of arrow from LM358IC to RV1.
    Is it a connection to RV1 above or below?

  • warLORD's picture
    Oct 31, 2017

    Sir . In ur circuit above.. how many seconds that the byzzer activate

  • DurgaRao's picture
    Oct 31, 2017

    sir plz can you explain how to connect it on breadboard by stepwise by today

  • LAZu's picture
    Nov 22, 2017

    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

  • Almarufuzzaman's picture
    Nov 22, 2017

    Sir, why did you use Variable Resistor RV1 instead of a Fixed Resistor?

  • satheesh's picture
    Nov 22, 2017

    can you give me the Pcb( printed circuit broad) layout

  • emy's picture
    Dec 02, 2017

    when I do it on OrCad there is error because pin 5 is unconnected . Any help to solve this problem?

  • AISHA's picture
    Dec 04, 2017

    You have to place a no connection (NC) symbol to all unconnected pins in orcad, only then it will work


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