Laser Security Alarm Circuit using IC 555 and LM358

Security is main concern for various buildings, houses and offices. For this purpose, 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. We have also built some security alarms like this PIR sensor based motion detector and burglar alarm circuit. In this circuit we are going to build a laser security alarm 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


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 mono-stable 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.


Comments (95)

  • kumar's picture

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

    Nov 03, 2015
  • Norwell's picture

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

    Dec 15, 2015
  • uma 's picture

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

    Jan 27, 2016
  • Ajay's picture

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

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

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

    Feb 12, 2016
  • Abhishek's picture

    Adjust the Variable Resistor RV1 and check circuit connections.

    Feb 23, 2016
  • rohan's picture

    can you let the buzzer run a little longer that

    Feb 19, 2016
  • Jayant's picture

    yes, you can run the buzzer longer, by changing the R1 and C1, as 555 is running in Monostable mode. Check this article:  Monostable multibrator

    Feb 23, 2016
  • nathaniel's picture

    i need clap on and clap off switch

    Feb 21, 2016
  • raji's picture

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

    Mar 01, 2016
  • Smitha's picture

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

    Mar 01, 2016
  • priya's picture

    Why use 150ohm resistance

    Mar 13, 2016
  • Abhishek's picture

    @Smitha and @priya: Follow the circuit diagram

    Mar 28, 2016
  • tait's picture

    WHAT IS THIS 7805.

    Mar 20, 2016
  • Abhishek's picture

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

    Mar 28, 2016
  • Praveen's picture

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

    Apr 07, 2016
  • anusha's picture

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

    Apr 10, 2016
  • John's picture

    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

    Apr 22, 2016
  • devid's picture

    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.....

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

    you may also use toy laser light and ldr.

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

    Apr 27, 2016
  • Sadaf yousef's picture
    Sadaf yousef

    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

    Apr 28, 2016
  • Maddy's picture

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

    May 17, 2016
  • Meji's picture

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

    May 11, 2016
  • Maddy's picture

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

    May 17, 2016
  • mogtaba's picture

    What is the function of resistance R2?

    May 26, 2016
  • Abhishek's picture

    R2 and R3 are forming a Voltage Divider circuit.

    May 28, 2016
  • shivansh rastogi's picture
    shivansh rastogi

    what is RV1 and 10k written under it

    Jun 01, 2016
  • Maddy's picture

    Variable Resistor of 10k.

    Jun 11, 2016
  • Asad's picture

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

    Jun 06, 2016
  • Maddy's picture

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

    Jun 11, 2016
  • balaji's picture


    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.


    Jul 08, 2016
  • challe's picture

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

    Jul 13, 2016
  • zulfadzly's picture

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

    Jul 21, 2016
  • Abhishek's picture

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

    Aug 13, 2016
  • sowmya's picture

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

    Aug 21, 2016
  • saurabh adsule's picture
    saurabh adsule

    it can be used also in pcb and which plz tell

    Aug 29, 2016
  • Mohammed's picture

    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.

    Sep 10, 2016
  • Maddy's picture

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

    Sep 21, 2016
  • Hrithik's picture

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

    Sep 24, 2016
  • chirag's picture

    thanks the circuit worked !!

    Sep 30, 2016
  • student's picture

    sir can you give us the pcb layout for this?

    Oct 02, 2016
  • danish's picture

    sir if we remove LM358 ic ?

    Oct 14, 2016
  • Ugur ECE's picture
    Ugur ECE

    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?

    Oct 23, 2016
  • Maddy's picture

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

    Oct 27, 2016
  • Abu Sidhiq's picture
    Abu Sidhiq

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

    Oct 23, 2016
  • AditiBhatt's picture

    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??

    Nov 04, 2016
  • Abhishek's picture

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

    Dec 02, 2016
  • ahmed enaami's picture
    ahmed enaami

    Are there changes in the 555

    Nov 05, 2016
  • Swapnil's picture

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

    Dec 06, 2016
  • Maddy's picture

    You can use 9v battery directly without 7805.

    Jan 28, 2017
  • pavan's picture

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

    Dec 10, 2016
  • manar's picture

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

    Dec 23, 2016
  • Maddy's picture

    Use Proteus for drawing circuit and simulation.

    Jan 28, 2017
  • aishu's picture

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

    Feb 06, 2017
  • Muvin's picture

    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

    Feb 16, 2017
  • favas khan 's picture
    favas khan

    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.

    Feb 07, 2017
  • Subroto naik's picture
    Subroto naik

    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

    Feb 08, 2017
  • Maddy's picture

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

    Mar 04, 2017
  • Aniket's picture

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

    Mar 03, 2017
  • Shreepada Mishra's picture
    Shreepada Mishra

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

    Mar 15, 2017
  • abenthung's picture

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

    May 02, 2017
  • Exequiel's picture

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

    May 10, 2017
  • abenthung's picture

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

    May 13, 2017
  • sisemimwongo's picture

    awesome i like it

    Jun 15, 2017
  • Asaeli Baleimaleya's picture
    Asaeli Baleimaleya

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

    Jun 29, 2017
  • Abhishek's picture

    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. 

    Jul 05, 2017
  • Zillur Rahman's picture
    Zillur Rahman

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

    Jul 03, 2017
  • Kate's picture

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

    Jul 25, 2017
  • Revanth A's picture
    Revanth A

    Can i increase the buzzer time in the circuit.

    Aug 23, 2017
  • O'kkali Calme's picture
    O'kkali Calme

    great minds with great ideas like the project

    Aug 26, 2017
  • Mahima's picture

    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?

    Sep 23, 2017
  • AISHA's picture

     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.


    Sep 24, 2017
  • Lynn's picture

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

    Sep 26, 2017
  • Zia's picture

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

    Oct 08, 2017
  • angelo 's picture

    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

    Oct 13, 2017
  • satish's picture

    how to set the reference voltage in pot

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

    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

    Oct 26, 2017
  • Faiq's picture

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

    Oct 30, 2017
  • warLORD's picture

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

    Oct 31, 2017
  • DurgaRao's picture

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

    Oct 31, 2017
  • LAZu's picture

    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

    Nov 22, 2017
  • Almarufuzzaman's picture

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

    Nov 22, 2017
  • satheesh's picture

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

    Nov 22, 2017
  • emy's picture

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

    Dec 02, 2017
  • AISHA's picture

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

    Dec 04, 2017
  • Nehal's picture

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

    Dec 15, 2017


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