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    This laser door alarm is based on the interruption of Laser beam. A low cost Laser pointer is used as the source of light beam. When somebody breaks the laser path, the alarm will be generated for few seconds.

    Laser Door Alarm Circuit Diagram

    Laser Door Alarm Circuit Diagram

    How to make a laser alarm

    The laser door alarm circuit has two sections. The laser transmitter is a laser pointer readily available. It should be powered with 3 volt DC supply and fixed on one side of the door frame. The receiver has a Phototransistor at the front end. L14F1 NPN Darlington phototransistor is used as the laser sensor. IC1 is used as a voltage comparator with its inverting input tied to a potential divider R2-R3. So that the inverting input is kept at half supply voltage.

    The non inverting input receives a variable voltage based on the conduction of T1. The receiver should be fixed on the opposite door frame and should be properly aligned to the laser beam. Normally the laser beam illuminates the face of phototransistor and it conducts. This keeps the voltage at pin 3 lower than pin 2 of IC1.

    As a result, output of comparator remains low. LED and Buzzer remain off in this state. When a person crosses the door, laser beam breaks and T1 cease to conduct. Collector voltage of T1 rises and voltage at pin 3 of comparator increases and its output becomes high. This activates LED and buzzer. Capacitor C1 keeps the base of T2 high for few seconds even after the output of IC1 becomes low again. C2 gives current to the buzzer for few seconds even after T2 turns off.

    Caution: This circuit uses harmful laser rays. Do not look into the Laser pointer. It should not be placed in places accessible to children.

    attentionThe author D Mohankumar is not an active member anymore. Please take into consideration that the presented information might not be correct.
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    107 Responses to "Laser Door Alarm Circuit"

    1. Hi
      Thinking of modifying the laser based door alarm to keep the pesky herons off my gold fish!!! I will use it to trip a water spray. However, so that the circuit doesn’t trip every time a “nice” little bird come to have a drink at the pond I want to use 2 laser sources about 150mm apart (heron body height) which have to be broken simultainiously to trip the spray. So I don’t have to duplicate the circuit for each laser source, what would the circuit modifcation be to use two L1 4FI’s ? Or do I just parallel them up ????


    2. D.Mohankumar says: on May 1, 2010 at 1:28 pm

      Two laser diodes can be connected parallel in the transmitter but it is better to use two receivers with a common relay for driving the pump

    3. sir here in this circuit can we apply 3v directly to photo transistor L14F1.or photo transistor has to place between laser pointer and 100r 1w resistor
      can we use photo transistor only in stead of laser pointer

    4. Laser pointer is the transmitter. It requires 3 volt separate battery power supply. Laser pointer can be placed 20 meters (maximum) away from the receiver.

      • kok meng says: on May 24, 2012 at 7:29 am

        hey sir i would like to ask if I use this system to combine with RFID card system…does it works ? My college project need to combine between lock system and ID card system. please do me a favor on this :)

      • Roshan Sadath says: on August 19, 2015 at 7:25 pm

        where will you get the laser pointer.
        i have searched for it a lot of places but I havent found it

    5. are all the components in the alarm system easily available

    6. Arushit mudgal says: on July 2, 2010 at 8:57 pm

      sir i am not getting IC1 plz tell me what should i tell electrician what i want

    7. Capacitors C1 & C2 would make the circuit unreliable..the IC as well as transistor will see short until the respective capacitors are full charged.by adding hysterisis to the opamp with a resistor these capacitors can be omitted..a cheap Op will do the job well a Fet input Op is a waste here..Instead of laser beams Infra red is more safe here and cannot be seen and can be hidden and made invisible !!!

    8. dear sir
      i m also facing problem in this circuit.
      it is also not working properly
      please guide me
      i m really thankful to you.

    9. Nothing new in the circuit. Phototransistor triggers the comparator IC. only thing is phototransistor should conduct when laser beam hits on it. Leave the base of Phototransistor unconnected.Connect only collector and emitter.

      • respected sir .

        there is a problem that the photodiode is a very small element,how is it possible that the laser ray must incident on it. there is a remarhable distance between tx and other ckt.so what will i do??
        please reply me as soon as possible

      • sir what is optical sensor

    10. sir i want to modify the circuit to show light insteady of alarm.is it possible? and can’t i build a circuit that produces the laser beam separately insteady of a laser pointer?

      • D.Mohankumar says: on July 29, 2010 at 8:00 pm

        You can use a relay at the collector of T2 to switch on a lamp.It is safe to use laser pointer since its output is low. Building a laser driver is difficult as well as dangerous.We may accidently look into it .Avoid it. Laser pinter can give 20 meters or more range.

    11. and where do i connect pins 1 and 5 of the ic

    12. IC TLO 71 is an Opamp. Pins1,5,and 8 are not connected.Not necessary

    13. have failed to get phototransistor L14F1 is there another type i can replace it with?

    14. nouman nayyer says: on July 30, 2010 at 4:34 pm


    15. what photo trans can i replace for L14f1 phototrans?
      its not available in the electronics shop…

    16. why is that even if with or without phototransistor the buzzer and the LED are in its “on” state?
      whats is 100R? is it 100 ohms?

      • D.Mohankumar says: on December 8, 2010 at 1:34 pm


        Without photo transistor or without light on the photo transistor, more current from R2 flows into pin3 of IC1.This makes the output of IC1 high. When light falls on photo transistor, it conducts and drain the voltage from pin 3 of IC1.This keeps its output low.
        In circuit diagrams, Ohms value is represented as R or E since the symbol is difficult to add.

      • ahhh…thanks…..^_^

    17. my circuit is constantly showing the circuit in its ‘on’ state…. it is not shutting down even in the presence of light on the photo transistor….. plz help me out…!!! :)

      • D.Mohankumar says: on December 12, 2010 at 2:55 pm

        Check the collector current of Photo transistor in the presence and absence of light.It should be high in the absence of light.Check the pin connections of photo transistor. Base pin should be left unconnected.

    18. photo transistor is working fine….also pin connections are also fine…. lpz help me… how to correct it?? :(

    19. is there anny other phototransistor that can i use since L1.. phototransistor is not available here in our palce….

    20. the phototransistor L1 is no longer manfactured…is there any alternative???

      • D.Mohankumar says: on December 15, 2010 at 4:19 pm

        Photo transistor L14F1 is available in the market. It is very sensitive one. You can also use 2N5777 but its sensitivity is lesser than L14F1.LED type photo transistors are also available.The advantage of L14F1 is that, it is FET based and a focussing lens in it.

      • can we use l14g3 inplace of l14f1???????/

    21. how can this circuit be modified to give a constant on when the beam is broken unless the circuit is manually reset??

    22. can i use 2N5778???

    23. sir what kind of phototransistor we use for this project.

    24. sir
      i would like to make a project on remont controller based(which can be operated with remont controller),will u plz suggest me some tng abt that.

    25. i have a problem related to the laser pointer. give me the name of laser pointer that i can give to shopkeeper.

      • D.Mohankumar says: on February 17, 2011 at 4:50 pm

        China make low cost laser pointers are available in shops.

      • sir,i’m not getting the IC-TL071 in “multisim software”.So what i have to do so that i can run this circuit on multisim???

      • I was able to make a simple circuit that has no capacitors. Same concept.

        Sir can you make me a circuit that has relay with LED in the first terminal and buzzer with the other terminal?

    26. sir i want to make a rc circuit to control my door lock. can u help me??

    27. sir I wanted to ask that what should I do to get a continuous unstopped beep in case when laser beam is interrupted???

    28. last week i purchased another laser pointer and again had ruined it because I tried to use a regulated wall power. I used 2x 100 ohms resistor to limit the current to less than 23mA (this is the actual current passing in the original circuit using 3 button cell batteries (1.5Vx3=4.5V). I also include necessary capacitors but to no avail.
      Please help me have the correct schematic for this. Button cell batteries won’t last long, I need to connec from 220v wall outlet. I already had toy laser burglar alarm gadget, I only need a continous power supply. I need this to install this for my house a future farm. thanks



    31. Hello sir i tried this circuit but it shows constant high output . so i checked every component everything worked fine. them i checked opamp and found that output remains high at all conditions accept when pin2 is connected to supply & pin 3 is connected to ground . i don’t know what the problem is . i tried changing opamp but no use . please someone help me :-(

    32. is it necessary to use laser with 3v battery ? what if i use normal laser pointer directly?will connecting laser with 3v battery makes it more brighter or powerful?

    33. not specific to this laser door alarm,but is it possible to make a laser door alarm that will sound when beam is broke and continue to sound until beam is restored?also the activation and deactivation would need to be without delay within fraction of second of beam broken and restored?if possible any ideas where i can get one or get one made?



    35. hi
      I can’t get t1=l14f1
      how can I get it ?

    36. the alarm isn’t working should I wait until the capacitor is fully charged?

    37. Hi guys the circuit is working . only problem is laser does not provide sufficient illumination. so instead i used IR led and now it works fine!!

    38. Hi guys the circuit is working . only problem is laser does not provide sufficient illumination. so instead i used IR led and now it works fine!! But aligning ir led perfectly with phototransistor becomes a little difficult . you have to keep moving led near the phototransistor till the sound of buzzer goes little low them fix the led there so that it works properly

    39. Hi guys the circuit is working .If u want to completely off the buzzer sound when laser pointer beam fall on the phototransistor u remove the capacitor C2 from the circuit and then check.

    40. THANK U SIR

    41. hi this is zeeshan, an Engineering student, i have made this project and found some errors in this first of all according to my knowledge opamp is operated with two different supplies, so we have to use two 9v batteries, one positive terminal is connected to negative terminal of other battery which is now called ground. now positive one battery is connected to +vcc or pin 7 and -ive of second battery is connected to -vee( pin4).
      and those having problem with “ic” not found they can go to market and have any opamp ic and just connect a diode in series with 100r resistor to stop -ive voltages.

      circuit works well but cant sense too fast you have to break beem for approx .5 second to sound buzzer.
      anyone need help can contact


    42. CAN I USE OPAMP 741 OR LF 356 OR LM 358???

    43. sir, am facing the problem.Iam built laser door alarm,but it is not working.The speaker is not off WHEN THE BEAM OF LASER IS EMITTED. What is the reason for this problem sir.please reply

    44. can we use l14g3 in place of l14f1???????

    45. Sir
      L14F1 is not available at Trivandrum(Kerala)

    46. sir,we are working on laser communication.
      the operating frequency reqd is 64 khz.
      the laser diode at transmitting end is the normal beam point laser.At the receiving end the photo diode gives prper results uptil 4 khz.Can u help us as to what photo diode should we use.

    47. Sir, i use this circuit in my project assesment but teacher asked a question if i sleep continously on bed and some one enter inside the house ur alarm is create alarm but question is this alarm is created sound for few second like 2 second
      sir how i modify that problem in the circuit

    48. i got the output succesfully…….:-)

    49. hellow sir,
      to prepare an abstracti need more information can u pls tell any other link to know more about it

    50. hi, thanks for your circuit. I want to attach one more extra part in this. it is like when a person crosses instead of light can i get a message to my mobile that someone went thro?

    51. does anyone know how to use a laser diode with a 14-pin butterfly pinout? i just can’t find a sample circuit online (i have a few of these units of different power ratings). thanks!

      • Hai Mahesh

        Small Laser pointers available in the market… plz buy that one… it’s easy

      • thanks, hari, but that’s not what i am looking for. i need to work on the 14 pin ones so that i can scale up to 10W or 40W laser diodes — far more powerful than a laser pointer.



    52. how much cost it would be Laser Door Alarm Circuit

    53. dear sir,

      is it possible to use invisible
      laser beam in project if yes to
      how to get it ?

      and do we use more laser beam
      another door to attach same circuit ??

      plzz sir reply soon thats my project

      last semester.


    55. sir,can i use an LDR instead of photo transistor..

    56. my bc548 gets warm/hot what to do…???

    57. Abraham lincoln says: on July 14, 2013 at 7:32 pm

      sir, which ic is used in this.

    58. Abraham lincoln says: on July 14, 2013 at 7:33 pm

      it is indicated as ic1 in the circuit. can I know about it.

    59. Dear sir,
      i want to build this project for a use, please can i replace the buzzer with a relay?? can i use any type of photo transistor??
      sir your help will be appreciated.

    60. Sir,
      I would like to know what are the applications for this system?
      And also for explanation purposes is this much information sufficient?

    61. Sir,
      i want to use visible laser beam,and apply as laser game.which type of laser should be used??
      please specify details of laser source.

    62. Hey everyone .. I want to ask you in something
      I want to know after the op-amp output becomes high ! what is the work of t2 ? if it is a switch so how much volt that it gives to the buzzer and C2 ?? because I don’t have any voices here !
      does the buzzer work by voltage or current only ?

    63. Hello friends of this forum,
      I want to ask, can i replace the T1 transistor and use but a LDR?? i cant find such transistors where i live .

      • The LDR has a possibility of working, but it needs the proper filter omit ambient light while admitting the laser or IR LED wavelengths. The pull-up resistor may need to be adjusted. The laser illuminates such a tiny spot, that it may not work well with an LDR that has a large effective area.

    64. also can the Laser Pointer be any ordinary LED???

    65. for extending the alarm time we can use 555 timer


    67. Sir, how we can connect three terminal as two terminal of L14F1 in circuit

    68. Can we use ic 555 as comparator

    69. can i replace the buzzer with 12/24 volts fire alarm?

    70. Can we use L14G1 in place of L14F1. ..PLZ guide me sir..

    71. I made a same concept circuit before using a LDR only and no capacitor. It really works fine.

      Components LDR, Resistor vary with LED/Buzzer voltage and other resistor depends on your sensitivity preferences and Transistor, Laser pointer, toggle switch, PS/Batt, PCB.

      Forgot the transistor but I think I have a copy of my circuit in my laptop.

    72. Hello sir, cn we use LDR instead of T1.

      Plz rply soon

    73. Suraksha Dandin says: on March 16, 2015 at 2:47 pm

      Sir can u plz tell me the specification of opamp????

    74. hey biddiz
      LDR can easly work instead of phototransistor

    75. can we use ic ne555 instead of transistor

    76. sir plz tell me that do we have tu leave base unconnected of the phototransistor 2n5777 as l14f1 is not available so i have tu use this transistor

    77. can we use LDR instead of L14F1

    78. dear sir,thanks a lot for this kind of circuits,pls,send me circuit any wireless signal get away from 200meters,to activate relay

      thanks and regards

      nissanka (sl)

    79. can it be used to find the permissivity of cockpit wind shield byway of modifying given alarm circuit?

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