Photodiode Alarm Circuit

This Photodiode based Alarm can be used to give a warning alarm when someone passes through a protected area. The circuit is kept standby through a laser beam or IR beam focused on to the Photodiode. When the beam path breaks, alarm will be triggered.

The circuit uses a PN Photodiode in the reverse bias mode to detect light intensity. In the presence of Laser/IR rays, the Photodiode conducts and provides base bias to T1. The NPN transistor T1 conducts and takes the reset pin 4 of IC1 to ground potential. IC1 is wired as an Astable oscillator using the components R3, VR1 and C3. The Astable operates only when its resent pin becomes high. When the Laser/IR beam breaks, current thorough the Photodiode ceases and T1 turns off. The collector voltage of T1 then goes high and enables IC1. The output pulses from IC1 drives the speaker and alarm tone will be generated.

Photodiode Alarm Circuit

IR Transmitter Circuit

A simple IR transmitter circuit is given which uses Continuous IR rays. The transmitter can emit IR rays up to 5 meters if the IR LEDs are enclosed in black tubes.


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  • Ivan Petrov

    Hello , sir D.Mohankumar

    I’m student 2-nd year, studing electronics and i have to write a presentation about how to connect a photodiode to mircocontroller(port). Can you please help me with some information thank you a lot.

  • Rafee Baksh

    Hello, sir….i’m the student of ece 3rd yr…….i had read abt a project of photodiode…in a book of Electronics and device circuits of JB Gupta. and searched abt it, and found ur project , would u please mail me brief details about it on my Id … I’ll be thankfull to u.(

  • wise

    what kind of photodiode did you use? we made the circuit but when we hit the photodiode with a laser, the speaker would not stop and the LED would not work. please help

  • sounak

    sir can u plz send me the project ????
    plz send me at my email id plz


    sir photodiod is not available .is it worked with any other.

  • Sundar

    Thank you very much to publish the circuit. This is a part of my big project.

  • angie

    may you please send me more stuff on this project i’m working on my project write up. thank you sir

  • share

    good day! sir D.Mohankumar,

    sir, i am a 4th yr.electronics student and i’m in need of a project this semester… i found out that this photodiode alarm is a nice one. can you help me in doing it? can i ask for the whole information about this?, the materials, the exact value of the components and the computations if possible, just send it to my email.. i need your help.. i am hoping for a positive response towards my request… thank you very much..

  • ebin

    sir i made this circuit ,but it is not working.can u mail the reasons

  • dinaskumar

    its ok,i gt my answer

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