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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  • ARIF

    hello, may i know how to attach the alarm circuit with receiver circuit..or maybe it can attach with must i do? i hope you can help me.thank you

  • Shaik akram

    Can I get any kind of study material. plz
    needcit urgently

  • nucklu

    hey sir,
    iahve tried your circuit but my speaker is beeping as sson as i apply 9v battery :/ there is effect of the laser beam. Kindly guig me.
    thanks 🙂

  • Alan

    How would you go about adding more photodiodes ( 6 of them)? should it be connected in parallel or in series with the other photodiodes?

  • aniltvmin

    Dear Mohanji,

    Can you plz reply on mr.Jim Keith’s comment…??
    Did you tested your circuit?

  • Marco

    Can i put relay somewhere in this circuit?

    • Jim Keith

      Upon reading the comments and checking the circuit again, it is obvious that this circuit cannot work as shown. In my post “Quirky 555 Timer Reset Function” I show that the reset level voltage threshold is very low.

      To make it work, move the Green LED to in series with R2 and ground the emitter of T1.

      To add a relay, I would add a 2nd 555.
      Tie pins 2 & 6 to the collector of T1.
      Connect relay between pin 3 and common (with back diode across relay coil).
      Ground pin 1.
      Tie pins 4 & 8 to Vcc.
      Pins 5 and 7 are not connected.

      When light hits the photodiode, T1 turns on and pulls the collector of T1 to ground potential. The 555 with pins 2 & 6 tied together acts as an inverting Schmidt trigger and its output (pin 3) goes high and applies voltage to relay coil.

      I recommend a 12V power source so that you may use a 12V relay coil. With 9V source, you must use a higher current 6V relay coil.

    • Marco

      Hi Jim!
      Thank you very much for answering!

  • K.venkatesan

    Pls send the block diagram of this circuit sir.

  • supriya

    sir i have connected the circuit but it is not working properly.i dont know the exact reason the circuit is connected properly itself bt even in the presence of laser rays the buzzer is generated and the led does not glow either(i even tried by changing the led).cn u help me with the different possible faults with my u plz guide me its urgent.

    • Jim Keith

      I suspect your transistor is connected incorrectly. Looking at the flat with the pins down, pinout is CBE.

      This is confusing because the 2N3904 & 2N4401 that I generally use is opposite or EBC.

  • Anirvan

    Dear Sir,
    I am a reader of EFY and I find it very informative. I am a non engineering person but have a lot of interest in electronics. I want to
    know from you that are there proper Engineering jobs available in India ? Because you and EFY deal with electronics regularly I think you can
    answer my question better. I have a friend who had completed his engineering studies recenty and had applied in various companies for job, But
    he has been offered Administrative, managerial and Call Center jobs by most of the companies instead of Engineering job. So I have a question
    to you that are there really any engineering jobs available in India for engineering graduates,hobbyists or people with non-Engineering backgrounds.

  • Cj

    can you send us a photo of your completely assembled project? so we compare our design.

    thank you!