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    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.

    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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    61 Responses to "Photodiode Alarm Circuit"

    1. Hi thanks it is useful for me

    2. Dear sir:

      Is it correct to directly connect an 8 ohm speaker to the output of IC1? Does such a low impedance speaker will not be a excexive load on the output of IC1 and can damage it?

    3. Dear sir,
      I have made this project .It is not working.can u mail me which type of photodiode u have used.It is urgent.

      • D.Mohankumar says: on October 30, 2010 at 7:46 pm

        You can use any LED type photo diode with black casing.Connect it in the reverse bias mode.Green LED should turn on when the photo diode gets IR/Laser light. Try the circuit using the TV remote to give IR rays.

    4. May I know that the actual value fro capacitor that you used in this circuit?can i used on microcontroller

    5. may i know what the actual value for capacitor that you used in this circuit?can i used for smoke sensor application and apply on microcontroller?

    6. Capacitor 0.1uF is 104 and 0.01 is 103. Both are disc capacitors. You can use the circuit in micro controller also with suitable modifications

      • Sir,
        i m a student of 3rd yr n i wanted to know how can photo diode or photo transistor be connected with processor or controller if i want to read its status n perform the operation. i m confused between emitter and collector

      • can i use an LDR in replacement of the photo diode?

    7. hi D.Mohankumar
      tanx 4 your site
      would u please offer me some site which their project are
      about micocontroller
      as well as your site

    8. Hey D.Mohankumar
      tanx for the site
      pls sir am a final year student am doing your project on photodiode alarm pls sir can you send me more materials on this project

    9. pls sir am a final year student and doing your project on photodiode alarm can your help me with more materials on this project.Tanx

      • kanyisa says: on May 3, 2011 at 3:00 pm

        am 3rd year student and am undertaking a project proposal ma project uses a photodiode alarm and i want it to oprerate at different places if threre is any fault or damage being done plz help to this project what am i supporse to do in this regards?

    10. pls sir am a final year student and doing your project on photodiode alarm can you help me with more materials on this project.Tanx

    11. pls sir am a final year student and doing your project on photodiode alarm can you help me with more materials on this project. Tanx for this site

    12. Please sir am a final year student and doing your project on photodiode alarm can you help me with more materials on this project. Tanx for this site

    13. Give your mail ID

      • shakti gupta says: on September 4, 2015 at 4:40 am

        sir as i,ve required and connected all the devices as shown in the diagram but hear it showing no response hence give me some tips to correctio in it….

    14. Sir can the output time be extended??

      • D.Mohankumar says: on December 12, 2010 at 4:48 am

        Hello Rash
        To get long time alarm, use a monostable using 555 in the place of the astable circuit. Connection to pin 4 remains same. You can adjust time up to 3 minutes if 1 Meg resistor and 100 uF capacitor are used as timing components in the monostable.

      • Hailay nigus says: on June 14, 2012 at 12:44 am

        Hello sir! I construct the circute and the IRLED & PHOTODIODE are conected by leather(not electrically) the green LED alrady is ‘on’ when some one passed b/n the IR LED AND PHOTODIOD the green LED is ‘OFF’. But zer is continous sound on z speaker from starting no new sound even someone is pass what should i do? it is project

    15. it is a very useful in industrial.wonderfull things

    16. Can you mail me the detailed way you calculated the values of components in this circuit for 9V supply?

    17. Sir,can i know the exact value of capacitor C1,C2 andC3?

    18. its ok,i gt my answer

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

    20. 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..

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

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

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

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

    25. 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

    26. 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.(pier.rafee@gmail.com)

    27. 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.

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

      thank you!

    29. 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.

    30. 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 circuit.cn u plz guide me its urgent.

      • Jim Keith says: on November 20, 2012 at 1:25 pm

        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.

    31. Pls send the block diagram of this circuit sir.

    32. Can i put relay somewhere in this circuit?

      • Jim Keith says: on December 17, 2012 at 2:10 pm

        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.

      • Hi Jim!
        Thank you very much for answering!

    33. Dear Mohanji,

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

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

    35. 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 :)

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

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

    38. my photodiode part is working but the speaker is continously on so what to do

    39. Sir pls am working on a project concerning photodiode alarm can you help me with materials on it. Thank you.

    40. mr mohankumar can I get the circuit using monostable plz

    41. It is anybody here who owns the hole project? One of those who ask mr Mohankumar for it…or anyone who implemented it…or who explain me the entire functionality of the circuit….pls i need your help

    42. what is this sensor

    43. great project works PERFECTLY!!!!!! I USED BC 547 & 4.7kohm resistor since i didnt had bc 548 & 4.7k pot….. still its working great…

    44. This project is not working….i redesigned it for my personal use…i will put the circuit on site when i will have enough time….anyway ty Mr Mohankumar for the basic idea

    45. anyone is here plz tell me
      I m making this it works reversible ( means when light fall speaker sounds whenever someone cross it speaker not sounds plz me where is the fault its urgent………

    46. Sir which value of resister used in IR trancmiter circuit?

    47. I am a 2nd year student and i have choosen this as my project. Kindly, plzz email me detail working of this project

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