smoke sensor alarm circuit diagram

Smart Smoke Alarm Circuit

At the heart of this ultra simple smoke alarm is H21A1, which is a photo-interrupter module consists of a gallium-arsenide infrared LED coupled to a silicon phototransistor in a plastic housing. The slot between the infrared sender(LED) and receiver (phototransistor) allows interruption of the signal with smoke, switching the module output from ‘on’ to ‘off’.

When the smoke enters the slot, the infrared beam falling on the phototransistor is obstructed. As a result, the phototransistor stops working and the transistors (T1&T2), wired as a simple darlington pair, conduct to activate the rest of the circuit.

When the darlington pair conducts, the gate voltage of SCR BT169 (T3) goes high and the SCR is fired. Consequently, the gate terminal of mosfet BS170 (T4) receives positive voltage through resistor R4 and the active piezo-sounder (BZ1) energises to latch! Visual indicator LED (D1) lights up instantly.

The buzzer sounds to indicate the presence of smoke, unless you switch off the circuit by disconnecting the 9 volt dc input supply. For optimum performance, adjust trimpots P1, P2 and the value of resistor R1.

Smoke detector alarm circuit diagram

smoke alarm circuit diagram

H21A1 schematic


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  • Aditya m

    Sir, please upload a proper working circuit of this smoke detector.
    Because when the battery is connected to it,the buzzer automatically turns on,with out any obstacle inserted in the H21A1.The led doesnt turn on either…..
    i’ll be very greatful if you upload a proper working diagram of it or you can send also send it to my email (

  • kent

    pls sir can u explain the above in a way that i can understand better?

  • kent

    sir please can u post a diagram showing how to place a LED and a LDR in place of H21A1.

    • madhu

      can u explain the working of the circuit

    • Jim Keith

      Not a simple mod as there are many differences between a photo-transistor and an LDR. This is something on my wish list of projects to develop.

      What I want to do is to make a reflective smoke sensor–the led shines into a literal black box and the LDR looks for some reflected light–signature will be low, so all must work correctly.

  • Nes

    sorry sir i have another question about sensor H21A1. why the schematic H21A1 and the H21A1 in the circuit diagram is different? how to connect that sir?? i had measured the current at H21A1 that you had teach me and i got 2V across it..but my led and buzzer is not there another solution for me?

    • Nes is the same the schematics the transistor is in the pnp transistor but in the circuit diagram is in to connect that?sorry for asking this because my circuit is not really function and its not detect the smoke..i just want to make sure for npn and pnp of transistor connection in H21A1..

    • Jim Keith

      Schematics are the same for the H21A1 –just look more carefully

  • Nes

    sir i had place the paper and the voltage i got is 6.2V but i had tried to remove the paper and i had measured the voltage is 8.2 v..why there are a different when i put a paper into slot and when i remove it??

    • Jim Keith

      I am currently working on a simple updated circuit. At the same time, I will prove the concept to see if it is sensitive enough to really function as a smoke detector–I have doubts regarding its practicality. The photo-interrupter I have on hand is the H21B1 which is a photo-darlington. Operationally, the main difference is that the LED current will be lower as well as the photo-transistor pull-up resistor. Will attempt to post Jan 15.

    • Nes

      ok sir..i will do it first..thanks sir..if i have another problem,..i will asked u again..thank you…

    • Jim Keith

      Upon reexamination of the circuit, it appears that there is a problem in the MOSFET gate drive circuit–there is no positive discharge path for C2. Add a resistor across C2 (in the range of 100K to 1M) and see if that helps.

      Regarding testing of the photo-interrupter, it appears that the change in illumination is causing a change in photo-transistor conductance. The problem here is that it should indicate lower voltage when the paper is removed. Is it possible that your measurements are reversed?

      Also note that the pot setting (P1) should affect this voltage.

  • Jim Keith

    If the buzzer beeps and the LED is off, you must have a wiring error. They both must come on simultaneously because they are connected in parallel.

    To test the opto-interrupter, place a multimeter across pins 3 & 4 to measure voltage across the phototransistor. Then place paper in the slot to view what happens.

    • Nes

      sir..i had check my connection but my led turn on and my buzzer directly beep. i also had test my h21A1, which is i had put a multimeter as you teach me but the voltage that i had across it is 6.2 V..but how can i solve the led and buzzer problem?

    • Nes

      ok thank sir..i will check my connection first!!

  • Nes

    May i ask some question here sir..i had done this circuit connection and it seem function very well but i had some problem about this circuit which are :
    ..why when i connect battery 9 v my LED turn on and at
    the time my buzzer direct seem possible because there are no smoke passing through the H21A1…how can i solve this problem..sir…
    i really need your answer as soon as possible..pleaseee reply me sirrr!!!

    • Nes

      sorry sir..i had do all the trouble shorting tips that you had told me but only my LED is turn off and my buzzer is still directly beep..why this could happen??is there another solution that can i do?is that my h21A1 is not function or what? i also had test it using a smoke..but nothing had happen.. please help me sir..can you give another tips??
      please reply this as soon as possible!!

    • Nes

      thank you sir..i will try again to trouble shooting this circuit..
      if i have any questions or problem i will asked u again..thank for the reply…

    • Jim Keith

      Yes, shorting means to connect –or temporary connection for troubleshooting your problem

      Common is the same as ground in this circuit

    • Nes

      sir..i had do all the troubleshooting that u had gave on above but it the buzzer is still directly beep and the led still turn how can i solve it?is there another solution?
      can i ask another question which is:

      what is the meaning of
      Shorting gate to common of T4 should turn off buzzer.
      Shorting gate to common of T3 should turn off buzzer.
      Shorting base to common of T1 should turn off buzzer.
      is that, i should connect gate to ground or what?

      sorry sir…please reply soon as possible.
      i really need your help!!

    • Jim Keith

      Read and apply my comment of Oct 13 above.


    plz reply me fast…… my project is pending for the submission…!!!

    • Jim Keith

      Trouble shooting tips:
      Shorting gate to common of T4 should turn off buzzer.
      Shorting gate to common of T3 should turn off buzzer.
      Shorting base to common of T1 should turn off buzzer.
      Setting P1 at max resistance should turn off buzzer.
      If any of these steps fail, troubleshoot specific stage to find error.
      Make sure LED in photo-interrupter is forward biased–should have about a 2V drop.
      Placing a piece of paper in the slot is a simpler test than using smoke–when this works, you can attempt to set for higher sensitivity so it can detect smoke.
      P2 appears to be unnecessary–use 10K to 100K resistor instead.


    Mam May I used same value of variable resistor and as well as another resistor… …

    Now i am used same the all component (capacitor is 100microF) in circuit but circuit will not work properly when i connected battery at that time buzzer direct beep… not passing threw any smoke between the photodiode…. then what can i do???

    plz reply ….i dont understand what i could?

  • niharika

    To what values variable resistance should be set reply it fast

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