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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  • mohammed

    I connected the circuit and it is fine but when I make smoke not working when I put paper in H21A3 it is work

    let me what is the problem please

  • sacad

    sir can i found H21A1 in proteus becouse i need it quickly plz sir reply or if u know another one that i can found in proteus plz plz sir tel me…

  • mohammed

    I connected the circuit components in the breadboard but the problem when I connect it with the power the buzzer start ruing should be run when the sensor detect the smoke can you support where is the problem. type of sensor is H21A3 I cannot get sensor H21A1

    waiting replay as soon as possible please

    • Mihai

      Don’t bother with this piece of junk. You are wasting your time. Find a viable project. There are plenty more on the web.

  • mohammed

    could you please somebody explain the function of each part in the circuit

  • bhargav

    sir when we connect 9 volt battery buzzer is ringing
    what is the reason sir

  • mohammed


    can we connect another circuit that will receive the output of alarm circuit to automatically unlock exit door if somebody familiar please guide me

    I mean to make this circuit when the fire alarm run automatically the exit door unlock (the door which has electromagnetic lock )plz guide the best circuit to connect with output of Smoke Detector Circuit to unlock the door

  • angah

    hello..what can i replace the bt169 with?? because it not found in isis ..thanks

  • byambaa

    Hello,i not good english khkh.T4-BS170 transistor, K105 (FOYC) transistor can be replaced if? Its BS170 transistors are not available(not find).help me

  • Mihai Popescu


    Employ a resistor(say, 1kΩ or more) in series with emitter of T2 or P2. If P2 is at its minimum value (wiper is up–almost short circuit), then, the Darlington transistor(T1, T2) is subjected to all the supply voltage without a limiting current resistor. It will blow up for sure. Or the battery. Or both.

    The thyristor T3(BT169) can’t be turned on because there is no resistive path to ground(cathode via R4 to GND), so there will be NO latch-up!

    In order to complete the circuit (for conduction of the thyristor) insert a resistor instead of C2(which is useless, anyway). Otherwise T3(BT169) is useless!!!

    Suppose there is a resistive path to ground(and smoke too)….

    R3(100Ω) is waaaayyy too small. It will drain the battery in no time. In fact it will draw more current than the buzzer(and the LED) itself.

    BT169 needs only 200µA in its gate to turn ON. A proper value for R3 would be 15KΩ…27KΩ.

  • sreehari

    I had selected this circuit as my project and I am successful now as the circuit is working on pcb.
    I have to face Viva next and I have the following questions unanswered:
    Why is the LED not glowing when current is passing through the phototransistor which is high and glowing in the case of low current flowing due to the obstruction.According to my knowledge it should glow in both the cases.
    Anyone please help me!!
    please explain me the working of the circuit in detail or mail to else I will be detained If I don’t answer the questions in viva.

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