Fire Alarm Circuit

In this fire alarm circuit project, a thermistor works as the heat sensor. When temperature increases, its resistance decreases, and vice versa. At normal temperature, the resistance of the thermistor (TH1) is approximately 10 kilo-ohms, which reduces to a few ohms as the temperature ncreases beyond 100°C. The circuit uses readily available components and can be easily constructed on any general purpose PCB. Check out the fire sensor circuit.

Schematic of the Fire Alarm Circuit

fire alarm circuit circuit schematic

Timer IC NE555 (IC1) is wired as an astable multivibrator oscillating in audio frequency band. Switching transistors T1 and T2 drive multivibrator NE555 (IC1). The output of IC1 is connected to npn transistor T3, which drives the loudspeaker (LS1) to generate sound. The frequency of IC1 depends on the values of resistors R5 and R6 and capacitor C2. To prevent the thermistor from melting, wrap it up in mica tape.

The home fire alarm system works off a 6V-12V regulated power supply. LED1 is used to indicate that power to the circuit is switched on.
We have not tested this fire alarm systems project.


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  • ghanshyam sharma

    sir I doing this fire alerm as mini project so I must to sme information……plz send to my id

  • lizgrace

    can the speaker be replaced by a buzzer??

  • arvind

    i made this circuit bt after giving suply it will not works . plz help me ….

    • Jim Keith

      This is a repeat of a reply made in Sept.

      Welcome the club of disappointed hobbyists. Remember that unlike humans, electrons never make mistakes.

      First recheck your wiring avoiding assumptions like: I checked that detail before so it is OK.

      Verify that T1 is NPN and T2 PNP. Measure cold resistance of thermistor.

      Divide circuit in half and test part of it. For instance, tie pin 4 of the 555 to +6V to see if the audio annunciator functions.

      Measure input voltage to D1 with voltmeter and determine that it changes with temperature.

      To set the temperature threshold more accurately, it may be necessary to replace R8 with a 5K pot.

      Above all, do not give up easily because what you eventually learn will improve your future trouble-shooting skills.

    • arvind

      everything is correct still its not working…

  • sara

    please ask me. i make this project but not work. use bc549 and bc556 an replace sl100 with 2n2222 this can be my problem? i can measure thermistor with ohmmeter can i test it?

  • Ankit

    sir can u send me the pcb layout of this circuit… because i dont know how to make it and it is my project …
    if snd it i m so thankfull to you..

  • anshuman

    its really help me doing finish my project ,thank you and sir i want to learn more

  • Dorothy

    it is possible to make a special fire alarm basically for more marks in my project.

  • Aniket Ingale

    A have made this circuit but it not works

  • anushuya

    hello sir i need a complete discription abt tis prjct…. can we use this same circuit in AC rooms… what type of heating element should i use to explain…

  • x3p

    Hey. I’m in need of a circuit in addition to a fire alarm to warn of smoke and carbon monoxide as well. PLEASE. Help? Thanks