Simple fire alarm- thermistor circuit diagram

Here i-St@r comes with a simple and automatic fire alarm and detection system. This fire detection alarm circuit is based on thermistor. What is a thermistor? A thermistor is nothing but a variable resistor whose resistance changes with temperature. Fire detection is simply possible via this low-cost alarm circuit.

A simple potential divider arrangement using thermistor is capable of sensing the temperature (presence of fire) and alerting us with a warning signal. This is suitable for your home security systems. Also school students can do this fire detector circuit as their high school science fair projects. Here I will explain in detail about fire alarm circuit wiring with animation/ simulation of fire alarm.

Circuit diagram of simple fire alarm

fire alarm circuit diagram

Components required

  1. Power supply
  2. Resistors (1kΩ)
  3. Potentiometer (10KΩ)
  4. Capacitor (10µF, 16V)
  5. BC548 transistor
  6. High resistance thermistor (NTC)
  7. Diode (1N4001)
  8. 6V Buzzer

Working of fire alarm circuit

  • The thermistor and R1 forms a potential divider network which drives the transistor.
  • The heart of this circuit is a thermistor. Thermistors are low-cost, easily-available temperature sensors widely used for uncomplicated temperature measurements.
  • Thermistors are temperature sensitive resistors. When temperature increases resistance offered by the thermistor decreases and vice versa. At normal temperature, the resistance of the thermistor is around 10kΩ.
  • All resistors vary with temperature, but the semiconductor materials used for thermistors are especially sensitive to temperature.
  • The transistor is turned ON by the voltage drop across the resistor R1.
  • Consider the temperature of the atmosphere is around 25°C, and then the resistance of thermistor is high so the voltage across the thermistor is also high according to the basic ohm’s law V=IR.
  • The threshold voltage can be set by varying the potentiometer (R1).

  • At this situation voltage across resistor R1 is low and it is not sufficient to turn on the transistor.
  • As temperature rises, the resistance of thermistor decreases so that the drop across the resistor R1 increases which turns ON the transistor.

  • When the transistor is turned ON, the current from Vcc starts to flow via 6V buzzer which produces a beep sound.
  • Diode is used for enabling unidirectional conduction and the capacitor removes sudden transients from the thermistor.

Simulation of the fire alarm project

We have used an LED instead of buzzer for the simulation of fire alarm system.


Pinout of BC548

Related circuit

111 thoughts on “Simple fire alarm- thermistor circuit diagram

    • Hi pankj khedekar
      Please try a 10K potentio meter instead of 470 ohm resistor and set the threshold voltage by varying it.

      • Have u used the 1 k resistor.My buzzer is ON, even before fire.What are the corrections made in the circuit given here?

        • Hi vijayalakshmi,
          Sensitivity of this circuit can be adjusted using R1 potentiometer. Try to vary R1 and test the circuit.
          Our circuit is 100% working and I added Video Demo also.
          My recombination are
          1. Adjust R1 Pot
          2. Crosscheck the components

          Please let me know your further improvements in the circuit

    • You can use any thermistor, but change the potentiometer accordingly (with respect to resistance). Power supply 6V 500mA DC.

      • If I use any Thermistor, how do I change potentiometer accordingly. I have been connecting the circuit, but it does not work at all, buzzer sound at all times. Please tell me something………Thanks

  1. is it not okay to use 100 ohm thermistor sir? i tried to use it and the speaker turned on even without presence of heat

  2. Good Morning. What is the value of thermistor you used in this project? We can’t buy thermistor because we didn’t know the exact value of thermistor for this project. PLEASE REPLY AS SOON AS POSSIBLE. Thank you so much.

  3. I would like to know what is the difference between 10uF/16V and 10uF/63V.

    Also, can the latter one be used in this circuit?

    • Hi Shaurya
      Voltage rating indicates that the maximum voltage that the capacitor can operate, for 12V circuit you can use 16V and above voltage rated capacitors.

  4. hey, i think the page has been changed and the circuit diagram too. the last was the 470 ohm resistor used. my project worked on it. i need that old page urgently. please send me the old page as soon as possible.

  5. Hello sir, my circuit is not working can you tell me why. I have used 10k ohm variable resistor and then also it is not working. Reply pls….

    • this cirucit is not workung model because it has no proper bising for transistor. you can remove first of all capacitor and diode. after that give proper baise to bc547. u wii get success.

  6. Hi guys,
    i followed this guide for a class mini project, and i encountered a problem, my buzzer will sound even when there was no heat.
    the problem was the THERMISTOR itself, u see these are of two types, NTC vs PTC.
    NTC works by dropping the resistance as the temperature rises, PTC does the oppositte.
    so make sure u use the right type, here we need NTC.
    cheers and thanks to the Author for this guide

  7. i tried constructing the same circuit as shown in the video. sir, if you permit,can you tellin here the steps you did in constructing the circuit.?thankyou.

  8. Hi Shivangi,
    At this point we are helpless, due to our busy schedule we won’t be able to prepare the video you requested. May be we could do it at a later time, it seems that you’re a beginner in electronics.

    Please comment the problem your’e facing with the circuit so that we can figure it out. Never loose hope and keep trying. All the best.

    • sir i will sent you my circuit which i made… plzz give me your email id to send pic…plzz i tried so much but i fail so plzz he me and take out of me…….. 🙁
      if uh dont that only send me pic of joined circuit but plz take it clear….

        • I analyzed your circuit arrangement, please make the following changes
          1. Take preset connection from middle and anyone of the end points.
          2. Interchange connections to the buzzer.

          Now vary preset and check the output. Hope that helps.

          • i cant understand what you say ..plz explain mr in easy way…..which changes i do in circuit…tell me and where i use 1k resistance and 10k resistence??

  9. Sir I have maded this fire alarm by removing 10 uf electrolytic capacitor and changed 10 kilo ohm variable resistor with 1 kilo ohm variable resistor thank you sir.

  10. That’s a simple circuit and operation to detect fire. may you guys explain the block diagram of this circuit.

  11. I want to work on it this year 2015, But Sir is there any way of constructing it using the must least cost of materials,pls. i’m waiting because i need an advice through my email.Thank

  12. i simulated this circuit using pspice and i’m new to pspice simulation software,so.. wanted to know how the output of this circuit will look like in a CRO,,
    Thank you,

  13. I am using 100ohm thermistor and 22ohm resistance in the place of potentiometer. My buzzer is sound continuously. And thermistor is also becomes hot after some time. Please help me.

  14. sir,i use only 1okohm pot it does not give as variable type and thermister is very small in black colour this type of thermister can i use, and the problem is it produce light the thermister does not heat,pls reply me as soon as possible

  15. sir, i use 10k thermistenot and 10k pot also but it does work. the circuit is always on even i does not given a heat. can you given solution immediately.plssssssss.

  16. Hi friend
    I just ask from you’re this ckt working or not?(100&working) and I want to if it working good
    How much amp I need 6VDc & how much amp?

  17. Hi sir the circuit even when the thermister us not heated pls tell what to do
    For 10k thermister which value of pot should be used

  18. what is the set point of temperature for this fire alarm? I mean at what temperature this fire alarm will be activated because when a match stick will be ignited then also the thermistor will sense it and the circuit will be activated

  19. Hi. I have a 12 v buzzer available and I want to use it in the circuit. What changes will iI make to my circuit?
    Thank you

  20. For 10k thermister which value of pot should be used.if i use 10 k pot them what abut thermistor rating. also tell me how they are related.

  21. can u please take another clear picture of the circuit u built? Please it’s for a project and i need a reply as soon as possible.

  22. love this! but I am a newbie .please vcc stands for what? and what are d two terminals for d supply voltage?.thanks in anticipation

  23. hello,
    my company needs a industrial fire control systems.
    would you give us any information about your products?

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