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 increases as temperature decreases. 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 Simple fire alarm  thermistor circuit diagram

Components required

  1. Power supply
  2. Resistors (1kΩ)
  3. Potentiometer (10KΩ)
  4. Capacitor (10µF, 16V)
  5. BC548 transistor
  6. Thermistor
  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).
Thermistor potential divider working+2 Simple fire alarm  thermistor circuit diagram
  • 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.
Thermistor potential divider working+1 Simple fire alarm  thermistor circuit diagram
  • 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.

Simulation of Simple fire alarm circuit with LED Simple fire alarm  thermistor circuit diagram

Fire Alarm Circuit Video Demonstration

Here we are providing the practical demonstration of our Fire Alarm Circuit to assure you that the circuit is 100% working:). Enjoy!

More Circuit Videos @ CircuitsGallery Video Lab

Pinout of BC548

Related circuit

35 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.

  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

    • Hi Julian
      If you are using 100 ohm thermistor you should use 22ohm POT (Here is a possibility to burnout POT, due to high current, to prevent it use high wattage POT)

  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….

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