Simple Heat Sensor or Temperature Sensor Circuit

We have previously built the Fire Alarm using Thermistor and Fire Alarm System using AVR Microcontroller. Today we are building very simple Temperature Sensor Circuit or Heat Sensor Circuit. This circuit uses very few and basic components which can be easily available, anyone can build it right away. This Heat Sensor is not only simple but also effective; you can try it at home.


Here Transistor BC547 is used as a Heat Sensor. As the temperature of PN junction increase, transistor starts conducting to some extent. This ‘temperature’ property of transistor is used here to use it as a heat sensor.


Diode 1N4148 and variable resister of 1k ohm are used here to set a reference or threshold level for the sensitivity of heat. And the sensitivity of the circuit can be adjusted by rotating the knob.

temperature detector using transistor circuit diagram

Working of the circuit is simple, when there is heat or increase in temperature to the level where it crosses the threshold set by Pot, Then the collector current increase and LED starts illuminating slowly. We can also use Buzzer in place to LED. You also note that, before start testing the circuit, first set the Variable Resistor. When you rotate it completely in one direction, LED will be Off, and when you rotate it completely in other direction, LED will glow with full illumination. So set the Pot at the position, where a slight rotation will start a dim illumination in LED.


The temperature dependency of PN junctions in transistor can be understood by the formulae presented here. Base-Emitter voltage (VBE) drops approx. -2.5 mV/°C, negative sign indicates the Drop or decrease of voltage across B and E.


A NPN transistor much acts like a diode if we short the Base (B) and collector (C) of Transistor. In that case B-C acts as Positive terminal and Emitter (E) acts as negative terminal. And if we keep the voltage source constant, then the voltage across the transistor becomes the function of the temperature. For PNP transistor E will be positive terminal and B-C will be negative. Hence by shorting the B and C, we can use transistor as Temperature Detector. Below is the NPN transistor BC547 Pin configuration:

NPN Transistor BC547

Operating temperature of Transistor BC547 is up to 150 degree C, so it can be perfectly used at high temperature as a Heat Sensor. And we can also make a Fire Alarm out of it. 


Comments (13)

  • k venkatesh's picture
    k venkatesh

    the project was nice and the cost is less it will be useful to the students

    Sep 06, 2016
  • abhishek's picture

    i havent got the output brother

    Oct 30, 2016
  • Abhishek's picture

    Try adjusting the variable resistor (Pot)

    Dec 01, 2016
  • Nan Hee's picture
    Nan Hee

    can i get the list of the components? please.for our project purpose..

    Jan 18, 2017
  • adam Mohamed's picture
    adam Mohamed

    It was a nice and simple project. Such basic projects teaches alot.

    Feb 21, 2017
  • arar's picture

    can I get that list of components please for project purpose..thank you

    Mar 07, 2017
  • megha's picture

    nice project

    Mar 20, 2017
  • Manasa's picture

    Can I get the list of components for this project

    Mar 31, 2017
  • Evan's picture

    Great work! Is it possible to sent me the list of components you used so that I can be possible to build it for my school project? It would be grateful if you can! Thanks! :)

    May 03, 2017
  • Mehran's picture

    Great work! Is it possible to sent me the list of components you used so that I can be possible to build it for my school project? It would be grateful if you can! Thanks! :)

    Jun 02, 2017
  • kwabena reuben 's picture
    kwabena reuben

    where is the base of the transistor connecting? resistors got only two wires.

    Sep 30, 2017
  • Majora John's picture
    Majora John

    is it possible to use any transistor as our heat sensor except the one you mentioned above

    Oct 03, 2017
  • cogent laban's picture
    cogent laban

    i like your project

    Dec 19, 2017

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