Simple Heat Sensor Circuit


ByJayant 22

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. 


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  • k venkatesh's picture
    k venkatesh
    Sep 06, 2016

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

  • abhishek's picture
    Oct 30, 2016

    i havent got the output brother

  • Nan Hee's picture
    Nan Hee
    Jan 18, 2017

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

  • adam Mohamed's picture
    adam Mohamed
    Feb 21, 2017

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

  • arar's picture
    Mar 07, 2017

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

  • Manasa's picture
    Mar 31, 2017

    Can I get the list of components for this project

  • Evan's picture
    May 03, 2017

    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! :)

  • Mehran's picture
    Jun 02, 2017

    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! :)

  • kwabena reuben 's picture
    kwabena reuben
    Sep 30, 2017

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

  • Majora John's picture
    Majora John
    Oct 03, 2017

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

  • cogent laban's picture
    cogent laban
    Dec 19, 2017

    i like your project

  • Odokonyero Jackson's picture
    Odokonyero Jackson
    Feb 06, 2018

    what can i use to test the conditions of the transistor am using.

  • zenithmarvelous's picture
    Feb 07, 2018

    i really thanks the engrs, cuz u ar doing a great job here.... but can i order for ur packages?

  • sibin's picture
    Feb 19, 2018

    Hi i liked your project it is simple and good.I am thinking Instead of light why cannot we use piezotwitter

  • vibhavith's picture
    Mar 03, 2018

    is there any way to power up this circuit using a laptop by connecting a USB. urgent please reply.

  • Aswinth Raj's picture
    Aswinth Raj
    Mar 05, 2018

    Yes you can, just connect the positive and ground accordingly 

  • Arnab Biswas's picture
    Arnab Biswas
    Apr 06, 2018

    It has a great meme Content

  • Sven's picture
    May 13, 2018

    The picture of the transistor shows "B, C, D", but I think the author means "B, C, E".

  • Aswinth Raj's picture
    Aswinth Raj
    May 16, 2018

    Yes Sven it should have been B,C,E. It would have been a typo 

  • Elly's picture
    Aug 08, 2018

    Can i get the list of components please? Really need it a.s.a.p for my lab project purpose..

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