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You are here: Home / Mini Projects / Simple Thermostat Circuit Using Transistors

Simple Thermostat Circuit Using Transistors

Last Updated on May 26, 2022 by Swagatam 17 Comments

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The electronic thermostat explained here can be used to control the room temperature by appropriately switching (turning on and off) a heating device.

By: R.K. Singh

Operational details of electronic thermostat

The circuit employs a thermistor NTC (negative temperature coefficient) as the sensor device.

- As long as the ambient temperature stays higher than the value prefixed by the potentiometer, the relay correspondingly remains inactivate and the red LED may be seen lit.
- In an event of the ambient temperature getting lower than the set value, the relay is activated and the green LED is illuminated.

The potentiometer needs to be carefully adjusted in order to get the desired effects.

To adjust the proposed transistor thermostat circuit, the NTC is enclosed inside a glass tube and its leads are terminated out through long wires so that it can be placed over the desired location for the required sensing.

The circuit is set by placing the thermistor glass tube along with a mercury thermometer inside a container filled with melting ice water, and in the next procedure its placed at ambient temperature and finally close to a gas burner for implementing all the setting levels.

In each of the above mentioned cases, the point at which the green LED just lights up is located by gently manipulating the pot knob toward the maximum and marking it with a line over the knob dial in order to make the relevant temperature calibrations, these markings are then appropriately labelled with the respective temperatures which are recorded simultaneously on the associated thermometer.

The circuit operation is quite straightforward and can be understood by assessing each transistor cut off and triggering states.

For so long as the NTC resistance is very high (when the ambient temperature is low) causes the transistor T1 to go into saturation provided the potentiometer setting permits this.

Considering the above situation is enabled the transistors T1 T2 T3 and T4 saturate and also activate the relay.

The relay employed may be a double contact and each time it is activated two operations are executed, one pair of contacts to switch the LEDs and the other to activate the heater or the desired load.

The capacitor C1 makes sure sudden changes in the value of the NTC.

Circuit Diagram

Bill Of Material for the above transistor thermostat circuit:

  • Resistors:
  • R1, R4, R6: 10K,
  • R2: 12K,
  • R3: 6.8K,
  • R5: 33K,
  • R7: 470K,
  • R8: 2.2K,
  • R9: 560 ohms.
  • P = Potentiometer Linear 10K.
  • NTC: Negative temperature coefficient 10K.
  • Capacitors:
  • C1: 100nF,
  • C2: 47uF, 10V (electrolytic capacitor).
  • LEDs: 1 red, 1 green
  • Transistors
  • T1 and T3: 2N2222,
  • T2: 2N2907,
  • T4: 2N2905
  • Relay: 12V DPDT.
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About Swagatam

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials.
If you have any circuit related query, you may interact through comments, I'll be most happy to help!

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  1. Joseph says

    May 26, 2022 at 2:58 am

    Bonjour cher ami et mes très sincères remerciement.
    Voudriez vous m’indiquer à quel endroit précis peut on placer un second potentiomètre dans ce circuit? Cela permettra d’avoir un seuil de temperature haut et un seuil de temperature haut réglable à volonté.

    Joseph

    Reply
    • Swagatam says

      May 26, 2022 at 10:15 am

      Hello Joseph, you can add another potentiometer in series with R1, but it will also serve the same function as the existing potentiometer.

      Reply
  2. Yogesh says

    June 27, 2021 at 9:09 pm

    Sir, I want to make thermistor circuit with opamp, to activate the relay to run a exhaust fan accurately at a particular temperature.
    For eg: turning the relay on at 35°c

    Please help me, how can I achieve this task with the help of proper calculation, and also to avoid the unwanted switching of relay at minimum voltage variation.

    Reply
    • Swagatam says

      June 28, 2021 at 12:18 pm

      Yogesh, you can try the first circuit or the last circuit from this article:

      https://www.homemade-circuits.com/make-simple-refrigerator-thermostat/

      Reply
      • Yogesh says

        June 28, 2021 at 12:56 pm

        Thank you sir, (•‿•)
        I will surely try out any one circuit ,from these two circuits sir

        Reply
        • Swagatam says

          June 28, 2021 at 2:19 pm

          You are welcome Yogesh!

          Reply
  3. Jesse says

    March 9, 2021 at 8:22 pm

    I do not see a power supply.
    I design elsctronics from broken old junk.
    I would be ecstatic to have you email me. You are brilliant, Swagatam… I have reviewed much of your stuff online.

    Reply
    • Swagatam says

      March 9, 2021 at 9:08 pm

      thank you Jesse, yes the diagram did not have the supply lines clearly marked, but I have corrected the issue in the new updated diagram.

      Reply
  4. Marius Bogdan says

    February 3, 2021 at 5:21 pm

    I’m so glad I found Your site! Today, few are involved in the design of analog circuits. In Hungary,(in the past) many young people used to be interested in electronics, because it was not possible to buy many different electronic devices when they were ready. I started in 1969 with a 4-year electronics technical school, then Kando University (then it was only a 3-year college BSC) I am already retired, but thanks to my well-equipped laboratory (Tektronix, HP, R & S, etc.) instruments, still active today I’m. Both of my sons are electrical engineers. I’ve been a sound engineer for several years (my favorite) I like Your site, I’ve already found some interesting topics! I wish You much more success and good health! Marius Bogdan

    Reply
    • Swagatam says

      February 3, 2021 at 8:30 pm

      Thank you for visiting this and liking it! Your valuable feedback is much appreciated! Please keep up the good work!

      Reply
  5. Turan BENLİ says

    January 31, 2021 at 3:54 am

    Yayınladığınız projeleri beğeniyorum. Profesyonel değilim. Elektronik termostatın devresinde T4’ün collektör ayağında bulunan diyotun numarası belli değil belirtirseniz sevinirim. Teşekkür ederim.

    Reply
    • Swagatam says

      January 31, 2021 at 5:45 pm

      The relay diode is 1N4007

      Reply
      • Turan BENLİ says

        February 2, 2021 at 1:57 pm

        Merhaba, Çok Teşekkür ederim. Bu tür projeleriniz bizim için çok değerlidir. Sizlerin bu değerli bilgileri paylaşmanız bizlerin Elektronik dalda çalışmalarımıza çok büyük değer katmaktadır. Saygılarımla,

        Reply
        • Swagatam says

          February 2, 2021 at 9:44 pm

          Thank you very much for your kind feedback!

          Reply
  6. Gopalakrishnan says

    December 27, 2017 at 11:23 am

    Alternative two relay switching with one push on switch 12 volt operative circuit

    Reply
    • Swag says

      December 27, 2017 at 1:01 pm

      https://www.homemade-circuits.com/single-push-10-step-selector-switch/

      replace pin#10 with pin#4

      use only pin#2 as the output

      Reply
  7. Swagatam says

    March 21, 2017 at 4:45 am

    yes you can try that

    Reply

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