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.
Joseph says
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
Swagatam says
Hello Joseph, you can add another potentiometer in series with R1, but it will also serve the same function as the existing potentiometer.
Yogesh says
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.
Swagatam says
Yogesh, you can try the first circuit or the last circuit from this article:
https://www.homemade-circuits.com/make-simple-refrigerator-thermostat/
Yogesh says
Thank you sir, (•‿•)
I will surely try out any one circuit ,from these two circuits sir
Swagatam says
You are welcome Yogesh!
Jesse says
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.
Swagatam says
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.
Marius Bogdan says
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Swagatam says
Thank you for visiting this and liking it! Your valuable feedback is much appreciated! Please keep up the good work!
Turan BENLİ says
Yayınladığınız projeleri beğeniyorum. Profesyonel değilim. Elektronik termostatın devresinde T4’ün collektör ayağında bulunan diyotun numarası belli değil belirtirseniz sevinirim. Teşekkür ederim.
Swagatam says
The relay diode is 1N4007
Turan BENLİ says
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,
Swagatam says
Thank you very much for your kind feedback!
Gopalakrishnan says
Alternative two relay switching with one push on switch 12 volt operative circuit
Swagatam says
https://www.homemade-circuits.com/single-push-10-step-selector-switch/
replace pin#10 with pin#4
use only pin#2 as the output
Swagatam says
yes you can try that