Simple Thermostat Circuit Using Transistors

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.