This circuit has been designed and fabricated using the op-amp IC 741 and a 1k thermistor as the temperature sensor. It is based on the principle of Wheatstone bridge. The ratio arms R1 and R2 of the bridge are kept fixed (say unity). The voltage across the thermistor, i.e. across the arm R4, is compared with the voltage across the variable arm R3 which is kept fixed for a particular temperature (using the op-amp).
When the bridge is not balanced the output of 741 drives transistor BC147 to conducting state. Thus, the relay is energized and switches the heater on. As the temperature increases, the voltage across the thermistor decreases and when this voltage is equal to the voltage across R3 (i.e. the preset value) the output of 741 becomes zero. The transistor then stops conducting, the relay is ed-energised, and the heater is switched off. Band switch S1 gives the different ranges of temperature and potentiometer VR1 is used for fine adjustments.
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