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10 September 2009

Thermostat Circuit Using LM358

Thermostat is one of the common device used to control the temperature of a space be it a warehouse, a room, a hall or an office. This thermostat circuit will focus on the heating control of a space that uses electric heater as its source of heating. It basically consists of a comparator that controls the ON and OFF of the electric heater based on the sensor temperature. This thermostat is based on op-amp LM358 to control the operation this circuit. Look this figure of circuit.

The LM358 op amp is used as a comparator to sense the inputs of the reference voltage (PIN 3) and room temperature (PIN 2). The thermistor used is a NTC (negative temperature coefficient) type where its resistance will drop when the temperature increases and vice versa. It has a resistance of 20K ohm at 25 degree Celsius. When the room temperature drops, the thermistor resistance will go up and hence the output of the operational amplifier will be low. This cause the relay to turn OFF and the heater will conduct until the temperature of the room rises again. To calibrate this circuit, we using variable resistor VR1. Set the lever of the slide potentiometer or rotary potentiometer VR2 to 25 Celsius location. Place the thermistor at a space where the temperature is at 25 Celsius. By varying VR1, set the resistance at the position between the ON and OFF of the relay. Use a suitable contact relay rating according to the load of the heater.