Temperature Controlled Relay

This circuit is a simple temperature controlled relay. The relay activates when the preset temperature is reached.
Initially, thermistor (temperature sensitive resistor) R1 has 10 kilo ohms resistance at room temperature. As the temperature rises, its resistance drops. When the resistance of R1 is lower than the combined resistance of R2 and POT, comparator will output high and activates Q1 and the relay.
You can adjust the preset temperature by just adjusting the potentiometer. Example if you want to preset the control to activate when 50 deg centigrade is reached. Expose or place R1 to temperature and adjust the potentiometer until the relay activates.

Temperature activated circuit

Part List:
R1 - 10 kilo ohms thermistor (NTC 10K ohm 1% RAD)
R2 - 1.2 kilo ohms 1/4W
R3, R4, R5 - 10 kilo ohms 1/4W
POT - 10 kilo ohms potentiometer
D1 - 1N4001 or similar diode
Q1 - 9013, 2N2222 or similar NPN transistor
RLY - 12v relay
IC - LM393 or any comparator IC

Click image to enlarge:
9013 and 2N2222 pins
LM393 pins

 Source : http://www.simple-electronics.com

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