on the desired temperature. The circuit can be used for many purposes for example switch on the fan on the desired temperature or activate emergency temperature alarm etc.
The circuit can be powered by a 9 volt battery, adaptor or transformer. The two BC547B transistors are connected as a Darlington pair to increase the sensitivity or gain of the circuit. A 20K variable resistor is used to adjust the desired level of heat on which you want to activate the relay switch. Try to fit the thermistor a little bit away from the other components so it will not get their heat.

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The circuit presented here is a project of a temperature sensor relay switch circuit which will switch on the relay and any device connected to the relay when the thermistor will sense heat. You can connect any AC 110V or 220V or DC appliance with the relay to switch on automatically

Temperature Sensor Relay Switch Circuit