A very simple electronic thermostat circuit can be constructed using this electronic circuit that can be used for controlling a thermal resistor a fan or other device that is dependent  by temperature .

The temperature which torn on or off the controlled is set using a 50kohms potentiometer .

When the temperature is low than the preset temperature , the T2 transistor conducts and the relay activate the thermo-element (thermal resistor ) or if at the relay terminals is connected a fan , the circuit will turn off the fan .

The T1 transistor must be a npn type BCY58 type ( or similar ) and T2 transistor must be a pnp type BCY78 ( or similar type ) . The Th thermistor must be mounted in that place that needs a constant temperature ( controlled temperature ) . This electronic thermostat circuit needs to be powered using a 24 volts DC power supply , so you need to use a 24 volts relay .

The thermistor used in this project has a value of 5k ohms .