A Thermistor is used in the circuit for sensing the heat and two 5K variable resistors are used to adjust the circuit to activate the relay on the desired temperature. The purpose of using the 1N4007 diode across the relay is to limit the back EMF of the relay coil. The circuit can be operated with a 12 volt battery or power supply but you can also operate it with 6 volt battery or power supply but make sure to use a 6 volt relay also.
The figure shows a very useful project of a temperature control circuit using 555 IC. The circuit can be used to control any AC or DC device on the desired temperature. In the previous topics we have discussed the temperature circuits which are based on transistors like Temperature Alarm Circuit,