Heat Sensor, Automatic temperature control

The Thermistor, or NTC (Negative Temperature Coefficient) of 10K, is a standard type. Most types will work. The one in the diagram is a 10K model made by Fenwal . The resistance lowers as the surrounding temperature increases which affects the output (pin 6) and energizes the small relay and Led1(optional, just cosmetic and can be left out).
P1 is a regular Bourns trimmer potentiometer and adjusts a certain range of temperatures. I used a 50K, 10-turn type for a bit finer adjustment but any type will work.

R1 is there to protect the Thermistor (NTC) from a full 12V just in case trimmer pot P1 is adjusted all the way down to ‘0’ ohms. In which case it gets very hot and probably burn. R2 is optional in case your relays tends to ‘chatter’ a bit. It provides a bit of hysteresis when the set temperature of the thermistor reaches its threshold point.

This value may need to be adjusted anywhere between 120K and 470K (although I indicated different values on the schematic). Transistor Q1 can be a 2N2222(A), 2N3904, NTE123A, ECG123A, etc. Not critical at all. It acts only as a switch for the relay so almost any type will work, as long as it can provide the current needed to activate the relay’s coil. D1, the 1N4148, acts as a spark arrestor when the contacts of the relay open and eliminates false triggering. Feel free to use any other type, like a 1N4001 or something. Solder directly onto the ‘+’ and ‘-‘ relay terminals. If you need a ‘Frost’ sensor, just swap positions of the R1 and Th1 positions.