This is a simple circuit I made a while ago. My dad needed something to turn on a fan in a heat exchanger and this is what I came up with. This circuit was used for about a year and a half for said purpose without any problems, other than explaining to my dad how to use it. Basically, the circuit will turn on and off a relay based on two set points. Its simplicity also makes it useful for several other applications as well. I used an lm335 temperature sensor as input. The two pots set what temperature the relay will turn on and off at. Once the temperature goes above the high set point, the relay turns on and stays on until it drops below the low set point. Simple and effective.