The circuits below are variations on basic thermostat designs that can be used to control either heating or cooling devices.
Potential applications include such diverse jobs as opening flaps or controlling heat in greenhouses, powering automatic fans that only cut in when needed,
Thermostat circuits - Craig Forsythe
The 311 is a purpose-built comparator chip (these circuits work by comparing a set voltage with a changing one produced by the inversely temperature-sensitive thermistor). For this application, however, it's a bit like using a Ferrari to do the work of a wheelbarrow. The 311 is very sensitive and, unless significant feedback is provided, stray electrical noise will tend to make it oscillate (the relay will get jittery). ......