A diode, such as a IN4148, has a typical -2m V/C temperature coefficient at a 1 mA diode current. Ql and Q2 form a constant current source. D1 is the temperature sensor. ICl-a and -b are dc amplifiers, with ICl-c a temperature reference voltage supply. ICl-d is a comparator with variable hysteresis. R14, R15, and R16 are chosen depending on the thermostat range desired. Q3 is a relay driver (2N3904).
The relay used should handle the load current or an optoisolator triac combination can be used.