Notes:
The thermistor used has a resistance of 15k at 25 degrees and 45k at 0 degrees celsius. A suitable bead
type thermistor is found in the Maplin catalogue. The 100k pot allows this circuit to trigger over a wide range
of temperatures. A slight amount of hysteresis is provided by inclusion of the 270k resistor. This prevents
relay chatter when temperature is near the switching threshold of this circuit.