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.
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