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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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