Circuit Diagram
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.
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