Frost Alarm Simple Circuit Diagram


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.

Source - http://www.hobbyprojects.com/quick_circuits_reference/circuits/frost_alarm_circuit.html

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