Need to design a small disconnect circuit

Sounds like you need a small moisture sensor driving a FET. Moisture sensors usually are looking for (ahem) moisture, but there is no reason one shouldn't work for detecting frost or ice. The sensor is easy to make - two electrical contacts in close proximity but not quite touching. Put a small DC voltage across the contacts, and look for the small current that results when they are bridged by moisture. I'm sure Google will turn up hundreds of sensor designs.

Bill

Kinda like a howitzer for a mosquito.

But you are looking for Thermistor Probes if all you want is a sensor and they are readily available. Just do a search on that and see what you get.

Peter Wieck Wyncote, PA

thanks everyone for ideas. I will look up information. pls if you have any more ideas do let me know.

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There are a number of electronic thermostat kits available but if you would like to roll your own then the schematic on this page (lower portion)

might do the trick. While the author is driving a Peltier device this can be substituted with a suitable relay. Also, instead of the smd mosfet you can use a suitable TO220 device. With the existing values the circuit switches between -19C and +2C so you can play around with the resistor values in the sensor bridge to alter the range to suit your requirements.

Take a look at this simple solution:

or

But distinguish between freezing cold and icing up. 32F precedes icing. I'm not sure how you'd go about detecting ice - maybe some photoelectric device (a hunk of ice blocking a path or causing a change in reflectivity.)