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Freezer / Frost Alarms
Last Updated on: Tuesday, April 22, 2008 04:20 PM
Circuits Designed by Dave Johnson, P.E. :
  • Freezing Temperature Alarm
    This circuit turns on a beeper whenever the outside air temperature is less than zero degrees Centigrade.  The circuit draws only a few microamps from a 9 volt battery.  It uses a glass bead thermistor accurate to 1 degree C. 
  • Freezer Over-Temperature Alarm
    This circuit turns on a beeper whenever the inside temperature of a freezer is greater then zero degrees Centigrade.  The circuit draws only a few microamps from a 9 volt battery.  It uses a glass bead thermistor accurate to 1 degree C.
Links to electronic circuits, electronic schematics, designs for engineers, hobbyists, students & inventors:

Cold Activated Switch #2:  (schematic / circuit design added 9/02)

Cold Activated Switch:  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. (added 2/06)

Fridge Door Alarm:  (circuit added 9/04)

Frost Alarm #2:  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. (added 2/06)

Frost Alarm #3:  (schematic / circuit design added 9/02)

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