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Last Updated On:  Wednesday June 02, 2021

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Freezer Under-Temperature Alarm


Design a low power circuit which will sound an alarm, when the inside temperature of a freezer is above freezing or 32F.



I chose an accurate thermistor from Keystone as the temperature sensor for this circuit.  This device has a very specific resistance at a particular temperature.  According to the data sheet for the thermistor, the device will have a resistance of 361K at zero degrees C, which is the freezing point of water.  The thermistor is wired into a simple bridge circuit, which uses 1% resistors. The bridge is connected to a very low power voltage comparator.  The circuit forms a switch circuit, which changes state, when the resistance of the thermistor reaches a certain value.  At zero degrees C the voltage at the non-inverting input of the comparator exceeds the inverting input.  The output of the comparator then swings high, which drives the gate of a FET.  The FET drives a nice beeper from Star, drawing power directly from the 9v battery.  The whole circuit is powered with a stable 5 volts using a low power voltage regulator from Seiko.  The complete circuit draws only 9 micoamps, which means a 9 volt battery should run the circuit for 5 years or so.  Very thin wires from the thermistor can be routed through the freezer gasket to the rest of the circuit outside the freezer.

Freeze Under Alarm designed by David Johnson, P.E. 4/1/2005


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