Circuit Solutions Notebook
Last Updated On:  Monday October 03, 2011

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Freezer Under-Temperature Alarm

Challenge:

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

 Solution:

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.