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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.
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