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.