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Challenge:
Devise
a low power system that will sound an alarm inside a house, when the outside temperature is
below freezing (0C).
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 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. Thin wires from the
thermistor mounted outside can be routed through a tiny hole in the side of the house to the
rest of the circuit inside.
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