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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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