Freeze Alarm - 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 . . . [Circuit Solution by David Johnson P.E., 05/07/06]
Alarm - This circuit turns on a beeper whenever the
inside temperature of a freezer is greater then zero degrees Centigrade. The circuit draws only a few microamps
from a 9 volt battery. It uses a glass bead thermistor accurate to 1 degree C. . . . [Circuit by David A.
Johnson P.E., 11/05/14]
Temperature Alarm - The circuit below is designed to
activate a beeper alarm, whenever the outside air temperature is below 0 degrees C (32F). A 9v battery powers
the circuit. The average 9ua current is so low that the battery should last for many years. . . . [Circuit by
Dave Johnson P.E., 11/05/14]