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]

Freezer Over-Temperature 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]

Freezing 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]