Freezer Over-Temperature Alarm
designed by David Johnson, P.E.

A typical freezer can contain hundreds of dollars with of food.  If the device becomes unplugged or if the door is left open too long, the food inside can quickly become spoiled.  The hobby circuit below is a battery powered alarm system, which is attached to the outside of the freezer using a couple strong magnets.  Two small gage wires are fed from the alarm box to a thermistor temperature sensor, mounted inside the freezer.  The alarm beeper will be activated, whenever the electronic circuit detects a freezer temperature warmer than 0 degrees C (32 F).  The overall current is so low that the 9v battery powering the device will last for many years

Click on Schematic below to view PDF version of this Circuit