How to Build a Freezer Alarm

How to build a freezer alarm. The freezer alarm alerts you when the freezer temperature lowers and can be assembled from about 15 simple electronic components for less than $20.

| July/August 1982

Why take the chance of ruining the precious produce in your icebox, when you can learn how to build a freezer alarm. (See the freezer alarm diagram in the image gallery.)

How to Build a Freezer Alarm

Freezing is one of the best—and most popular—methods of preserving food . . . but unlike canning or drying, it's continually dependent on a sometimes undependable machine. And nothing is more disheartening than opening a freezer door, only to discover that all your goodies have completely thawed. This agonizing experience can be avoided, though, if you equip your appliance with MOTHER's "freezer fever" alarm.

The device requires two basic elements: a thermometer (which continuously monitors freezing compartment temperature) and a comparator circuit (which decides when the thermometer reading has gone too high). Thanks to modern electronics, these can be assembled, from about 15 simple electronic components, for less than $20.


The part of the alarm that actually goes into the freezer consists of a germanium transistor (which allows more current to pass as temperature increases) with a 1-megaohm resistor soldered between its base and emitter leads. Only two of the transistor's leads (the emitter and collector) need be connected to the comparator circuit, but they must both be properly polarized. Color-coded speaker wire works well for this task.

The transistor must also be well protected from moisture, so—once you have the resistor and speaker wires soldered to the leads—be sure to apply a liberal coating of silicone sealer (the bathtub variety) both around its base and on the exposed wires.

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