Well, I wish I could claim this little bit of cleverness, but alas, it's 
something I found online. I am, however, describing it in my own words and 
adding a small modification that makes it even simpler than the original 
version. It would be hard to come up with a simpler or cheaper solution, and 
though it may not be quite the sophisticated electronics project you had in 
mind, it appears to meet all of your stated requirements.

Parts list:

9V battery
9V battery snap
piezo beeper (the kind that beeps on its own with just a DC voltage)
some small gauge wire
a small disposable cup made of paper or thin plastic
a small piece of adhesive tape
some table salt and tap water

Connect the negative lead of the battery snap to the negative terminal of the 
beeper.

Connect a wire to the positive terminal of the beeper, and another wire to the 
positive lead of the battery snap. How long these are is up to you. You may 
want them long enough to put the battery and beeper outside the freezer so you 
can hear the beeper and so you don't subject the battery to the frezing 
temperatures.

Strip a little off of the free ends of the wires. You don't need much. About 
1/16" should do fine.

Twist the insulated part of the wires together near the free end, leaving the 
bare ends close together but not touching.

Tape this to the inside wall of the cup so that the bare wire ends touch the 
bottom of the cup.

Mix up a small amount of salt water in another container.

Put the wired cup in the freezer and lean it on something so that it's sharply 
tilted to one side with the wires towards the top.

Pour in some salt water such that it's all to one side of the cup and doesn't 
touch the bare wires.

Close the freezer and allow time for the salt water to freeze.

Once it's frozen, sit the cup upright and connect the battery to the battery 
snap. You're done.

If the freezer stops working, the salt water will start to melt and will flow 
down into the bottom of the cup, touching the wires and completing the circuit.

When not sounding, it's an open circuit. So un-alarmed battery life is limited 
only by the shelf life of the battery.

Piezo beepers use little power. The one I use on my rat trap draws just over 
5mA when connected to 6V, and is reasonably loud. A fresh 9V alkaline battery 
should be able to power one of those for several days. So if you arive home too
late to hear the alarm, you're almost certainly too late to save the food 
anyway.

Adjustability is achieved by controlling the salt mixture, and happens to be 
almost exactly the range you specified. 0C for no salt, about -21 for fully 
saturated salt water. Shouldn't be too hard to get the desired temperature 
empirically. I'd expect you'd want the densest mixture that will reliably 
freeze in order to get the earliest alarm.

I guess if you wanted to do this in a neater package, you could build it using 
a small sealed container with some silicone or a waterproof glue to seal the 
hole that the wires pass through. A plastic 35mm film container might work if 
you glue the lid on so it doesn't leak. Turn it on its side and freeze it, then
stand it upright to arm. A clear container would be best, so that it's easier 
to tell when it's frozen.


 GM> CHANGING THE SUBJECT....

 GM> Does anyone have a circuit of a deep freeze failure alarm? I
 GM> recently had to empty about $60 worth of stuff into the garbage
 GM> when we found the power plug on the freezer had been bumped and
 GM> the thing wasn't working.

 GM> An alarm that went off before the stuff thawed out would have
 GM> saved it all.

 GM> I thought I remembered seeing such a gadget as a project in one
 GM> of our electronics magazines some years ago, but I can't find it
 GM> in any of the copies I've saved.

 GM> I have some ideas of my own, but I thought I'd toss the matter
 GM> into the ring to see whether anyone has already got something.

 GM> The requirements are:

 GM> 1. Battery powered

 GM> 2. Alarm to go off if temperature rises above a preset limit.

 GM> 3. Preset limit to be adjustable between 0 deg C (32 F) and -20 C
 GM>    (-4 F) or thereabouts.

 GM> 4. Long battery life in the non-alarmed condition.

 GM> 5. Alarm to be able to operate for several hours without draining
 GM>    the battery, in case the house is unattended when the alarm
 GM>    goes off. Alarm must be very low drain or operate in short
 GM>    beeps.

 GM>    ,-./\
 GM>   /     \ From Greg Mayman, in beautiful Adelaide, South Australia
 GM>   \_,-*_/    "Queen City of The South"    34:55 S  138:36 E
 GM>        v

 GM> ... To keep milk from turning sour: Keep it in the cow.
 GM> ___ Blue Wave/386 v2.30




Bye bye!
  William

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