Step 1: Get Some Parts Together
A Temperature Sensor -- I used one called TC622EAT from Microchip for this because it seemed ready made for this application. It simply needs an external resistor to set its trip temperature and it comes in a transistor like package perfect for putting at the end of some longer cord and run inside your freezer. I think most temperature sensors would work however, just as long as they operate in the right temperature range, and if they do not have the trip set feature, that just means that you have to get a little more creative ie add a relay or something like that.
A Resistor -- Just to set the temperature of the TC622. There is a little formula to figure out the resistance needed for the desired trip temperature in the datasheets on the sensor on the Microchip website.
An Op-Amp -- This temperature sensor can't output the current needed to switch on the alarm that I had for this, so an Op-Amp was used as a buffer so that the alarm would go off.
An Alarm -- Here I cheated, and used a window alarm that I bought from Canadian Tire for 1.50, after stripping some of the unnecessary parts from it.
Misc Cable and some batteries -- for the temperature sensor lead, I will use a piece of IDE cable, because its flat so it can go between the seal for the freezer better, and I have a lot of them lying around unused. the temperature sensor that I have needs about 4.5 volts minimum, so I will use 3 AA batteries.
Step 2: Put It All Together
The end of the sensor cable was done up with heat shrink tubing to make it really neat as it would be in the freezer. I got lazy with the op amp and the connections are just soldered right onto the pins.
The batteries are just taped together with some wire makeing them seriesed until I can find a battery clip or something like that for them.
Step 3: All Finished
Also, you don't really want to put the whole thing in a freezer because I believe that the cold will kill the batteries faster than normal.
Although the alarm isn't intensely loud, it should be loud enough that if you picked your temperatures right, you should have enough time when you hear it to do something about it before all your food goes bad. Maybe somebody unplugged it by accident or something easy like that.
I have done a few tests with the setup in the second photo. I haven't pulled the plug on the freezer, only moved the sensor in and out, and it worked as I hoped that it would.
This little project is easy and cheap, and I believe that it has the potential to save hundreds of dollars in spoiled food.