It seems that by far the most popular temperature controller design is the Ranco/Love type that "overrides" refrigerator controls by providing it's own thermal sensor and killing power to the fridge. They are a bit pricey, and it seems just a tad convoluted to go about things in this way. The fridge has a perfectly good thermostat inside it already, and the only problem is that it's not in the range we'd like it to be in. Right?
I would think it would be possible to cut the leads going to the thermister in the fridge's own thermostat unit, and add a small piece of circuitry (e.g. a pot, and maybe a few other components, honestly I haven't thought this through completely) to provide a manual 'offset' which could be tuned to push the operating range of the fridge's thermostat into the right zone for beer fermentation. Then, a bit of trial and error, a thermometer, and a large jug of water could determine exactly what setting on the fridge's thermostat dial (usually not given in degrees anyway) corresponds to what temperature. Of course this wouldn't be as fancy as a digital display, but it might be possible to implement for a few bucks in parts.
Has anyone tried this? I'd love to get some feedback from the electrical savvy folks on here. I suspect the exact design might depend on the specific unit to be re-wired, but these fridge thermostats can't be all that different from each other can they?