Despite conventional wisdom, the Guinness is usually NOT served warm on the Emerald Isle - there were few places I drank in that relied on "cellar" temp to cool their beer. According to the Guinness Storehouse, the Guinnness should be served at 8° C (46°F), which means it needs to be colder than that in the kegerator, as the laws of thermodynamics definitely apply to beer kegs. A guy in Donegal (where they serve the finest pints!) told me he keeps his kegs at 3°C (37.4°F), figuring by the time the pour ends it'll be close to factory specs.
Usually we keep fine homebrewed kegs of ales, pale ales, barley wines and stouts on tap at the Check 6, but lately the Guinness has become de rigeuer. The problem is, I never really paid that much attention to the temp of my kegerator. My beer always seemed cold enough and no one ever complained, but when I stuck an old school fridge themometer in there recently, I was shocked at how warm it was :-)
So why build this? Fridge thermometers work just fine but you have to open the kegerator every time you want to check the temp. And every time you open the door it screws up the controlled thermal environment. Using this little guy allows you to track inside temps which will help you find the proper setting for your kegerator thermostat.
Enter the Arduino + DS18B20...