late entry ...
I built one of these from buying a Raspi and not knowing what else to do with it ... it cost me less than $100 in parts (temp sensors, wire, solder, box to house it) ... I already had a small wine fridge that the wife let me use for the ferm chamber, and a 40w light globe in a tin, as a heater (sitting on a small candelabra so it doesn't scorch the bottom of the fridge).
I actually had it up and working before I brewed my first beer, due to my understanding at the time that Temperature control during Fermentation was key to great beer, and I am a world class geek
It is set up so I can check it on the browser on my phone, which I did every 30 seconds for my first few brews ... I think I only went to work to be able to check it from a distance
I also incorporated a microphone in the system, so I can hear the bubbling, and in fact measure the time between bubbles so I can estimate if activity is increasing/decreasing...
god it got tiring !!!
... had drain off a bit of the geek OCD-ness, and try being a normal boy again ... plus once the resulting beers started to get opened, I lost track/interest in fiddling constantly... I had some serious drinking/tasting to participate in ...
.... now, I just set the temp, and come back and look next weekend ... maybe a little peek at it if it is a hot day, just to make sure it is keeping things just so ... but otherwise I love how easy it is to walk away from, safe in the knowledge that temp is maintained brilliantly.
*Be warned, if there is an electronics geek potentially living inside you, you will lose a chunk of your life to something like this ...
I tell this story as a warning, and hope that I can save just one of you from all the time wasting associated... not in building it, or setting up the chamber, or chasing purchases on ebay for the parts, or even using it ... just from checking the bloody thing every nano second ... be warned, it will take you down my friend, you will find yourself one day sitting on the can pressing the browser refresh like a person possessed, sweating/swearing on your low 3g/4g signal ... but I am hoping this tale gets to you before you get that low.