hey guys, longtime lurker, brewer and EE here who started in automated control systems. Wanted to say the arduino / pi combo is the way to go as it is a great analogy to the PLC / SCADA combo at the heart of a lot of industrial automation.
these relay boards will certainly work, they take a 5V connection and then you can apply 3.3V to control each relay. The thing to watch is probably current draw, i'd probably use another 5V source - having all eight on at once might be a decent draw. in this case, the 3.3V - 5V interfacing is taken care of. Adafruit makes a simple bi-direction interfacing chip for the pi for this specific purpose.
however, this 3.3V - 5V disparity is not the only reason to use an arduino for IO, thought it does make voltage screwups much cheaper, just a new chip in the arduino vs. a whole new pi. The real advantage is that linux is too slow/sloppy to properly control IO, but is clearly the winner in terms of http (web serving), logging (data acquisition) and supervisory control (changing setpoints, starting/stopping processes, etc.).
my ultimate setup will go as follows:
- start with just raspberry pi controlling ferm temp (currently following this tut: http://learn.adafruit.com/send-raspb...i-data-to-cosm
- expand to control keezer and ferm with arduino/pi combo
- finish electric single tier stand (BCS) and use pi to run web interface showing keezer and ferm temps as well as an interface into the BCS during brewing.
next pi project is a desktop arcade enclosure (MAME) which, IMO, goes great with friends and freshly brewed beer.
keep this thread going and good luck to all