Hey thanks all for the quick replies. I'll just put a reg on my shield, I have the room. If all 4 relays were to pull in at once, which they shouldn't but could, it sounds like I'd have a voltage drop that could glitch or crash Arduino. Thanks for the filter advice, I have a few small rad caps hanging around, I'll add one.
Caleb, I'll be outlining more of the project on my home brew forum, http://www.brewcommune.com under the equipment forum. In a nutshell I want to monitor and log temperatures and be able to control pumps and burners if I want. I have wanted to learn how to program for a long time but i am a very hands-on learner and of all the online tutorials I can't seem to get past "Hello World" before I get bored. By choosing the Arduino platform and picking an application I really know and understand I felt I had an opportunity that would keep my attention and propel me forward. I was right, I've learned a ton already and I'm just scratching the surface!
My final project will have a browser based application that will have a graphic representation of my brewery. I'll be able to click on various objects to turn pumps on or off and set critical temperatures. All the while it will read from Arduino and log important temperatures so I can go back and see what happened. I'll build a second one to monitor and control fermentation temperatures including having a Lager routine to drop my fermenter into lagering temp on a specific schedule.
I have 6 LM34's that will read mash, hot liquor, boil, vorlauf return, chilling H20 in and out. The mash and hot liquor sensors control 2 of the relays and the relays control direct spark ignition controls and valves for those two burners. The other two relays control the 2 pumps. All the relays have override switches so I can control the entire thing manually. Mostly I want this for the logging part but hey, why not control something if you can. I've had a digital HLT for a while now and it's like having a helper just sit and watch that vessel. When I brew, I brew back to back 10G batches so things get a little hectic as I'm finishing sparge and brining batch one to a boil while I'm trying to refill and reheat my strike water for batch 2.
It's been a very challenging project so far and I can't wait to get it incorporated into my actual brewery. I just finished making the temperature probes out of the LM34's and my first shield and sketch is working more or less as anticipated. I've chose some different switches and connectors so I'm etching a version 2 shield.
I think the Arduino is absolutely the most bang for the buck I've ever got out of a toy under $35! Oh and sorry to everyone else for all the beer-geek jargon :-?