@Chris: the 24V AC is really only required for sprinkler valves, the Pi itself, runs on 5V DC as you know, so there is a switching converter on OSPi to rectify and step 24V AC down to 5V DC to power RPi. If you have a 12V DC power supply, it’s probably better to step it down to 5V DC directly to power RPi. The reason is that I suspect 12V DC-> 24V AC ->5V DC is inefficient and not a very robust solution especially for WiFi. So what I am suggesting is to split 12V DC into two paths: one goes into the interver to generate 24V AC required by the sprinkler valves; the other goes into a 5V DC step-down converter to power RPi.
Another option, which is probably simpler, is to use 12V DC to directly power everything, including sprinkler solenoids. While most sprinkler valves are designed to work with 24V AC, they CAN operate under 12V DC too (see my blog post here: http://rayshobby.net/?p=9529). If you use this approach, you can feed 12V DC directly to OSPi (OSPi’s switching converter can work with any input voltage between 7V-40V DC). However, you do need to replace the 8 triacs onboard by NPN transistors or N-channel MOSFETs, because triacs will not work with DC current (more technically, they can turn on DC devices but once on they cannot turn them off).
My BS and MS are from UMass and I lived in Amherst for 5 years.
Awesome, I live in Amherst, MA. You can email me directly at [email protected]