We chose an Arduino Uno.
That's a Python logo!
(No, that's not a picture of it)
We were provided with an Arduino Uno at the start of the project and saw no need to change. The Arduino, equipped with a Protoshield designed by Olin's own Prof. Brad Minch, connects to each of the 4 servos, as well as our solenoid and potentiometer. An ATX supply provides enough power to run the servos.
The solenoid is screwed into the sprinker valve.
Our sprinkler valve runs on 24VAC. Rather than finding a new DC solenoid to interface with the valve, we designed a simple circuit to actuate the solenoid. The circuit uses a commonly avaliable 2N7000 MOSFET to drive a 5V relay which switches a 24VAC transformer that the team found (we think it originally powered some sort of stereo equipment). The MOSFET and Arduino are protected with a diode accross the relay.
It's on github.