Instructables member drj113 has put together a powerful Networked DIY Watering Controller. It has a AtMega168 at its heart and uses a RTC to keep accurate track of time. Where I live there is no restrictions on lawn watering but this is a good flexible system that can save you costly fines if your area imposes lawn watering restrictions.
“The sprinkler controller is built on the base of my Adruino Ethernet controller. The Arduino ethernet controller provided a basic network connected ATMega328 chip, and had a number of spare I/O lines. I extended the PCB design to provide 6 solid state outputs, each able to drive +12v at 300mA, which is enough for a watering solenoid. Each solenoid output is driven from a NPN transistor, as shown by the attached picture. To turn on the transistor, the port pin is simply driven high. I decided to use individual transistors, as they cost $0.03 each, and if they are destroyed, they are individually replaceable easily.”
