I have some unsolicited Food For thought 😀
I am a Happy User of the DIY Temp Controller http://www.brewpi.com/
“BrewPi runs on a Raspberry Pi and an Arduino. The Raspberry Pi runs the web server and a python script to log data and manage settings. The Arduino reads the temperature sensors and activates the cooler, heater and other actuators. It also has an LCD and knob to display and change settings.”
This is a great setup, It makes the system very reliable and flexible you get all the best parts of both systems. this also allows you to add more than 1 arduino for greater functionality.
I would love to see Open Sprinkler adopt this configuration. it opens the door to all sorts of cool things like easy web based firmware updates, and logging.
It would be cool to be able to add weather station sensors to this system so the webpage showed you weather conditions and make it able to upload to weather underground.
Or control greenhouse fans , Light shades , heaters , chicken coop doors , really this system could be truly amazing.
the sky is the limit and there are so many relative function and applications that OP could be the go to controller for.
I am curious about the decision to go with the hybrid solution: a Pi + Arduino. Is it because the Pi is running out of pins or has no ADC on board? Assuming the number of pins are sufficient and adding an ADC chip (like OSPi does), why not just use a single Pi to handle all the tasks?