As a followup, rather than hardwiring a rain sensor to your sprinkler controller, I could actually pull data from your internet connected personal weather station assuming you’ve got one that’s reporting to Weather Underground. That would actually be pretty trivial, and would be MUCH more complete than just a rain sensor because I could do different things based on temperature, humidity and exactly how MUCH rain was received.
That’s not to say I shouldn’t support hardwired rain sensors anyway… Just a thought…