Re: All beside the point.
I assembled something to do this a few years ago. Used a PIR of the type used to control a flood light to control a socket into which I'd plugged a 12v transformer which powered a 12v water valve (from a washing machine). This controlled the water supply to a impulse sprinkler. This worked pretty well once I figured out how to shade the PIR from the sun as it had to face south. Also had a bit of fun finding a PIR with adjustable sensitivity (got it from RS in the end).
It wasn't pretty but it worked. There is an all-in-one device that does the same thing:-
but its good to feed your inner Heath Robinson :-)