Seems this problem is a common one.
I used pieces of wire about ever 2 inches years ago.
I wanted that planter of herbs.
I goggled
sprinkler motion detector and here is what I found
http://pondparts.com/subcat274...
The top one is $60, but below that is one just right for what you need I think.
It doesn't water, but is motion sensored and will train them.
