They worked reasonably well, but the cats have learned that if they run quickly past one, it won't hiss. So now I'm thinking of modifying them so they use an IR beam, and a beam interruption would trigger the hiss.
since the sprayers have a configurable spray width I think you could get pretty far with some sort of wide-angle sensor (a few ultrasonic range finders, or maybe a Kinect?) placed at one long-edge end of the lawn and use that to trigger sprayers laid out perpendicular to the camera.
They worked reasonably well, but the cats have learned that if they run quickly past one, it won't hiss. So now I'm thinking of modifying them so they use an IR beam, and a beam interruption would trigger the hiss.
The eternal battle goes on.