Here’s a cat protector project from LuckyResistor. Check out the project page from prototype to the final device:
if you own a cat, you know even if your cat is well brought-up, laws are only valid as long you are at home or even in the same room. the goal of this project was to create a small device, which detects movement in protected areas (e.g. tables) and speak your usual phrases in your voice to the cat.
Honestly, I prefer the blender version.
I’ve tried some ideas for that and my cat’s care level was exactly zero in most cases. Except for the piezo buzzer where it actually attracted the cat to investigate the strange sound. Next thing I’m going to try is a splinkler system.
I saw one automated system within the last few months for keeping neighbor cats out of a garden using a little jet of water or a sprinkler. IIRC, it worked very well. I suspect cats would call it torture if they could.