Curiosity killed the cat, but this project only sprays them with water.
Tinkernut posted this humorous project useful for animal control in the kitchen. It uses an Arduino/PIR combo to sense motion and activate the auto trigger on a Raid spray bottle (of course, after emptying and thoroughly cleaning the Raid bottle.) When strategically placed on a kitchen counter, it sprays water at any errant feline that happens by.
Via Tinkernut.

It’s somewhat amusing how the Arduino between the PIR sensor an the motor is entirely redundant…
Not entirely, it does keep the motor going for a few seconds after being triggered, but yes you could do this without the Arduino and just using a few extra components.
Ah, but precisely therein does the ludicrousness lie. These modules already do hold their output active a variable amount of time after being triggered; the better versions of this sensor come with the relevant two pots already installed (mine did), but if you look closely on the vid, at 1:20 the unpopulated “Delay” pot footprint is clearly visible, helpfully labeled as such. And it does _exactly_ what the Arduino is set up to…
Ah yes, but you see he just bought shares in Arduino ;)
This is awesome! Now I can stop the neighbors from crapping on my lawn. Just need to step up the pressure to something like a water mains hose :p
You must have some weird neighbours!
Oh god! Somewhere along the line I forgot the “dog” … this is way better!