Now before you call RSPCA please read all of this
I have a cat flap which is used by my cat, see exhibit A - Rico:
Unfortunately there is a lot of other cats around, which I wouldn't normally mind but they all seem to be getting in through the cat flap and eating my cats dinner. Which wouldn't be a big deal if they didn't make such a mess of it. Rico is not a fan of collars so a magnetic key or IR cat-flap is not an option. We could go for face recognition but he has an evil (well, not really) twin in the area.
Now, despite his mean appearance in the above photo Rico is a lover and not a fighter, and is not able to defend his domain effectively. So what I need is a fairly humane yet effective cat boobie trap.
Something entirely automatic would be indiscriminate and would affect Rico as well, so i was thinking of something that I could trigger myself as I hear the cat flap flapping (meaning an intruder is legging it). The cat flap is in the door at the side of the house, so ideally I'd want something that turns the passageway by the side into a bit of a waterfall for a few seconds at a press of a button. All thats needed now is to design such a contraption.
So help Rico and post your ideas.