Our neighborhood, like many in the states I imagine, is crawling with feral cats. For the most part, I don't mind them; they keep the pigeon population down, they don't cause a lot of damage or raise much ruckus. (I do wish they would quit reproducing, though) We managed to rescue a kitten recently and she is adapting well to her new, cushier life LOL
Unfortunately, there is one male that insists on marking our front door. Every. Freaking. Night. There are few smells in the world more pungent than that left by a territorial cat. My boyfriend was at the end of his rope and wanted to trap the cat to turn over to animal control - a solution that didn't sit well to me as I know that amounts to a death sentence. So, to the drawing board where I came up with my pressure switch triggered Sonic Critter Deterrent. It went off 4-5 times the first night, and we haven't seen or smelled the cat since.
Step 1: The Supplies
Door alarm (got mine at the local dollar store): $1.00
2 pieces of cardboard or box (I found this box that the post office left at the office last year to introduce their program, it is a size we never use): Free
Aluminum foil: Pennies
Weather stripping (this was left-over from fixing a laundry room door): Free if scrap, A buck or so if purchased
A small amount of wire will be needed, your soldering gun, some spray adhesive, a bit of copper or aluminum tape