I am making a bike alarm for my bike. I haven't started yet. Basically, I'm thinking of a pin to be connected to the metal post (to which the bike is locked.) I want to detect if that connection (to the ground via the metal post) is broken. Is that possible?
How about fastening a wire to the post, and then the other end, on the bike, goes into a "jack" (the sort of thing you plug headphones into). With the wire inserted, it makes a connection, and the Arduino knows the wire is there. As the thief steals the bicycle the plug (or loose wire) is pulled from the jack, breaking the connection. i.e.