Go to a salvage yard, and get the air shock compressor off an old Cadillac Fleetwood. (Late 70's to mid-80's) It's already got a pressure-control switch built in, and you can use a small Freon tank as a reservoir. The control valve and air tubing for the horn can be had at any truck parts store, and you're in business!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!