What kind of valve is on this tennis ball cannon in this picture?
describes how it works pretty well even thought this one is not in fact solenoid operated. This one is operated manually via the blow gun which is just being used as an inexpensive lever operated valve, but the main part of the valve works on the same principle illustrated in the Wikipedia article, just mentally replace the solenoid with the lever valve.
It's a pneumatic valve with a compressed air tank to power the cannon. Any kind of pneumatic valve should do the trick, just make sure you use the correct fittings for whatever valve you buy. The red lever piece is from a standard pressurized air system for a mechanic's shop. It is probably only necessary as a safety mechanism to discharge the air without shooting the ball.
