If you're like me, you're working on an arcade cabinet and waiting for buttons to arrive in the mail, and thinking to yourself, "gee, I sure love Pong, but there's no way I can bring down some paddle-slicing fury with a joystick." It's a common thought. It's also why I ultimately decided to include spinners (paddles) on my arcade machine. You can too, and here's how to do it for cheap.
1 old ball mouse
1 peanut butter lid (yes, really)
a bunch of small coins
some old Legos:
-1 four or six-peg axle
-2 small right-angle gears
-1 large wheel
-some spacer bits
Step 1: Cheap Spinner Interface
Old ball mice are a great cheap solution for spinners, like the ones for playing Breakout, Pong, and some non-ball-based driving games. Not only can you find them for free, but they don't require any drivers, and they come with two optical encoders (one per axis), so you can make controls for two players out of one mouse.
Just cut out the bare parts you need. That means keep the plastic that holds the encoder shaft in place, and try to leave enough extra to support the electronic bits. It doesn't hurt to have a good flat area to glue the whole thing down with either.