This project is the Arduino code for a thingy that uses a cheap eBay analogue joystick module as an input device for computers that want a quadrature mouse signal.
This project is aimed at the early pre-ADB macs (128/512/Plus) and the Apple Lisa, but with some adaptations it would work with, for example, the Acorn Archimedes (the main difference being that you would need to add another two buttons).
I use this on an Arduino Pro Mini in a small remote control-sized case.
Pin assignments are at the top of the code and are twiddlable there.