GenCDi lets you plug a standard Sega Genesis / Mega Drive controller into a Philips CD-i and use it as a CD-i pointing device. An Arduino Pro Micro (ATmega32U4, 5V/16MHz) reads the Genesis pad through a male DB-9 connector. It talks to the CD-i through half of an 8-pin mini-DIN cable.

The two-layer PCB is small (32.5 × 52.3 mm) and all through-hole, with just three parts: the Pro Micro, the DB-9 connector and the cable wires. A two-part 3D-printable case, with its FreeCAD source, holds it together with four #4-40 screws.

The firmware is a short Arduino sketch using the SegaController and cdi-controller-library libraries. The D-pad moves the cursor, A and B are the two CD-i buttons, and C presses both.

This is just sharing the project from https://github.com/itsmattkc/gencdi by MattKC, where you’ll find the original design files, build instructions and his two videos about it. All credit goes to the original author.