tsb made open source converter for do-it-yourself solution to connect DB9 game controller (C64 joystick) to TheC64 Mini. Invention is that this adaptor also has “Menu” button from Mini's joystick. It means that you can return to Carousel menu from game, without using Mini's or some other multi-button controller. Project is utilising Arduino Leonardo (Micro Pro) board.

“With this converter you can use an old Arcade joystick to a C64 Mini. It is converting the signal from the old type connector (DB9/Atari-based) to USB.
It can be used on a stock C64 Mini machine, without any modifications.”

• https://www.tsb.space/projects/c64-mini-arcade2usb-converter/
  • “C64 Mini/Vice – Arcade2USB Converter” - main project web page, with detailed instructions how to build it
• https://gitlab.com/iamtsb/c64mini-arcade2usb-converter
  • Gitlab project files
• http://thec64community.online/thread/311/opensource-db92usb-adaptor
  • related thread at TheC64 Community forum