Gil Yankovitch built JoyFon, an open-source gamepad that plugs directly into a smartphone's USB-C port and sits beneath the screen, replicating the Game Boy form factor for vertical-orientation mobile gaming.
The hardware runs on a Raspberry Pi Pico with support for either the RP2040 or RP2350. It enumerates as a standard USB HID gamepad, meaning no drivers, no pairing, and compatibility with any host that accepts HID input. The 3D-printed enclosure accounts for screen coverage, a deliberate tradeoff that works cleanly with 4:3 aspect ratio emulated content. Later hardware revisions added shoulder buttons to the original four face buttons, d-pad, start, and select layout.
The full build files and firmware are on GitHub. What's worth reading is the design rationale behind the enclosure geometry and the USB-C positioning, which determined the entire form factor. This is a solved problem executed with unusual precision for a one-person project.
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