The SayoDevice O3C is a three-key Hall-effect keypad designed specifically for rhythm games like osu! and Geometry Dash, and it has quietly become a hit on AliExpress with over 10,000 units sold. The device uses OUTEMU magnetic Hall-effect switches with rapid trigger support and a customizable actuation point as low as 0.05 mm (0.002 inches), enabling extremely fast and precise keystrokes that matter in timing-sensitive gameplay.
At its core, the O3C runs on a WCH CH32V307WCU6 RISC-V microcontroller clocked at 144 MHz with 128 KB SRAM and 192 KB + 288 KB flash memory. The keypad connects via USB 2.0 Type-C and supports an 8,000 Hz polling rate for minimal input latency. The hot-swappable keys work with any compatible magnetic Hall-effect switches, and the device includes a programmable rotary knob that can be mapped to functions like volume control or list scrolling. A 0.96-inch IPS color display shows key travel information, press counts, or custom images and text, with the first line personalizable by the user.
Configuration happens entirely through a browser-based interface that eliminates the need for dedicated drivers. The web tool handles firmware updates, Hall-effect switch calibration, actuation and release parameter adjustments, rapid trigger settings, key remapping, and RGB lighting customization. Firmware files are hosted on the company's CDN, and setup guides are available through the r/SayoDevice wiki on Reddit and various YouTube tutorials. The keypad measures approximately 6.5 cm by 5.0 cm (2.6 by 2.0 inches) and features ABS plastic keycaps with laser printing.
The O3C works out of the box on Linux systems as a standard HID keyboard device using the built-in hid-generic and usbhid drivers, with confirmed compatibility on distributions including Ubuntu, Arch Linux, and Linux Mint. Beyond the official firmware, the device has attracted a modding community with projects like o3cpatch offering custom features including USB commands to retrieve raw analog data and custom menu backgrounds, built on reverse engineering documentation that details the device's AES-256-CBC encrypted firmware structure.
The SayoDevice O3C is available in multiple color options for $27 (€25).
