Cheapino is an open-source split 36-key keyboard focused on affordability and efficiency in hardware design. What sets it apart is the use of a Japanese duplex matrix wiring scheme that allows both halves of the keyboard to be controlled by a single RP2040-Zero microcontroller, cutting the number of required GPIO pins roughly in half compared to conventional split keyboard designs. This design choice reduces complexity and cost while still offering features like hotswap sockets and firmware customization via QMK and Vial.
split 36-key keyboard with a Japanese duplex matrix and RP2040 control
At its core, Cheapino is a split keyboard with 36 keys total, arranged in a compact layout. The keyboard is split into two halves, each connected via reversible PCBs and linked by RJ45 cables. Instead of requiring two microcontrollers—one for each half—the design uses only one RP2040-Zero MCU to handle both halves. This is made possible by the Japanese duplex matrix, a wiring scheme that cleverly multiplexes key inputs to reduce the number of GPIO pins needed.
The PCB design and case models are done entirely in OpenSCAD, making it easy for users to customize or print their own enclosures. The keyboard supports both direct soldering of switches and hotswap sockets for easier assembly and modification.
Firmware-wise, Cheapino integrates with QMK, a widely used open-source keyboard firmware framework, with added support for Vial. Vial enables users to remap keys and edit the keymap in real-time without reflashing, improving the developer and user experience.
The entire project is licensed under Creative Commons Attribution, allowing anyone to modify and build upon the design for personal use.
the Japanese duplex matrix: hardware efficiency with tradeoffs
The Japanese duplex matrix is the technical highlight that makes Cheapino stand out. Traditional split keyboards usually require a separate MCU per half because each half independently scans its own matrix. This doubles not only the MCU count but also firmware complexity and costs.
In contrast, the Japanese duplex matrix interleaves the row and column wiring of both halves so that a single MCU can scan the combined matrix by switching between the halves via the RJ45 connection. This halves the GPIO pin count and eliminates the need for a second microcontroller.
Under the hood, this means the MCU toggles control lines to select which half of the keyboard matrix is active at any given time. The firmware must handle multiplexing and key scanning across the combined matrix, which adds some complexity to the scanning logic but is well supported by QMK.
The tradeoff is clear: you gain a simpler, cheaper hardware build and reduced BOM (bill of materials) cost but at the expense of a more complex matrix scanning routine and slightly trickier PCB routing. This approach also limits the design flexibility compared to fully independent halves.
The code quality of the firmware integration is solid, leveraging the battle-tested QMK ecosystem and the Vial extension for smooth runtime remapping. The PCB and case designs are cleanly organized in OpenSCAD files, making modifications straightforward for those familiar with 3D modeling and PCB layout.
explore the project
The repository is organized primarily around the hardware design files and firmware configurations:
PCB design and case: These are OpenSCAD files that define the physical layout and enclosure. You can customize or 3D print your own case using these.
Firmware: QMK configuration files tailored to support the Japanese duplex matrix and the RP2040-Zero MCU. Vial support is included, enabling dynamic keymap editing.
Documentation: The README outlines the design philosophy, wiring scheme, and assembly instructions.
Since no explicit installation or quickstart commands are provided, the best way to try out Cheapino is to start by reviewing the documentation and browsing the OpenSCAD files to understand the physical design. The firmware folder contains the QMK keymap and configuration, which you can compile and flash using the standard QMK build tools if you want to test or customize the keyboard firmware yourself.
verdict
Cheapino offers a practical approach to building an affordable split keyboard by cleverly using the Japanese duplex matrix to run both halves on a single RP2040 MCU. This hardware optimization reduces cost and complexity for DIY keyboard enthusiasts who are comfortable with PCB assembly and QMK firmware customization.
The tradeoff is that the wiring and scanning logic are more complex than a straightforward split with dual MCUs, which may require deeper understanding of keyboard matrix designs and firmware internals. It’s not a plug-and-play solution for beginners but rather a neat project for those who want to experiment with efficient hardware design and enjoy the flexibility of open-source firmware.
If you’re into custom keyboards and want to build a compact split layout with minimal hardware overhead, Cheapino is worth exploring. The open-source OpenSCAD designs and QMK/Vial support provide a solid base to build on and customize to your preferences.
→ GitHub Repo: tompi/cheapino ⭐ 1,095 · OpenSCAD