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A fully custom, 3D-printed, Bluetooth split mechanical keyboard built without a PCB.
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A fully custom, 3D-printed, Bluetooth split mechanical keyboard built without a PCB, running Rust firmware on nRF52840 microcontrollers.
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:::info
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@@ -12,87 +13,183 @@ A fully custom, 3D-printed, Bluetooth split mechanical keyboard built without a
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## Description
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This project is a custom-built, wireless split mechanical keyboard based on the ergonomic Corne physical layout. To maximize hardware complexity and avoid prebuilt kits, the keyboard uses no PCB; instead, the switch matrix is entirely handwired. The core of the project is writing a custom embedded Rust firmware from scratch using the embassy-rs framework to handle matrix scanning, key debouncing, and Bluetooth Low Energy (BLE) communication.
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This project is a custom-built, wireless split mechanical keyboard based on the ergonomic **Corne** layout (4 rows × 6 columns per half, plus a 3-key thumb row on each side). There is no PCB: every switch is handwired into a diode matrix and mounted in a 3D-printed plate!
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Each half uses a **SuperMini nRF52840** (Adafruit nRF52 Bootloader, UF2 drag-and-drop flashing). The firmware is written in **Rust** and built on the **RMK 0.8** framework with a **dual-binary BLE split** architecture: the left half acts as the **central** (USB/BLE to the PC, full keymap), and the right half acts as the **peripheral** (scans its matrix and sends coordinates over Bluetooth). Configuration lives in `keyboard.toml` (pins, matrix map, three layers).
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The original plan was to implement matrix scanning, debouncing, and a BLE HID keyboard stack manually with **embassy-rs** and Nordic’s softdevice bindings. After roughly a week of attempts where the nRF52840 radio/HID path never came together reliably, the project moved to **RMK**, which still uses Embassy and modern Rust async patterns under the hood but provides a proven split-BLE and keymap pipeline.
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## Motivation
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I chose this project to gain deep, hands-on experience with both bare-metal electronics and modern embedded software. On the hardware side, handwiring the diode matrix forces a complete understanding of how electrical grids operate. On the software side, writing the firmware in Rust provides the opportunity to learn embassy-rs for asynchronous embedded programming and to understand how to implement a BLE HID (Human Interface Device) stack from the ground up. Also I want to some watch movies in the summer on my tv and I need a keyboard to type on the internet so that the search is way faster.
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I chose this project to get real experience on both sides of a custom keyboard: bare-metal wiring and modern embedded Rust.
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On the **hardware** side, handwiring forces you to understand rows, columns, diodes, ghosting, and how few GPIO pins can serve many keys. On the **software** side, I wanted to learn how a keyboard actually becomes a BLE HID device not just flash someone else’s binary.
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A practical goal too: a compact wireless keyboard for the TV/PC so searching and typing away from the desk is faster than a remote.
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## Architecture
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##Architecture
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### Hardware
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- The system architecture consists of a custom electrical matrix and an asynchronous Rust software stack.
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-**Layout**: Split Corne — 21 keys per half (18 in the main 3×6 grid + 3 thumb keys).
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-**Matrix**: 4×6 per half; **1N4148 diodes** (column → row) to prevent ghosting.
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-**Controllers**: Two **SuperMini nRF52840** boards (nice!nano v2–class clones).
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-**Power**: **3.7 V LiPo** per half with a slide power switch.
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-**Structure**: 3D-printed switch plate and case; **24 AWG** solid wire for row/column buses.
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- The Hardware Matrix: 42 mechanical switches are handwired into a matrix of Rows and Columns. Diodes are soldered to each switch to prevent electrical "ghosting" when multiple keys are pressed.
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### Firmware (final)
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- The Microcontroller: An NRF52840 development board acts as the central brain, connected to the matrix rows and columns via its GPIO pins. It is powered by a 3.7V LiPo battery.
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| Role | Binary | UF2 output | Responsibility |
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|------|--------|------------|----------------|
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| Left half |`central`|`NEW_LEFT.uf2`| BLE central, matrix scan (cols 0–5), USB/BLE HID to host, full keymap |
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| Right half |`peripheral`|`peripheral.uf2`| BLE peripheral, matrix scan (cols 6–11 via `col_offset = 6`), sends `(row, col)` to central |
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- The Rust Firmware: The software utilizes embassy-rs for async execution. A polling task continuously scans the GPIO matrix to detect state changes.
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**Pin map (both halves, local wiring)** — inner column toward the split, outer toward the pinky:
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- Bluetooth Communication: Using the nrf-softdevice crate, the microcontroller advertises itself as a BLE Keyboard and transmits the processed keystroke data over the air to a host computer.
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| Row | MCU pin |
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|-----|---------|
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| Row 1 (top) | P0_31 |
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| Row 2 | P0_29 |
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| Row 3 | P0_02 |
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| Row 4 (thumb) | P1_15 |
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| Col | MCU pin |
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|-----|---------|
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| Col 1 (inner) | P1_06 |
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| Col 2 | P1_04 |
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| Col 3 | P0_11 |
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| Col 4 | P1_00 |
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| Col 5 | P0_24 |
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| Col 6 (outer) | P0_22 |
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**Keymap (3 layers)** — defined in `keyboard.toml`:
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-**Layer 0**: QWERTY Corne base; left thumb `MO(1)` (symbols), right thumb `MO(2)` (nav/F-keys/BT).
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-**Layer 1**: Numbers and symbols (`\``~``!` … `[]``{}`, etc.).
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-**Layer 2**: F-keys, arrows, paging, BLE profile/clear (`User1` / `User2` / `User5`), and modifier combos (`WM(...)`) for shortcuts.
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**Build & flash**
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```text
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cargo build --release --bin central → NEW_LEFT.uf2 (left only)
Family ID for UF2: `0xada52840` (Adafruit nRF52). Flash memory layout reserves the bootloader region (`FLASH` from `0x1000`).
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### Software path (what actually happened)
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1.**Week ~1 (failed path)**: Custom `embassy-nrf` + softdevice/BLE HID experiments—matrix ideas worked in isolation, but getting a stable BLE keyboard device on the nRF52840 did not, despite many rebuild/flash cycles.
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2.**Final path**: **RMK 0.8.2** with `rmk_central` / `rmk_peripheral` macros, `keyboard.toml`, and **Vial-compatible** storage. Minimal application code (`central.rs`, `peripheral.rs`); complexity moved into configuration and build tooling.
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## Log
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<!--write your progress here every week-->
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<!--Progress log (~3-day intervals). Hardware and bring-up took the majority of the project time.-->
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### Week 5 - 11 May
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### 5–8 May
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Finalized component research and ordered all necessary hardware (microcontrollers, switches, batteries, keycaps) -> Waiting for their delievery.
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Finalized the Corne layout choice and BOM. Ordered SuperMini nRF52840 boards, Gateron Milky Yellow switches, 1N4148 diodes, LiPo cells, wire, keycaps, and power switches. Started the 3D model/plate while waiting on shipping.
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### Week 12 - 18 May
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### 9–11 May
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So far, I've 3D printed the baseplate for the switches to snap into. I went with a hand-wired matrix setup, including diodes for reliable input. I've also soldered a power switch and a battery, which I tested with a multimeter. I connected the columns and one of the rows to the microcontroller just to check the continuity. This is all for the first half of the build; I plan to wrap up the other half later this week.
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Parts still in transit. Read nRF52840 docs, Embassy book chapters, and split-keyboard matrix guides. Sketched row/column routing on paper so the handwire would not be guesswork later.
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### Week 19 - 25 May
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### 12–15 May
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## Hardware
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Printed the switch plate and case pieces. Began the **left half** handwire: diode orientation (column → row), column buses along the inner edge, first row soldered. Multimeter continuity checks after every few keys—caught one reversed diode early.
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The hardware relies on a 3D-printed chassis to hold the switches, eliminating the need for a PCB. The logic is handled by NRF52840 chips due to their low power consumption and excellent Bluetooth capabilities.
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### 16–18 May
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Finished the left matrix and installed the LiPo + slide switch. Smoke test: power only, then tweezers on row/column pins. Connected one row and one column to the SuperMini to confirm a single intersection could be detected. **Right half not started yet**—wanted one working grid first.
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### 19–21 May
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### Schematics
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Completed **right half** wiring using the same pinout as the left (per-half local row/col numbering). Set up the Rust toolchain (Windows + WSL), `thumbv7em-none-eabihf` target, `flip-link`, and `cargo-binutils`. First flashes used a single-binary approach; behavior was inconsistent on the split.
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### 22–24 May
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### Bill of Materials
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**Hardest software stretch:** tried to implement the keyboard **without RMK**—Embassy executor, GPIO matrix scan, debouncing, and BLE HID using Nordic stack crates (`nrf-softdevice` / related paths). Spent about **a week** here: device would compile and flash, but BLE advertising/HID reports never behaved like a real keyboard on the host. nRF52840 + softdevice + HID from scratch was a wall; pivoted to **RMK** as a maintained framework that already solves split BLE and keymaps.
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<!-- Fill out this table with all the hardware components that you might need.
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### 25–27 May
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The format is
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```
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| [Device](link://to/device) | This is used ... | [price](link://to/store) |
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Regenerated the project around **RMK 0.8** dual binaries (`central` / `peripheral`). Configured `keyboard.toml` (matrix pins, `matrix_map`, BLE addresses). **Left controller** refused to take new firmware for a while (old image kept running); later fixed with proper UF2 flash and `clear_storage`. Right half flashed as `peripheral.uf2` and paired over BLE. Hit Windows path length limits during builds moved/cloned the repo to a shorter path.
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```
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### 28 May
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-->
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Left matrix still typed garbage: all keys repeating one character, then a scrambled QWERTY-like grid. Root causes: (1) **stale keymap in flash** (RMK Vial storage overriding `keyboard.toml`), (2) **wiring** not matching the assumed column order. Rewired to the documented pin table; `clear_storage = true` once to prove `keyboard.toml` was live (`Esc` / `Q` test grid). Confirmed **Corne Split** BLE name and stable per-key output.
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| Device | Usage | Price |
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|--------|--------|-------|
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|[SuperMini NRF52840](https://shorturl.at/LceaZ)| The microcontroller |[45 RON](https://shorturl.at/Ukoo9)|
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|[Liter Energy 120mAh 3.7V LiPo (401230)](https://shorturl.at/YeYWx)| Power supply |[100 RON](https://shorturl.at/ZWF5S)|
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|[Gateron Milky Yellow Pro](https://shorturl.at/b2qIj)| Mechanical linear keyboard switches |[125 RON](https://www.gateron.co/products/gateron-ks-3-milky-pro-switch-set)|
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|[1N4148 High-speed Diodes](https://www.vishay.com/docs/81857/1n4148.pdf)| Prevents "ghosting" in the keyboard matrix |[16 RON](https://rb.gy/56u5x7)|
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|[24AWG Solid Core Copper Wire](https://www.optimusdigital.ro/en/kits/11950-plusivo-silicone-wire-kit-24awg-6-colors-9m-each-0721248989734.html)| Routing the rows and columns |[24 RON](https://www.aliexpress.com/item/1005010758626132.html?spm=a2g0o.order_list.order_list_main.17.b1b61802RMNohO)|
|[SS12F15VG4 4MM Slide Switches](https://leeselectronic.com/qc/product/31191-slide-switch-chassis-mount-on-on-spdt-1a-125vac.html)| Physical power toggle for the batteries |[36 RON](https://www.aliexpress.com/item/1005010262474497.html?spm=a2g0o.order_list.order_list_main.25.531a1802SqISZa)|
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### 29 May
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Removed `clear_storage` (kept `clear_layout` while iterating). Implemented full **3-layer Corne keymap** (base, symbols `MO(1)`, nav/F-keys/BT `MO(2)`). Rebuilt `NEW_LEFT.uf2`; left + right halves work together over BLE. **Project functionally complete.**
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## Hardware
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The hardware relies on a 3D-printed chassis to hold the switches, eliminating the need for a PCB. Logic is handled by nRF52840 SuperMini boards for low power and BLE.
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### Schematics
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### Bill of Materials
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| Device | Usage | Price |
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|--------|--------|-------|
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|[SuperMini NRF52840](https://shorturl.at/LceaZ)| Microcontroller (one per half) |[45 RON](https://shorturl.at/Ukoo9)|
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|[Liter Energy 120mAh 3.7V LiPo (401230)](https://shorturl.at/YeYWx)| Power supply (per half) |[100 RON](https://shorturl.at/ZWF5S)|
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|[Gateron Milky Yellow Pro](https://shorturl.at/b2qIj)| Linear switches |[125 RON](https://www.gateron.co/products/gateron-ks-3-milky-pro-switch-set)|
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|[1N4148 diodes](https://www.vishay.com/docs/81857/1n4148.pdf)| Anti-ghosting in the matrix |[16 RON](https://rb.gy/56u5x7)|
|[SS12F15 slide switches](https://leeselectronic.com/qc/product/31191-slide-switch-chassis-mount-on-on-spdt-1a-125vac.html)| Battery power switch |[36 RON](https://www.aliexpress.com/item/1005010262474497.html)|
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## Software
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### Libraries used (final firmware)
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| Library | Description | Usage |
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|---------|-------------|-------|
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|[embassy-rs](https://github.com/embassy-rs/embassy)| Async embedded framework for Rust | The core framework used to run asynchronous tasks (like matrix scanning and Bluetooth handling) without blocking the CPU |
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|[embassy-nrf](https://crates.io/crates/embassy-nrf)| Hardware Abstraction Layer (HAL) | Used to safely interact with the NRF52840's physical GPIO pins and timers |
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|[nrf-softdevice](https://github.com/embassy-rs/nrf-softdevice)| Rust bindings for Nordic's Bluetooth stack | Used to configure the BLE radio and implement the HID protocol to talk to the PC |
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|[defmt](https://github.com/knurling-rs/defmt)| Highly efficient logging framework | Used for debugging the matrix scanning logic over a USB connection |
**Complete.** Both halves type over BLE with a three-layer Corne keymap. Future tweaks are `keyboard.toml` edits + reflash `NEW_LEFT.uf2` on the central only; the peripheral only needs a reflash if matrix pins or BLE pairing metadata change.
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