Keyboard

PKB — Wireless Split Mechanical Keyboard

PKB (Printed KeyBoard) is a custom 42-key wireless split mechanical keyboard designed from scratch in KiCad 10 and SolidWorks, powered by dual Seeed Studio XIAO nRF52840 microcontrollers running ZMK Firmware.

Why I Made It & How It Works

Why I Built PKB

Traditional keyboards force your wrists to angle inward and make your fingers stretch across 6+ rows to reach numbers, symbols, and navigation keys. I wanted to design a compact, fully wireless split keyboard where:

1. Each half can be positioned at shoulder width for natural ergonomics.
2. Every letter, symbol, bracket, arrow key, F-key, and Numpad key is within one row of the home row using 3 tap-dance layers (126 effective key bindings across 42 physical switches).
3. Both halves fit on a single panelized PCB connected by mouse-bite breakaway tabs (BO1–BO3) to make fabrication and assembly clean and cost-effective.

How It Works

• Microcontroller & Wireless Split (Seeed Studio XIAO nRF52840): Each half runs a XIAO nRF52840 (U1/U2). The left half acts as the central controller (connecting to up to 4 devices over Bluetooth Low Energy or USB-C), while the right half pairs wirelessly over BLE to the left half.
• I2C GPIO Expansion (PCF8574AP): Because the XIAO nRF52840 has only 11 GPIO pins, the 5 matrix columns (COL0–COL4) are driven directly by the XIAO (D1–D5), while the 5 matrix rows (ROW0–ROW4) are read through an NXP PCF8574AP 8-bit I2C I/O expander (U3/U4) on a shared I2C bus (SCL on D9, SDA on D10).
• Dual 128×32 OLED Status Displays (J3/J4): Each half includes an I2C SSD1306 128×32 OLED display showing live battery percentage (%), active layer name (QWERTY, CODE / NAV, NUM / FN), and USB/Bluetooth profile status, with automatic 30-second idle blanking.
• Per-Key RGB Underglow (42× SK6812MINI) with High-Side MOSFET Power Cut (Q1–Q4): Each half features 21 addressable SK6812MINI LEDs driven by hardware SPI (D0) and powered from VBAT through an NMOS+PMOS (Q1/Q3, Q2/Q4) high-side load switch (D8) so LED quiescent draw can be completely cut off to preserve battery life.

PCB & 3D Renders

Full Panelized PCB Layout (KiCad)

3D PCB Render

Schematics

Both the left and right halves share a modular hierarchical schematic (side.kicad_sch):

Left Half Schematic (/left/)

Right Half Schematic (/right/)

CAD & Enclosure Design

Custom 3D-printed enclosures modeled in SolidWorks with integrated battery compartment and cable routing channel (available in .STEP, .SLDPRT, and .3MF formats inside CAD/):

Left Half Enclosure (left_cad.png)	Right Half Enclosure (right_cad.png)

Hardware Pinout

Firmware & Keymap (ZMK)

Pre-built .uf2 firmware binaries are automatically compiled by GitHub Actions and published to GitHub Releases:

• Left Half (Central): pkb_left-seeeduino_xiao_ble-zmk.uf2
• Right Half (Peripheral): pkb_right-seeeduino_xiao_ble-zmk.uf2

Tap-Dance Thumb Keys

• Left Thumb: 1 Tap = Space | 2 Taps = Toggle Layer 1 (CODE / NAV)
• Right Thumb: 1 Tap = Backspace | 2 Taps = Toggle Layer 2 (NUM / FN)

Layer 0: QWERTY (Base)

+-----------------------------------------+   +-----------------------------------------+
|   Q   |   W   |   E   |   R   |   T     |   |   Y   |   U   |   I   |   O   |   P     |
|   A   |   S   |   D   |   F   |   G     |   |   H   |   J   |   K   |   L   |   ;     |
|   Z   |   X   |   C   |   V   |   B     |   |   N   |   M   |   ,   |   .   |   /     |
| LCTRL | LSHFT | LALT  | LGUI  |  ESC    |   | RGUI  | RALT  | RSHFT | RCTRL | ENTER   |
+-------------------------------+---------+   +---------+-------------------------------+
                                | SPC/NAV |   | BSP/NUM |
                                +---------+   +---------+

Layer 1: CODE / NAV (Double-Tap Left Thumb)

+-----------------------------------------+   +-----------------------------------------+
|   `   |   [   |   ]   |   -   |   =     |   | PG_UP | HOME  |  UP   |  END  |  DEL    |
|   ~   |   {   |   }   |   _   |   +     |   | PG_DN | LEFT  | DOWN  | RIGHT |  ---    |
|  ---  |   \   |   |   |   <   |   >     |   |  ---  |  ---  |  ---  |  ---  |  ---    |
|  ---  |  ---  |  ---  |  ---  |  ---    |   |  ---  |  ---  |  ---  |  ---  |  ---    |
+-------------------------------+---------+   +---------+-------------------------------+
                                | SPC/NAV |   |   ---   |
                                +---------+   +---------+

Layer 2: NUM / FN (Double-Tap Right Thumb)

+-----------------------------------------+   +-----------------------------------------+
|  F1   |  F2   |  F3   |  F4   |  F5     |   |   /   |   7   |   8   |   9   |   -     |
|  F6   |  F7   |  F8   |  F9   |  F10    |   |   *   |   4   |   5   |   6   |   +     |
|  F11  |  F12  |RGB_TOG|EP_TOG |OUT_TOG  |   |   0   |   1   |   2   |   3   |   .     |
| BT 1  | BT 2  | BT 3  | BT 4  | BT_CLR  |   |  ---  |  ---  |  ---  |  ---  |  ---    |
+-------------------------------+---------+   +---------+-------------------------------+
                                |   ---   |   | BSP/NUM |
                                +---------+   +---------+

Build & Assembly Guide

Step 1: Depanelize the PCB

1. The left and right keyboard halves are manufactured as a single panel joined by three mouse-bite breakaway tabs (BO1, BO2, BO3) along the center spine.
2. Gently flex or snip along the perforated mouse-bite slots using flush cutters to separate the Left and Right PCBs, then lightly sand the tab edges flush.

Step 2: Solder Reverse-Mount RGB LEDs (D63–D104)

1. Flip the PCB to the bottom (B.Cu) layer.
2. Solder the 21× SK6812MINI-E reverse-mount addressable RGB LEDs per half so their lenses shine upward through the rectangular switch LED cutouts.
   • Important: Align the notched/beveled corner (Pin 3 = GND) of each SK6812MINI-E with the silkscreen marker on the PCB.
   • Use a temperature-controlled soldering iron at $280\text{--}300,^\circ\text{C}$ (or low-temp solder paste + hot air) to avoid overheating the plastic LED packages.

Step 3: Solder Matrix Diodes (D1–D42) & SMD MOSFETs (Q1–Q4)

1. Matrix Diodes (D1–D42): The ComboDiode footprints support either 1N4148W (SOD-123 SMD) diodes on F.Cu or B.Cu, or 1N4148 (DO-35 through-hole) diodes. Orient every diode's cathode band (|◁) toward the square pad (K).
2. High-Side LED Power Switch (Q1–Q4): Solder the SOT-23 NMOS (Q1, Q2) and PMOS (Q3, Q4) transistors onto their designated SMD pads near the microcontroller area on each half.

Step 4: Solder Passive Components & IC Expanders (U3, U4)

1. Solder the 100 nF decoupling capacitors (C1–C42) next to each RGB LED, plus the 100 µF bulk radial capacitors (C43, C44) across the VLED power rails.
2. Solder the axial resistors on each half:
   • $4.7\text{ k}\Omega$ (R1–R6): I2C SDA/SCL pull-ups and /INT pull-ups.
   • $10\text{ k}\Omega$ (R7, R8) & $100\text{ k}\Omega$ (R9, R10): MOSFET gate bias/pull-down resistors.
   • $330\text{ }\Omega$ (R11, R12): RGB DIN data-line series resistors.
3. Solder the PCF8574AP DIP-16 I2C I/O expanders (U3, U4)—or DIP-16 IC sockets—ensuring the Pin 1 half-moon notch matches the PCB silkscreen.

Step 5: Solder Hot-Swap Sockets, Switches & Peripherals

1. Kailh MX Hot-Swap Sockets (SW1–SW42): On the bottom (B.Cu) side of the PCB, solder all 42 Kailh Choc/MX hot-swap sockets onto their wide SMD pads, making sure they sit flat against the board and do not obstruct the switch center hole.
2. Seeed Studio XIAO nRF52840 (U1, U2):
   • Before soldering the XIAO down, solder two short wire leads onto the battery + and - pads on the underside of the XIAO nRF52840 and route them to the PCB's battery input pads (J1/J2) and slide power switch (SW43/SW44).
   • Solder the XIAO nRF52840 onto the PCB using pin headers or low-profile Mill-Max sockets with the USB-C port facing outward toward the top edge of the board.
3. 128×32 SSD1306 OLED Display (J3, J4): Solder the 4-pin OLED header (GND, VCC, SCL, SDA) so the OLED sits above the XIAO footprint.
4. Slide Power Switch (SW43, SW44) & LiPo Battery (J1, J2): Solder the 3-pin slide switch and the 2-pin JST-PH 2.0 mm battery connector.

Step 6: Flash ZMK Firmware & Pair Halves

1. Download the latest .uf2 files from GitHub Releases:
   • pkb_left-seeeduino_xiao_ble-zmk.uf2
   • pkb_right-seeeduino_xiao_ble-zmk.uf2
2. Connect the Left XIAO nRF52840 to your computer via USB-C and double-click its onboard Reset button—it will mount as a removable drive named XIAO-SENSE.
3. Drag and drop pkb_left-seeeduino_xiao_ble-zmk.uf2 onto the drive. Repeat with pkb_right-seeeduino_xiao_ble-zmk.uf2 on the Right XIAO nRF52840.
4. Power both halves on at the same time; they will automatically bond to each other over BLE. Pair your computer or phone to PKB via Bluetooth!

Step 7: Final Case Assembly

1. 3D-print CAD/3mf/left.3MF and CAD/3mf/right.3MF in PLA or PETG (0.2 mm layer height, 15–20% infill).
2. Place the 3.7 V LiPo battery into the recessed pocket at the base of each printed enclosure and plug it into the JST-PH connector (J1/J2).
3. Seat the assembled PCB into the enclosure, secure it with M2 screws through the mounting holes, press your 42 MX mechanical switches into the hot-swap sockets, and install your keycaps!

Repository Structure

├── CAD/
│   ├── Keyboard.step          # Full 3D assembly in STEP format
│   ├── 3mf/                   # Printable 3MF files (left.3MF, right.3MF)
│   └── Raw/                   # Native SolidWorks part files (left.SLDPRT, right.SLDPRT)
├── kicad/
│   ├── Keyboard.kicad_pro     # KiCad 10 project file
│   ├── Keyboard.kicad_sch     # Top-level schematic sheet
│   ├── side.kicad_sch         # Left/Right modular keyboard half schematic
│   └── Keyboard.kicad_pcb     # Panelized 2-layer PCB layout
├── firmware/
│   ├── boards/shields/pkb/    # ZMK shield definition, devicetree, Kconfig, and C init
│   ├── config/                # Keymap and ZMK configuration
│   └── build.yaml             # GitHub Actions build matrix
├── images/                    # PCB, schematic, and CAD screenshots
└── production.zip             # Ready-to-manufacture Gerber, Drill, and CPL files