VectorSplitK

VectorSplitK

An open-source, wireless, high-profile 42-key mechanical split keyboard. Built around the powerful Seeed Studio XIAO nRF52840, the VectorSplitK features an aggressive columnar stagger, custom-modeled flat profile keycaps, and a robust dual-case mechanical assembly. This project is my first split keyboard. The PCB was made entirely in KiCad and the CAD was done in Autodesk Fusion.

Full day-by-day design log, decisions, and dead ends: JOURNAL.md

Project stats: 14 build-log days · ~40h 20m total design time · 100% designing progress, 0% building progress (as of the latest journal entry)

On the name: Follows the author's personal naming convention for electronics projects (used previously for hackpad and devboard) — a descriptive word, a short form of the project's defining trait ("Split"), and a single trailing letter — giving VectorSplitK.

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Renders

Standard Top View

Isometric Split Profile

Rear Profile (USB-C & Alignment)

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Hardware Architecture

1. Electrical Schematic

The electrical system uses hierarchical sheets in KiCad to define identical left and right sides. It resolves typical multi-sheet duplicate ground issues by utilizing sheet-specific power flags (${SHEETNAME}_GND).

Main Sheet

Left Half Matrix

Right Half Matrix

2. Footprints

• Switch Footprints: High-profile acheron_MXH:MX100H (1.00u) and acheron_MXH:MX150H (1.50u) hotswap-ready switches (42 total, 21 per side).
• Diodes: Diode_SMD:D_SOD-123 (42 total, 1 per key).
• Mounting Holes: MountingHole:MountingHole_3.2mm_M3_DIN965_Pad (8 total, 4 per side).
• Resistors: Resistor_SMD:R_0805_2012Metric (4 total, 2 per side).
• Integrated Circuit: modified-XIAO-nRF52840-SMD:modified-XIAO-nRF52840-SMD (2 total, 1 per side).
• Testing Points: TestPoint:TestPoint_Pad_D2.0mm (4 total, 2 per side).
• Mousebites: Clean PCB snapping using PCM_marbastlib-various:mousebites_5p5mm_easysnap (3 break-offs).

3. Printed Circuit Board Assembly

Designed for rugged reliability and high-profile ergonomics using the custom footprints above.

KiCad PCB Window

KiCad PCB Render

KiCad PCBA Render

KiCad PCB Back Side

KiCad PCBA Back Side

• Design files: /PCBA

4. Enclosure & CAD Assembly

Designed from scratch in Autodesk Fusion, featuring integrated battery slots, precise USB-C ports, and secure brass threaded insert sockets.

file structure of fusion

top case and bottom case together in fusion

section analysis in fusion showing the screw assembly

• Tolerance Offset: 0.2 mm case-on-case fit alignment.
• Plates: Structural upper case acts as a switch plate, anchoring switches securely.
• Keycaps: Custom-modeled 1.00u and 1.50u flat profile MX-stem keycaps tailored for 3D printing.
• CAD files: /CAD

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Libraries & References

KiCad libraries used:

• mito-keyboard — base keyboard library
• marbastlib — mousebite & hardware footprints
• Panelization — panelization footprints
• Seeed XIAO KiCad Library — XIAO nRF52840 schematic symbol

Layout & edge-cut inspiration (referenced for switch arrangement and board outline conventions):

• mito-keyboard
• TaoTeChing
• OVERRIDE-X3D-Split-Keeb

Keycap reference model: Flat MX Keycaps — Printables, used as a base to project and re-model custom 1.00u/1.50u profiles.

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Design Notes

Columnar stagger layout: Switches placed on a 20 mm grid; second and fourth rows shifted 10 mm up, the middle row shifted 20 mm up, with board edges drawn 5 mm out from the outermost keys.

Hierarchical sheet grounding: KiCad hierarchical sheets duplicate net names by default, which merged the left/right ground nets into one. Fixed using sheet-specific power flags (${SHEETNAME}_GND), producing distinct Left_GND and Right_GND nets per half.

Case construction (Fusion 360): Built symmetrically — one side of the PCB modeled, then mirrored — with:

• Screw hubs extruded directly from the PCB's mounting hole positions
• A battery cavity sized to the WLY394058 LiPo and centered relative to the four mounting screws
• A USB-C cutout on the lower case wall, offset slightly oversized for clearance

Keycap modeling: 1.00u caps built by lofting projected upper/lower faces of a reference keycap, then hollowing and adding a stem + cross-hub. 1.50u caps were derived by splitting the 1.00u shape, spacing the halves 9 mm apart, and bridging the gap. Stem geometry was verified by cutting the actual switch stem shape directly into the keycap model to guarantee fit.

Upper case (switch plate): Modeled by projecting all switch bodies into a sketch, offsetting 1 mm around each switch and 1.5 mm along the walls, then extruding to the wall thickness (4.05 mm) with screw-hole pillars extended up from the PCB. Outer walls were thinned via a −0.8 mm facial offset on the curved edges for a lighter profile. Corner/cutout dimensions (14.70 × 14.70 mm switch openings, 0.705 mm corner radius) were carried over from the author's earlier hackpad build.

Design refinements: The SoC position, PCB mounting holes, and upper-case mounting holes were all iterated to (1) clear the USB-C cable path symmetrically on both halves, (2) keep mounting holes equidistant from the three nearest switches for insert clearance, and (3) keep the upper-case screw geometry in sync with the final PCB layout.

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Fabrication & Testing

• Electrical Rule Check (ERC) run after schematic completion to catch connection errors before layout.
• Design Rule Check (DRC) run after routing, with remaining clearance/edge errors resolved before export.
• Silkscreen: Front Fab and Back Fab converted to Back Silkscreen; User.Drawings converted to Front Silkscreen and User.Eco1 to Back Silkscreen; testing points manually labeled and text centered relative to switches. Author name, project name, and build month/year added to the front silkscreen for the final revision.
• Gerber export: Full fabrication output exported as VectorSplitK_Gerber.zip.
• Fab test: Gerbers test-uploaded to JLCPCB to validate manufacturability ahead of ordering.
• STEP export: PCB exported at multiple levels of detail (full detail for reference, minimal/simplified for clean CAD import) to support Fusion 360 case modeling.

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Bill of Materials (BOM)

Full sourcing sheet with vendors and pricing: /Bill of Materials

Electronics & PCBs

• 2x PCBs (Left & Right snap-apart panels)
• 2x Seeed Studio XIAO nRF52840 MCU boards
• 2x WLY394058 3.7V 1000mAh 1S LiPo Batteries (3.9 mm thickness for slim fit)
• 42x Mechanical switches (MX compatible)
• 42x Kailh MX Hotswap Sockets
• 42x SOD-123 SMD Diodes (1N4148)
• 4x 0805 Metric SMD Resistors (2 per side)

Hardware & Case Assembly

• M3 Screws (Specific lengths to clamp top and bottom cases)
• M3 Brass Knurled Heat-Staking Inserts (Pressed into bottom case pillars)
• 1x Set Custom-printed 1.00u & 1.50u flat keycaps (STEP files included)

Sourcing

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Firmware

The board runs wireless ZMK firmware on the XIAO nRF52840, using a custom shield built on the upstream seeeduino_xiao_ble board support.

Matrix

5 columns × 5 rows per half (21 keys/side, 42 total — a 4×5 grid plus 1 thumb key), with diode-direction = "col2row" matching the SOD-123 diode orientation on the PCB.

Config

Firmware lives in /zmk-config, structured as its own ZMK user-config repo: zmk-config/ ├── build.yaml ├── config/ │ ├── vectorsplitk.keymap │ ├── vectorsplitk.conf │ └── west.yml ├── boards/shields/vectorsplitk/ │ ├── vectorsplitk.dtsi │ ├── vectorsplitk_left.overlay │ ├── vectorsplitk_right.overlay │ ├── vectorsplitk.zmk.yml │ ├── Kconfig.shield │ └── Kconfig.defconfig └── .github/workflows/build.yml

Pushing this folder as its own GitHub repo triggers a GitHub Actions build via build.yaml, producing vectorsplitk_left-seeeduino_xiao_ble-zmk.uf2 and vectorsplitk_right-seeeduino_xiao_ble-zmk.uf2 under the Actions tab, no local toolchain required.

Known caveat: the matrix transform assumes COL_0 is the innermost column (closest to the thumb key) on both halves. If keys type in mirrored left-right order on first flash, reverse the column order in vectorsplitk.dtsi — a firmware-only fix, no rewiring needed.

Flashing

1. Double-tap Reset on the XIAO nRF52840 to mount it as a USB drive.
2. Drag the matching .uf2 (left or right) onto the drive.

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AI as a Design Collaborator

While the hardware engineering, PCB routing, firmware architecture, and 3D CAD modeling were completed manually, AI (Gemini, ChatGPT, and Claude) was used as a technical assistant throughout the project—not to generate designs, but to help research, troubleshoot, validate decisions, and improve documentation.

• Brainstorming: Helped refine the project name (VectorSplitK).
• Research & Validation: Verified switch footprints, component choices, and battery selection.
• Debugging: Assisted in resolving KiCad schematic and hierarchical-sheet issues.
• Design Review: Discussed engineering trade-offs, BOM costs, and manufacturing decisions.
• Firmware & Documentation: Helped architect the initial ZMK firmware configuration, reviewed the PCB matrix for firmware setup, and improved the project's technical documentation.

Project Integrity: All schematics, PCB layouts, routing, CAD models, and mechanical design were created manually in KiCad and Autodesk Fusion. AI was used only for research, troubleshooting, review, firmware guidance, and documentation.

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License

This project is licensed under the MIT License - see the LICENSE file for details.

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