Cirrus40 Keyboard

The Cirrus40 is a 40% split mechanical keyboard, with an ortholinear layout and a focus on usable rotary encoders.

Design Considerations

• A 3x thumb cluster where each key is equally usable and comfortable to reach.
• Rotary encoders under the palms are nicely reachable without moving hands from the homerow position. One-handed use together with homerow modifiers (think: switching tabs, workspaces; word-by-word navigation, selection, and deletion). Hardware debouncing for the perfect input signal; no more missed transitions.
• A full 3d printed case protects the internals.
• An optional 3d printed 30deg tenting stand for rock-solid tenting. The tenting stand integrates perfectly with the case.
• An ortholinear layout. Why not columnar stagger: it's comfortable at rest, but increases separation between the pinky and ring fingers in motion.
• A hot-pluggable wired USB-C interconnect between halves. USB-C cables are widely available.
• Support for both wired and wireless operation.
• Simple tenting using M3 screws.
• No toys: no LEDs, no displays.
• Only widely available and inexpensive parts.

The Cirrus40 combines some of the favorite aspects of my previous builds (the Cheapino's thumb cluster and the Lotus58's key layout and USB-C interconnect). I got a bit fixated on optimizing encoder precision and ergonomics, which is why you'll see the hardware debouncing circuit, the below-palm position, and the strong recommendation for quality Alps encoders.

Firmware

QMK: qmk_firmware. My own keymap is available as an example at qmk_userspace.

ZMK: Contributions welcome. zmk-config hasn't been updated for Cirrus40 Revision 2 yet.

Required Files

All files required for manufacture are located in the most recent release directory.

• PCB Gerber Files: For manufacturing the printed circuit boards.
• 3D Case/Stand Files: STEP files for printing the case and optional tenting stand.

Ordering guide

TLDR: PCB and case manufacturing from JLCPCB, electronics components from LCSC, and the rest from AliExpress with Choice shipping.

• jlcpcb.com:
  • For the PCB: it's safe to stick with default values.
  • For the case: I used 9600 Resin (Spray Painting-Matte-Black) for the case and PA12-HP Nylon for the tenting stand with good results. Please let me know if you experiment with other materials.
• lcsc.com: See doc/bom.csv for more info. Note that the USB-C connector I used is currently out of stock and NOT included in doc/bom.csv.
• aliexpress.com: Great for switches, keycaps, encoder heads, the RP2040 ProMicro MCU, MCU pin headers, screws, spacers, nuts, etc. I've had good experiences with "Choice" shipping.

Note: jlcpcb.com and lcsc.com offer combined shipping; this is not recommended, as it was more complex, more expensive, and slower when I tried it.

Build guide

A detailed, step-by-step build guide is not yet available. Contributions on this front are very welcome!

However, the build process is straightforward for those with some experience in hand-soldering keyboards. The soldering difficulty is medium-to-easy; the surface-mount components are in a large (0805) package and are suitable for hand-soldering. The USB-C connector is also simpler than it looks: the wiring is such that all data pins can simply be bridged together (i.e. precise drag-soldering is not strictly needed).

I usually do the USB-C connector and SMD components first, followed by diodes and hotswap sockets; and finally the MCU plus socket and encoders. All components (except the encoder) go on the bottom side of the PCB. Consult the kicad files if something is unclear. A multimeter is good to have around for verification and error search.

Bill of Materials

A single keyboard build costs around 80EUR including shipping and customs from JLCPCB and LCSC.

Core Components

Part	Quantity	Notes
Reversible PCB	2	From the Gerber files
3D Printed Case	1	From the STEP files. Tenting stand is optional.
MX switches	36
MX hotswap sockets	36
EC11 rotary encoders	2	ALPS EC11E15244G1 is strongly recommended.
Encoder knobs	2
1N4148 or 1N4148W diodes	40	SOD-123 or SOD-323 package
100nF capacitors	4	0805 package
10kOhm resistors	4	0805 package
8.2kOhm resistors	4	0805 package
MCU Sockets / Pin Headers	2 sets	For socketing the microcontrollers

Wired Build Components

Part	Quantity	Notes
Pro Micro (RP2040) MCU	2
Ferrite beads	4	0805 package, e.g., 220Ω@100MHz
USB-C mid-mount connector	2	e.g., LCSC C963373
ESD protection diode	2	e.g., LCSC C558418

Wireless Build Components

Part	Quantity	Notes
nice!nano v2 MCU	2
LiPo Battery	2	301230 (110 mAh) size recommended
Power slide switch	2	e.g., LCSC C431540

Hardware

Part	Quantity	Notes
M2x4mm screws	20	Use M2x6mm for the 4 case holes if using the tenting stand
M2x7mm spacers	10
M3 screws and nuts	4 (opt)	For basic, non-stand tenting

Release history

Released files can be found in the releases/ directory.

• releases/20251220: Revision 2. 3x5+3 with a full 3d printed case and 30deg tenting stand. Hardware encoder debouncing.
• releases/20251001: Revision 1. 3x6+3 with a sandwich case.

Revision 1 Gallery