2 layers139.1 × 107.7 mm
2 layers133 × 91.4 mm
Cirrus40
schuay/cirrus4019A 40% split mechanical keyboard with a focus on usable rotary encoders
- Size
- 133 × 91.4 mm
- Copper layers
- 2
- Thickness
- 1.6 mm
- Parts
- 71
- Nets
- 66
- Pads
- 571
- Tracks, vias
- 482, 83
- Pours
- 211
- Routed
- Yes
- KiCad
- KiCad 9.0 (file 20241229)
- Created
- 1 Oct 2025
- Updated
- 7 Jun 2026
Overview
Cirrus40 is an open-source printed circuit board by schuay, from the schuay/cirrus40 repository on GitHub, published under the CC-BY-4.0 licence. It is a 2-layer board of 133 × 91.4 mm with 71 parts on 66 nets, routed with 482 tracks and 83 vias.
It is built around the SRV05-4 and MSK12C02. Its parts include 21 diodes, 20 switches, 3 ICs and 2 connectors. It belongs with the keyboards and input boards and the AVR (ATmega, ATtiny) boards of this library.
Main components
2
SRV05-4
SOT-23-6L-55℃~+150℃ 15V 1pF 4A 5V 5uA 6V 80W ESD Four channels IEC 61000-4-2、IEC 61000-4-5 Unidirectional SOT-23-6L ESD and Surge Protection (TVS/ESD) ROHShongjiachengESD And Surge Protection (TVS/ESD)
MSK12C02
SMD,8x2.8mm-20℃~+70℃ 1.4mm 10000 times 12V 2.8mm 50mA 8mm SMD(SMT) Tab SPDT Surface Mount, Right Angle SMD,8x2.8mm Slide Switches ROHSSHOU HANSlide Switches
README
README.mdCirrus40 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
Parts
71| Qty | Value | Footprint | References |
|---|---|---|---|
| 21 | DIODE | footprints:Diode-dual | D1, D2, D3, D4, D10, D11, D12, D13, D14, D15, D16, D17, D18, D19, D20, D21 +5 |
| 18 | SW_Push | footprints:SW_MX_HotSwap_Reversible | K1, K2, K3, K4, K5, K6, K7, K10, K11, K12, K13, K14, K15, K16, K17, K18 +2 |
| 4 | 10kOhm | Resistor_SMD:R_0805_2012Metric_Pad1.20x1.40mm_HandSolder | 10kΩ, 10kΩ, 10kΩ, 10kΩ |
| 4 | 470nF | Capacitor_SMD:C_0805_2012Metric_Pad1.18x1.45mm_HandSolder | 100nF, 100nF, 100nF, 100nF |
| 4 | 8kOhm | Resistor_SMD:R_0805_2012Metric_Pad1.20x1.40mm_HandSolder | 8.2kΩ, 8.2kΩ, 8.2kΩ, 8.2kΩ |
| 4 | Ferrite Bead | Capacitor_SMD:C_0805_2012Metric_Pad1.18x1.45mm_HandSolder | FB1, FB2, FB3, FB4 |
| 2 | MSK12C02 | footprints:MSK12C02 | S1, S2 |
| 2 | SRV05-4 | footprints:SOT-23-6 | U1, U2 |
| 1 | Conn_01x02 | footprints:PinHeader_1x02_P2.54mm_Vertical | J2 |
| 1 | MountingHole_3.2mm_M3 | MountingHole:MountingHole_3.2mm_M3 | |
| 1 | ProMicro | footprints:ProMicro_Reversible_orig | U3 |
| 1 | RotaryEncoder_Switch_MP | footprints:RotaryEncoder_Alps_EC11E-Switch_Vertical_H20mm | ENC1 |
| 1 | USB_C_Receptacle_USB2.0_16P | footprints:Molex-216990_GCT-USB45xx_HRO-TYPE-C-31-M-13x-PiMicro_1_DOUBLESIDED | J1 |
The 64 most common of 71 parts.
Questions
How big is the Cirrus40 PCB?
It measures 133 × 91.4 mm and has 2 copper layers, 1.6 mm thick.
What parts does the Cirrus40 use?
71 parts, built around the SRV05-4 and MSK12C02. The full list of its parts is on this page, with links to each part's symbol and footprint where PCBKit's library has them.
Where can I download the Cirrus40 KiCad files?
From the schuay/cirrus40 repository on GitHub, at the commit this page was made from: the board is pcb/pcb.kicad_pcb and its schematic pcb/pcb.kicad_sch. PCBKit opens the board in the browser, with nothing to install.
Can I use the Cirrus40 design in my own project?
Its repository publishes it under the CC-BY-4.0 licence: you may reuse it on that licence's terms (keep the author's notice, and for a copyleft licence share your changes alike).
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