USB-C plug
Part of the board, so it plugs straight into a port without a cable.
Kjern 32u4 line
Kjern-32u4HID-C
Designed and assembled in Bergen, Norway
Kjern-32u4HID-C
Kjern-32u4HID-C
A USB HID board with the plug built in.
ATmega32U4, USB-C plug, ESD protection and a resettable fuse. Designed and assembled in Bergen, Norway.
A common question
Why not use an ESP32‑C3?
USB
The C3 can't act as a keyboard.
The ESP32-C3's USB port only does serial and JTAG. The ATmega32U4 has a full USB device controller, the same chip as on the Arduino Leonardo, so it can show up as a keyboard, mouse, gamepad or MIDI device using the standard Arduino libraries.
Inside
The main parts
The board is 18 by 32 mm. There's also a small easter egg on it somewhere, but you'll have to find that yourself.
The parts
Polyfuse
500 mA. It trips on a short and resets once it cools down.
USBLC6-4SC6
ESD protection on D+ and D−, placed right behind the plug.
Reset
Tactile reset switch next to the plug.
Status LED
Connected to PC7 by default, so it is controlled from firmware.
16 MHz crystal
The clock source. USB needs an accurate clock to work.
ATmega32U4
The microcontroller, with 32 KB flash and full-speed USB built in.
AMS1117-3.3
On the back. Gives a 3.3 V rail for sensors and displays.
Power
From 5 V to 3.3 V
- VBUS5 V arrives on the plug
- Polyfuselimits current to 500 mA
- USBLC6protects the data lines
- 3V3-ENfeeds the regulator
- AMS1117drops it to 3.3 V
- Header3.3 V out on two pins
Solder jumpers
Seven solder jumpers
- HWB_PUcutdisconnects the HWB resistor
- HWB-LOWbridgeties HWB to GND
- FUSE-BYPbridgeskips the polyfuse
- EMI-DISbridgeties the USB shell to GND
- 3V3-ENcutturns off the 3.3 V rail
- AVCC-BYPbridgeskips the 10 Ω filter
- LED-SET3-wayfirmware status
Testpoints
12 labelled testpoints
Six of them, MOSI, MISO, SCK, RESET, 5 V and GND, are grouped on the back in a 2 by 3 grid, so a pogo-pin jig can program the board.
Drag the board to look around.
Press play for a 75-second tour of the board.
Or read it insteadCompared
Kjern-32u4HID-C and ESP32-C3
Both are small, cheap boards, but they're made for different things. The C3 is for wireless projects. This board is for projects that need to act as a USB device.
Kjern-32u4HID-CTypical ESP32-C3 board
Native USB HIDKeyboard, mouse, gamepad, MIDINo. USB is serial and JTAG only
Arduino Keyboard and Mouse librariesWork as-is, Leonardo-class chipNot supported
ConnectionUSB-C plug on the boardNeeds a cable
Logic level5 V, plus a 3.3 V rail3.3 V
ProtectionESD clamp and resettable fuseVaries by board
Configurable in hardware7 solder jumpers, 12 testpointsRarely
WirelessNone, by designWi-Fi and Bluetooth LE
Use cases
What people build with it
Macro pads
Shortcut keys and stream-deck style controllers. No drivers needed.
Custom controllers
Gamepads, flight sim panels and arcade sticks.
MIDI controllers
USB MIDI for knobs, pads and faders.
Automation
Test fixtures, kiosk buttons and repeated keyboard tasks.
Schools
Students can build a working USB device without dealing with wireless.
Hobbyists
The jumpers and testpoints make it easy to experiment with the hardware.
Tech specs
Specifications
- Microcontroller
- ATmega32U4
- Memory
- 32 KB flash
- USB
- USB-C plug
- Protection
- ESD and fuse
- Power
- 5 V and 3.3 V
- I/O
- 15 GPIO
- Solder jumpers
- 7
- Testpoints
- 12 labelled
- Size
- 18 × 32 mm
- Firmware
- Ulriken
- Status LED
- Configurable
- Made in
- Bergen, Norway
8-bit AVR in QFN-44, clocked by a 16 MHz crystal.
2.5 KB SRAM and 1 KB EEPROM.
Full-speed USB 2.0 straight from the chip.
USBLC6-4SC6 on the data lines, 500 mA polyfuse on VBUS.
AMS1117-3.3 rail on the back, switchable with 3V3-EN.
2 by 10 pads on a 2.54 mm grid, with 5 V, 3.3 V and GND.
Boot strap, fuse bypass, shield bond, 3.3 V enable, AVCC filter, LED source.
Three on the front, nine on the back, six laid out as an ISP header.
Plus about 10 mm of plug.
Keyboard, mouse and raw HID out of the box. MIT licensed.
Firmware status on PC7, a plain power light, or off.
Designed, assembled and tested in-house.
Status
Current status
First batchLate October
Last round of testing after several hardware revisions.
Pre-orders25 placed
Placed before the first production run.
FirmwareOpen source
The firmware is MIT licensed.
Made inBergen, Norway
Designed, assembled and tested in Bergen.
About
About Kjern
Kjern is a small electronics company in Bergen, Norway, run by me, Phillip Rødseth. I'm 15. I design the boards, write the firmware, and assemble and test each unit. I want to focus on making the hardware and work with shops that already know how to sell it.
Questions can go to kontakt@kjern.no.