umlaut_kb is a Linux driver for the 4-keys keyboard with symbols: ä, ö, ü, ß.
The keyboard itself is based on the TI MSP430F5529 LaunchPad. I don't publish it's firmware sources here.
I created umlaut_kb to extend my laptop's keyboard functionality to achieve the complete German alphabet.
The device requires English International keyboard layout.
Here is a fancy photo of how it looks like:
