From @Sven Gregori:

4chord MIDI

the USB MIDI keyboard dedicated to play all the four chord songs, from Adele via Green Day and Red Hot Chilli Peppers to U2 and Weezer. Thanks to MIDI, you can be any instrument – and all of them at once. Yay!

 Built around an AVR ATmega328 and Objective Development’s V-USB library, 4chord MIDI acts as a regular USB MIDI instrument. It supports playback in every key and five different playback modes:

• simple triad chord (root, third, fifth)
• triad chord + third + fifth + third as quarter notes
• triad chord + third + fifth + octave as quarter notes
• root note + third + fifth + third as quarter notes
• root note + third + fifth + octave as quarter notes

The playback tempo can be set between 60 and 240 bpm.

Here is the board in action:

The design files and source code are available on GitHub:

sgreg/4chord-midi