alt Hacker NewsThat is fantastic, I love it!
If I may submit an extremely pedantic music nerd bug report: at 46s in the video demo (https://www.youtube.com/watch?v=qboig3a0YS0&t=46s), the display should read Bb instead of A#, as the key of C minor is written with flats :)
(The precise rule is that a diatonic scale must use each letter name for exactly one note, e.g. you can't have both G and G# in the same key, and you can't skip B. This has many important properties that make music easier to read and reason about, such as allowing written music to specify "all the E's, A's, and B's are flat" once at the start of the piece instead of having to clutter the page with redundant sharps or flats everywhere.)
The device is fantastic indeed!
Regarding flats and sharps: one could ignore the Pythagorean stuff and go full well-tempered dodecaphonic, thinking purely in terms of semitones in the intervals. This toy sort of nudges towards this. It would be fun to add 12 small LEDs along the faders, and show the number of semitones with them, relative to the previous fader's position.
On one hand, the fact that the same sound can be named A# and Bb may be puzzling for a kid (they could differ on a violin, I suppose); OTOH if the kid later learns formal music notation, this becomes helpful, so your comment holds.