Wiring individual LEDs to the MAX7219 wasn't quite as straight-forward as wiring an 8x8 LED matrix (though quite why, we're still not sure). But, after a bit of fiddling about and debugging by plugging just one set of wires in at a time, we eventually got some working firmware for our light-up keyboard.
It's all pretty encouraging - dial in a chord or scale and all the available notes correctly light up! There are a few modes of operation. At the minute we're using a rotary dial to select everything, but in time, we'll use the second keypad to select the current key/scale, and plenty of different buttons to quickly and easily change between minor, major, full scale, chords etc.
In "chord" mode, it's pretty obvious what's happening: the chord shape is repeated in two places on the keyboard (originally we were going to just display it once, but then thought we've no idea whether the player would want to use their left hand or their right hand to play the chord - so put a chord shape under each!).
In "show full chord" mode, all of the notes that make up the select chord appear across the entire keyboard. This is a bit like showing a scale, but not quite. For example, a C major chord (based on the triad of C-E-G) is very different to the C major scale (consisting of the seven notes C-D-E-F-G-A-B). So what's the point?
Chords/triads have lots of different inversions.
A C major chord doesn't have to start with the lowest note on C.
You can play a C major chord as G-C-E. This is known as the "second inversion" (the first inversion is played E-G-C). So by displaying all the notes of the chord and repeating them multiple times over, you can simply play any consecutive three notes and you'll get a C major - even if you're not starting on the root note C.
Full scale mode is pretty self-explanatory: select a scale and the entire scale is displayed across the entire keyboard. This should make it easy for players to jam along, much like a guitarist might do, when playing a solo. If you know the song is, for example, a shuffle/blues in A, there's no reason why you can't dial in "A pentatonic" and you can pick pretty well any notes out of the scale and produce something vaguely melodic (that's how a lot of guitar players do it anyway!)