The standard pianoforte with recessed black keys makes it easy to play in diatonic, equal-temperament key-of-C. Every other key is hard. G has one sharp; D has two sharps; B has five sharps. Even to explain what "has sharps" means is a mouthful.
This code is meant to hook into an instrument with undifferentiated keys / frets / stringboards, each of which can be lit from below.
Taking (for starters) input from a knob, the program defines which keys (for starters) in an 88-key "piano" to light up the (for starters) diatonic notes. Turning the knob changes the "key" (ambiguous word but here I refer to the which-sharps-and-flats key), which defines which "keys" (hammers) should be back-lit.
A button can switch between diatonic and pentatonic.
Later we can add more instruments (eg: guitar, violin).
Need to find a library to hook the hardware in.
Need to find undifferentiated keys that can be back-lit.