Uhm ..... if i recall correctly, the pro version of this instrument read the image of the hand that intersect the beam (or if you prefer it simple, the reflex of the beam on your hand when you intersect the beam), in scanning cycles, and also varies intensity when the reflex is near the reading sensor, and decrease it when it's far, but i'm not completely sure if this part can be done just with a simple theremin circuit on the base that cover the beam emitters .....
So, you can do the same thing both with more emitters (turning on them in cycles and reading if there's a reflex when the correspondent module is on), or with a scanner, shifting the beam in a position, reading, shifting again, reading, and so on, in a cycle (this for read the notes, for the intensity, or you use a program that calculate the reflex intensity compared with all the others and with a sample one, or a capacitive sensor like the theremin one)
But still remain the pc interface, that is not so easy to DIY ..... and also worse, if you want to build it non-pc based, like the analog prototypes
:wtf: