[sdiy] 12 x N matrix scanning PCB

[David G. Dixon]

Today I completed and tested the final revision of my 12 x N keyboard matrix
scanning circuit, and all works perfectly.  Layout and BOM for a one-sided
PCB measuring 4" x 6" is available upon request -- please email me offline
if interested.

Essentially, it is Ray Wilson's MFOS keyboard scanning circuit, adapted to
keyboards with their matrices arranged by octaves.  This involves swapping
one of the 8-channel muxes for a 16-channel mux, expanding the 6-bit R/2R
ladder to 7 bits, and adding another 3-bit ladder to subtract voltage from
the first to get 12 equal voltage steps per octave rather than 16.  I've
also eliminated one extraneous schmitt trigger (and, thereby, one entire hex
inverter chip), as well as the entire contents of page 3 of the schematic,
thus giving (more typical) 5V gates and triggers.  Finally, I've added a
5-octave selector switch.  I'm using it with a couple of keyboards ripped
out of a crappy old Farfisa organ I found on the curbside during junk day --
a pretty cheap option for controlling an analog synth!