You could use mBlock to program an Arduino that examines an LDR
looking at switch. Think a simple bar with an LDR/switch spaced
same distance as switches on mixer.
Arduino looks at each LDR, measures it, and sounds a unique tone note
for each switch detected on. If you dont want unique tone/switch thats
easy to change.
I added a cal routine for ambient light levels, and additional LDR for that.
So I set a switch LDR must exceed the ambient light level by 50 (thats
1.25 V in the A/D reading on Arduino). You would have to "play" with
this value (the "50") to get it right for the specific LDR and light coupling
to LDR bar.
Written in mBlock, where block like objects dragged into design window
and configured. When you finish and hit the upload button mBlock generates
the Arduino code from the block layout and config and programs the Arduino.
This creates a stand alone solution, run off a battery, walwart, whatever.
I would use an Arduino Nano, thats ~ $3, mBlock is free.
Regards, Dana.