Pretty straightforward build. 4 multiplexer output pins into the Analog in ports of the Arduino 1,2,3 and 4.
Pitfalls: If you have an Arduino Leonardo: use Serial1.print to write to the TX on the 2nd pin for MIDI. The normal Serial.print writes to the USB TX. (very useful for debugging)
Pitfalls: I have a significant voltage drop therefore I use the first pin on the first multiplexer to read the analog value and use it as a reference when mapping.
ex: midi_value = map(analogValue, 0, myREF, 0, 127);
myRef is read in the setup() function from 4051 chip 0, address 0.
And the final “product”.