MIDI+Expression breakout board Build Instructions

This little board adds MIDI in and out via 3.5mm (1/8“) TRS connectors, along with an expression pedal input. It's all simply connected to the pi-Stomp Core board with a 5 conductor ribbon cable.

TRS connectors were chosen because they have a much smaller footprint, are easier to plug/unplug and seem to be the future of serial MIDI.

You can use simple TRS to DIN adapter cables for connecting to equipment with the classic 5-pin DIN connectors.

Just make sure they are listed as “Type A” which have the more common connection scheme than “Type B”. You can also build your own.

This should be a very straight forward build. One could likely assemble without instructions, but it's still a good idea to view the photos and assemble in the approximate order shown. At least note that the Diode and IC are polarized.

Bill of Materials

ID Description/Value
R1-R4 220 Ohm Resistors (1/6W 3mm)
R5 33 Ohm Resistor (1/6W 3mm)
D2 1N4148 Diode (there is no D1)
MIn 3.5mm (1/8”) TRS Jack
MOut 3.5mm (1/8“) TRS Jack
Exp 6.3mm (1/4”) TRS Jack
S1 8-pin DIP Socket
U1 6N137 or 6N138 OptoIsolator

  • Bend the legs of the R1 thru R4 (220 Ohm) and insert into the side of the board with the corresponding silkscreen label
  • Splay leads to hold in place

  • Solder
  • Repeat for R5 (33 Ohm)

The diode footprint is wider so bend the legs a few mm from the diode body

It's polarized. The Black band on the diode should correspond to the band on the board silkscreen.

The notch on the socket end should be oriented to match the board silkscreen.

The IC is polarized. The dot on the IC which identifies pin 1, should correspond to the notch on the socket and board silkscreen.

Since the connections to the pi-Stomp board are by a male header, you'll typically want to connect it with female jumpers. You could install a male header onto the MIDI board then attach with female-female headers, or use a jumper cut to the length you desire and solder that to the MIDI board.

  • Separate the 5 conductors (colors don't matter)
  • Strip and “tin” each conductor

  • Insert all 5 conductors into the board and hold in place
  • Solder

All Done

Installed into a 1590J enclosure. For mounting the Expression jack, it's generally best to have one washer on the inside, one on the outside along with the beveled washer and threaded shank.

“-”, “+”, Rx, Tx are connected to the equivalent pins on the pi-Stomp board and “Exp” to Analog input 7.

For the Expression pedal, an “adc_input” definition should be added to the “analog_controllers” section of ~/data/config/default_config.yml. Feel free to chose another midi_CC msg number (0 trhu 127)

  - adc_input: 7
    midi_CC: 77
    type: EXPRESSION
  • midi_expression_breakout_board_build_instructions.txt
  • Last modified: 2024/01/26 05:45
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