A MIDI IN header to assist development of synthesizers and light FX

Overview

As I got deeper into developing MIDI controllers and synths, I realized the need for a robust MIDI IN header. Laying out the standard MIDI IN optocoupler with the supporting components was getting old in a hurry. The feeble nature of the connection of the DIN-5 to a solderless breadboard also had to go.

Thus was born Midler, because she sits in the middle between my controllers and my synths. Not to be confused with MIDI THRU, Midler receives MIDI IN on one side, and serial output on the other.

pencil sketch of enclosure design
size view of midi port on finished enclosure
  • Robust wooden enclosure for a strong DIN-5 connection
  • Standard MIDI IN circuit (6N138 optocoupler, diode, resistors)
  • 3 pin header to the synth
    • Synth GND
    • Synth Vcc (5V)
    • MIDI Data In
  • Power Thru compatibility
    • DIN pins 1 and 3 provide GND and 5V
    • Schottky diode (1N5817) back feed protection
    • DPDT switch for synths that cannot handle the power
  • LED indicators
    • Power Thru power
    • Synth power
    • MIDI activity (with NPN transistor inverter)

Enclosure and Construction

raw block of wood
wood sanded smooth
wood pocket carved on CNC mill

Midler is housed in a wooden enclosure. Started with some leftovers in the scrap bin, milled and sanded square. Routed pockets in top and bottom to form a hollow enclosure held together with wood screws. Holes for switch and LEDs milled in top. Text engraved in the top and side. Stained with Varathane Special Walnut and sealed with Polycrylic. Text painted with Rustoleum Gloss Black Enamel.

stained block of wood
closeup of engraving fail
gcode to adjust coordinates

The motherboard sits on standoffs attached with M3 screws. The screw heads are carefully counter sunk to keep from scratching the table where I do my MIDI development.

Layout and artwork done in Inkscape. Text and artwork gcode generated by F-Engrave. Gcode for pockets and holes generated by Easel.

There really are a lot of details going on in 3D CAD / CAM. Very easy to miss a step, leading to lots of “trick shots”. Much like a golfer who routinely ends up in the sand, you get really good at digging out of holes.

Case study 1 - use the right endmill to carve the pockets. In the CAM software, I specified the 1/4 inch endmill. The carve actually came out like a pocket, but the circuit board doesn’t fit!!! Doh, I used a 1/8 inch endmill. This one had an easy fix, a quick CAM job to run the outline of the pocket with the right endmill in place. LIKE A GLOVE with a nice .75 mm gap.

size up the pocket
pocket fail
pocket win

Case study 2 - the toolchain can be tricky going from layout to design to gcode. If you forget to export .png from Inkscape with “page” selected (rather than default “drawing”), then you fail the margins in F-Engrave. In this case, the margin failure resulted in the text being low and left, and the artwork falling off the edge. Smack forehead. Inspecting the outcome before I carved the holes, I caught the error and manually adjusted the coordinate space to keep the holes lined up. The face plate doesn’t look quite as nice as I wanted, but it is a development tool only, and it serves a good reminder to export whole “page”.

Schematic and Circuit

Eagle .sch schematic file.

midler schematic
solderless breadboard
permanent breadboard