A 1x11 MIDI Thru box that optionally provides 5 VDC on unused MIDI pins


Grumpenspiel has one MIDI controller generating the messages and lots of synths generating noise and lights. Schmidt Trigger Hex Inverters split the MIDI IN signal and provide 11 clean outputs. An optocoupler keeps the Power Thru electrically isolated from the controller.

Since most Grumpenspiel components are low current 5V devices, Power Thru can provide power/gnd on two unused MIDI pins. Each output has a power switch, in case a standard MIDI device is connected that cannot handle power on those pins. Doing it this way reduces the number of connectors and wires needed for the majority of Grumpenspiel, saving more complex setups for only the most power hungry parts.


Power Thru is inspired by Morocco Dave’s MIDI Thru Box V2. He made a great project and a fantastic write up. Helped quite a bit to learn from his lessons.


  • Standard MIDI IN Optocoupler circuit
  • MIDI activity LED indicator
  • Two power circuits, each with:
    • DC input jack
    • Power indicator LED
    • Bulk Capacitor
  • Hex Schmitt-Trigger Inverter (SN74AC14)
  • The first inverter receives MIDI IN signal and the output is fed to the remaining 11 inverters.
  • Outputs are fed to 11 MIDI OUT ports.
  • Each MIDI OUT port provides:
    • Standard MIDI Out on pins 2 (gnd), 4 (+5V), and 5 (TX)
    • Non-Standard, special for Power Thru, power is supplied through pins 1 (+5 VDC) and 3 (GND)
    • A DPDT switch on each port can disconnect pins 1 and 3 in the event the MIDI synth on that port does not like power over MIDI cable.
    • LED indicator for the power switch.


Eagle .sch schematic file is available in the GitHub repository.

Image of schematic coming “soon”


Wood and paper