$5 USB MIDI adapter with ATmega32u4

This article will detail how to build a USB MIDI adapter (one-directional: you connect the adapter with USB cable to your computer, and it receives notes and pedal data from your keyboard’s MIDI OUT and transmits them to your computer) with ATmega32u4, 6N137 optocoupler, a few resistors and spare MIDI connector or cable. Read on for details!

Preamble

Last year I wrote how you can turn Teensy LC into an inexpensive USB MIDI adapter. I used it to replace a non-working Chinese MIDI-USB adapter that did not send controller messages (i.e. piano pedal) properly to PC.

However, since the Teensy still costs $12, and you have to get some additional components, it doesn’t make sense to use it in a dedicated MIDI adapter, since you can get a working USB MIDI adapter for around $15 from Thomann and probably many other places.

But how about other boards? After making my Teensy LC adapter, I actually tinkered with Adafruit Pro Trinket, and got that working as well (the standard Trinket does not have hardware UART so V-USB and software UART is somewhat risky). But there is even a better board for this: The ATmega32u4 board I previously showed how it can be made into USB HID mouse. Let’s see how to turn it into a USB MIDI adapter!

Required hardware

To build this, you will need roughly $5.40 worth of components:

  • SparkFun Pro Micro (clones with ATmega32u4 can be had for $3.50 from AliExpress, eBay, etc.)
  • 6N137 optocoupler, which can be had for ~$0.30 a piece (I suggest sourcing a couple in case you burn one by accident)
  • 330 ohm and 2 kohm resistors ($0.10) and 1N4148 diode if you want to be safe
  • MIDI connector or alternatively you can just cut a short cheap MIDI cable and wire it to your project, let’s say $1.50

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