#MIDI Host to TRS A - Novstion Launchpad to Teenage Engineering KOII

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indigo leaf
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I didn’t mention, I am a software engineer by trade (web) so I am interested in learning as well rather than just the utilitarian aspect.

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I’m wondering if there’s a way to actually see activity in my 3.5mm jack - voltage fluctuations or something?

dull path
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@indigo leaf, if you want, you can wire the power from the board's USB jack to power the USB host jack. This is what the Feather RP2040 w/ USB Host board does, along with some power protection circuitry: https://www.adafruit.com/product/5723, but in a pinch I've also just wired up the 5V coming off the board's USB to the USB host jack. It does work, even if it's not especially "legal".

The USB protocol requires a device (your board) to send a "configuration descriptor" to the computer that tells the computer how much power the device needs. By "piggybacking" the Launchpad (or whatever) onto your board, you're trusting your board told the computer it won't draw more than it said. This is pretty safe usually, but for some high-current devices like the LaunchPad X (which doesn't have a low-power "iPad mode" like older LaunchPads), you may need to tweak the USB configuration descriptor sent to the computer. This is not possible yet in CircuitPython. But again, you can just try it, and see if your computer complains. (I would invest in a "USB Power Meter" like this https://amzn.to/3SJjKs4 to see how much power you're drawing)

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As for watching the MIDI signals, you would normally use an oscilloscope or logic analyzer to do that, if you're looking to parse them. The voltage fluctuations are too fast to do it by eye, really. In a pinch, you can hook up an LED + resistor to a MIDI output and watch the LED flicker when you send MIDI.

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Or if you want to parse it without an oscilloscope or logic analyzer, just about any microcontroller can read the 31.25kbps standard serial that MIDI is, as long as you have a proper optoisolated input circuit. The MIDI Friends learn guide doesn't cover MIDI input, but Adafruit sells a MIDI FeatherWing that has a MIDI input with optoisolator https://www.adafruit.com/product/4740. You can also build one pretty quickly out of a 6-pin optiosolator chip and a couple of resistors. You can look at the MIDI FeatherWing for the schematic to crib from.