Midi IN fot Audio KIT v2.2

[mattia714]

How can I build a MIDI IN interface for Audio KIT v2.2 (es8388).
I would like to build the drum computer, but I don't know how to realize the hw part.

[MichaelPNolan]

So first you need an optocoupler ... and then I can help you find a circuit.
There are 8 pin DIP type chips ... and there are 4 pin common ones. I've gotten it to work with both types,
The cheaper 4 pin type you might have in some kit works at 5v but you can use a resistor divider to set it to 3.3v or thereabouts. 6N138 is a good one that will operate at 3.3v and then you simply take the output which is serial and use one pin on the ESP32 Audio Kit ... i've made 2 of those. The older and newer one with es8388 work.

http://auspyro.com/wordpress/2017/12/21/stm32-based-midi-synth/

This shows some basic circuits for 5v but i used this
https://arduino.stackexchange.com/questions/56279/how-do-i-properly-receive-midi-with-arduino-with-6n138
I made some resistors (r1, r2) a lower value (50%) and used Vcc 3.3v
R3 can be 220 ohm or 330 ohm
You don't really have to connect the 3rd pin of the 5pin connector to earth. Some circuits are more elaborate and use a jack connector instead of a 5pin connector. You can get 5pin to stereo jack connectors for midi. You can also just have terminal blocks and break it out to those 2 inputs that drive the optocouple LED.

[mattia714]

Thanks a lot for the answer.
Just another thing: which pin I have to use?
The one marked as RX0 (see attached pic)

?

[MichaelPNolan]

TX0 and RXO are also used when you are loading the software - in the code you can find a #define constant
MIDI_RX_PIN and define it somewhere like in config.h that is part of most of Marcus projects.
you can use probably any of the GPIO pins like 21,22,19,23,18,5
I've used 21 - so long as you aren't pulling the midi signal higher than 3.3v because that is the voltage this board works on.

[marcel-licence]

I tried to create little schematics of the MIDI interface: https://github.com/marcel-licence/ML_SynthTools/blob/main/extras/midi_input.md