Diodes for dummies

Just to know, what does D1 diode across midi input actually do? And the real question is could you swap in an LED there and make it a MIDI activity. Indicator?


  • It's for reverse polarity protection and normally does not conduct (so it wouldn't be a good indicator).

    A better indicator would be on the other side of the opto, and you'd probably need to do some pulse stretching too.  Your MIDI messages are pretty short by human eye standards so unless you were getting a lot of them you might not notice.
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