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Thread: Noob Q: T3.6 MIDI Input

  1. #1
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    Noob Q: T3.6 MIDI Input

    Ok, shameless newbie question time again.

    I need DIN MIDI Input for a project (no MIDI Out needed), but I'm focussing mostly on the software side; the electronics is way out of my comfort zone. I'm trying to interpret the schematics but I have no idea how to translate that into my breadboard setup:

    Click image for larger version. 

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    Q1: BoM...

    I assume I need:
    - PC900 Optocoupler
    - 270 Ohm resistor
    - 220 Ohm resistor
    - 0.1uF capacitor
    - anything else?

    Q2: How do I create the 'loop' (?) between pins 1 and 2 of the Optocoupler? What does that look like in real life?

    Q3: What is that bit of circuit coming out of pin 6 of the PC900? Is that a fork going into both Ground and +3.3 V?
    Last edited by buramu; 09-09-2017 at 10:29 AM.

  2. #2
    Senior Member
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    There's a diode in there as well, see below.

    Depending on what your Midi source is up to you may be able to directly connect the Teensy with a slightly different resistive network, depends on how the grounds are setup so if in doubt wire as shown, except you have 3.3V in place of 5V.

    The component forming the loop on pins 1 and 2 is a diode wired in reverse to keep voltages sane.

    on pin 6 there is a Cap and a positive connection. So you power power the device through pin 6 and 5 and read out put on pin 4.

  3. #3
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    Thanks, I think I have all the necessary components now.

    I made a quick pseudo-breadboard drawing. Does this look OK? I'm particularly unsure about the Diode, the Capacitor and the R270. All three look like short circuits to my untrained eyes.

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  4. #4
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    Well R270 can't be a short circuit, it's a resistor (though it is possible to use a resistor whoes value is too low for a given design)

    With the diode it's in backwards, dealing with reverse current flow, and in series with the resistor

    With the cap there is potential for a short there if you put it in backwards, but a cap is physically an open circuit, being two plates with an insulator between them.

    Would suggest that you may want to look at
    https://en.wikipedia.org/wiki/Ohm%27s_law
    and related pages for how resistors capacitors and diodes work at least in a DC (steady state) enviroment. So for example check what voltage your midi device produces, subtract the diode drop from your part data sheet and you can check how much current will flow through that resistor (and tell if it's low enough to be safe and high enough to turn on the isolator).

    Setting up parts of your design in something like
    https://www.circuitlab.com
    May also be helpful to sanity check things. If anything you build in the sim draws more than 100mA it's a sign that you either have a design problem, or have missed some element of the simulation.

  5. #5
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    Thanks a lot for the explanation and links. I have my setup working now - and I learned a thing or two in the process!

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