MIDI input/output status LEDs

[belzrebuth]

Hi all,
I was wondering how to add midi input and midi output indicator LEDs to a project very similar to this one here:
https://www.usb-audio.com/gm5board/gm5_midi_public.pdf

From what I understand I can't just tap in the optoisolator's current loop so I'd probably need something that its input accepts both the bipolar MIDI signal and is high impedance too so as not to disturb the circuit.
Some current output capability (about 20mA max) will be needed to be able to drive the leds without problem.

A mosfet comes to mind but I imagine it would take more calculations than a digital device to make it work.

I think it's common to use hex inverters or nand gates for this kind of job but I'm clueless on what might be the ideal circuit for this.

There is a discussion here about it:
http://midibox.org/forums/topic/17046-leds-on-midi-inout-on-the-core32/
But this thread doesn't actually concludes to a solid circuit.

What might be the ideal approach to this?
Thanks:)

[Siwastaja]

Ideal approach:

Because MIDI is a digital communication bus, it always connects to a microprocessor of some sort.

Use that processor to drive an LED, using any IO pin.

This way you can easily do things like stretch pulses, meaning you can turn the LED on for, say, 100ms when you send a single packet, so someone can actually see the LED. This is not the case if you just drive the LED with the data signals.

[belzrebuth]

Unfortunately I'm locked out of the MCU since I'm using the GM5 chip..
I can only do things with the data signals.

I've also found this

Which looks pretty simple in theory but would indeed make the LEDs blink too fast I imagine.
I suppose I could try that if there's not any suggestion..

[ratatax]

Maybe just a small capacitor will keep the LED up long enough ?
Or drive a transistor with a capacitor and resistor on its gate to keep it up without having to provide much current directly

[Renate]

The problem with blinky LEDs is that even a simple circuit would practically be more circuitry than the MIDI router itself.
Did you want an LED for each input and each output of the matrix?

I presume that this device uses an on-screen display to show you what the state of the crosspoints are.
Would on-screen blinkies be sufficient?
You could add another HID interface to the USB device and spit out status that way.

You could patch the software, pick some unused uP pins and do the LEDs that way.

You're kind of stuck between a screen-centric vs. box-centric device.
You could go all the other way and build something with 25 LEDs for the matrix + 5 input LEDs + 5 output LEDs + a bunch of pushbuttons.

[oPossum]

A 74HCT123 can provide a low load logic level input that can be connected to the MIDI I/O pins of the GM5 and provide a pulse stretched output that can drive a LED. An LMC556 could also be used, but I think the parts count would be higher.

[Renate]

A CMOS 74HC14 hex Schmitt trigger will get you 3 LEDs using two inverters each.

[belzrebuth]

Yep; that's what I did use in the end
Seem to work fine so far..