To comment on what was said above. You can theoretically skip the optocoupler, yes, but preferably not using pins 4 and 5. Pins 4 and 5 are connected to +5V and the signal, respectively. You would have to ideally connect pin 2 to ground, and pin 5 to the input. With this way you probably need to add a pull-up resistor to guarantee that it would work with all MIDI sources. But doing this risks introducing hum since you now have a ground loop. This is what the MIDI standard originally set out to eliminate.

Yah, made the guy send notes. It's so awesome! Especially when you assign some real instruments to your track - orchestral and real drums playing 8bit song I nearly laughed my head off

From what I read on the arduinoboy page sending notes requires some special version of LSDJ that has MIDIOUT sync mode?

no

Ok reassembled everything using one of my UNOs (put the allegedly broken pro mini aside) and it seems to be working. I mean I see it sends ticks when it is supposed to, but how do I send notes? Anyways I'll carry on with googling and trial and erroring, just letting you know that the thing full worked finally (cause I'm so sure everybody cares lol).

PS
I noticed one weird thing though. From what I see on my breadboard (and the thing works, so I think it was assembled correctly) Trash's schematic shows MIDI IN jack from the front side (the side where you plug in the MIDI cable) while the MIDI OUT jack from behind (where the connectors to solder to are). Is that correct or have I gone mental (finally)?

now it's bad advice coupled with misinformation.

Not really. Optos are not necessary, but only suggested. They are nothing more than a small fuse-type of device in between the sender and receiver in case the sender is bunk. Yes, it would be stupid to go without in commercial device, but we can assume FoM isn't going to hook up his arduino to anything other than his MIDI keyboard and/or his PC. Besides, I mentioned it only for testing purposes.

I can only presume that I've accidentally burned the TX pin, but how...

Oh bad luck won't leave me So I assembled MIDI OUT and tried it with LSDJ and Arduino stopped working. Reseting doesn't help and I can't load the sketch anymore it says stk500_recv(): programmer is not responding

this is bad advice

Never mind. I didn't see the second page.

If you trust the wiring of pin 4 and 5 of your midi keyabord, you can skip the opto altogether and connect the keyboard to the arduino.

They are all absolutely identical (sorry I don't have any camera atm not even phone). I examined them all under the magnifying glass to spot minor differences in prints, but no - all the same. The print is:

F*
6N138
1012T1

*Fairchild Semiconductor logo

It's actually not a print but sort of engraving. I bought them a while ago from this chinese store (http://www.ebay.com/itm/10-PCS-6N138-DI … 1351707421). Now I see that the couplers I have are not the same as on the picture (picture has prints as opposed to engraving). So it was probably the lowest quality factory with 70% of faulty chips

Could you arrange them by which ones work and which don't and take a picture? Or describe what's printed on each.

Final verdict - 4 of 6 optos I have DO NOT work. Two do. I thought I tried them all but I somehow missed these two. Mystery solved. Now I'm gonna finish MIDI OUT part and then finally get to soldering the stuff to the perfboard, preparing the case and stuffing everything inside. My head is in once piece this time.