So i have all the parts for my arduinoboy except for the 270ohm resistor (backorder). My question is this: can you swap that out for a lower/higher ohm resistor like another 220 (which i have a ton of) or will this just screw up how the arduino reads the signal?
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clovis CA
funny shit, that what happend to me too! haha, what i did is get a 220 resister and a 47 ohm resistor and just link them together =]
worked for me =]
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matt's mind
If = 12mA
Rl - 270ohm
Vcc is 5V.
its the limiting resistor. from the 6n138/6n139 datasheets.
there was a handy website i came across that had a table for the different optocouplers and the limiting resistors needed (and an explanation why), but i can't find it at all now 
edit - with the x0xb0x i remember this resistor was suggested to be changed if MIDI wan't working right. this was so long ago, i don't even remember the opto it uses... but, it should be able to be done. 6n138 is super common, there are a lot of alternate wirings available online but the suggested and most common all use 270ohm for the limiting resistor (to compliment the MIDI requirements i suppose)
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Rhode Island
Hah yea I should have remembered I could throw some in series or parallel to get the right resistance. I had been looking at some other schematics for midi in and they had used 280ohm. Wasn't too sure on the amperage so it's why I asked here.
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Sweeeeeeden
kitsch: Actually, you've misread what he said. He said he had 220 ohms but not 270 ohms. 220 ohms is used as the limiting resistor while 270 ohms is used on the secondary side as a pullup. I once played around a lot with that to find that adding a 1.2k (IIRC) resistor from pin 7 (the unconnected pin) to ground could improve signal quality when stealing power from over the link cable. However, I didn't document the tolerances for the 270 ohm resistor. My bet is that 220 ohms will work OK.
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Rhode Island
Ah great, thanks. If I can find a 40-50ohm resistor I'll plop it in series otherwise I'll just go ahead with the 220. Thanks nitro
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Sweeeeeeden
Actually, I realize the extra resistor was a 4.7k, for those who might be in need of that fix.
http://blog.gg8.se/wordpress/2009/07/26 … -glitches/
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Rhode Island
Ya 220 works just fine. Thanks for the info on improved signal quality. I will surely be using this