MIDI question

Hello. I'm building a MIDI interface...

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....and I'm planning on using off-the-shelf Radio Shack components to build my own opto-isolator. Does anyone know of any reason why I should not?

Reply to
MikeMandaville
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You will certainly learn more by creating your own parts. But what you learn may be that you should have bought the specified parts, to start with. ;-)

What Radio Shack components do you intend to try as substitutes? You can make a pretty fair opto isolator with the emitter and detector pair from an old mouse. Getting them well coupled is half the battle.

--
John Popelish
Reply to
John Popelish

See

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I built that MIDI pipe organ about 15 years ago. I used 6N137s for the isolators. I guarantee you'll have your hands full sorting out all the MIDI stuff without having to putz around with possible faults from a poor opto-isolator GG

Reply to
Glenn Gundlach

An opto-isolator costs about a buck. How much time and effort are you going to expend to save a buck? For midi you need to pay attention to a couple of things: First, the isolator has to operate on 5mA, many are not reliable with that low current. Secondly you have to insure an adequate rise and fall time for the 31Kb/s signal. Many are too slow and will round the waveform especially at low current. A part often used for midi is the 6N138. It has the right parameters and is cheap. Bob

Reply to
Bob Eldred

It's been years since I've seen any MIDI construction articles, but back then they all tended to specify a specific opto-isolator, with comments about how it needed certain specs to match the MIDI specs.

I have no idea how critical it was, and indeed there were construction articles that used average opto-isolators.

But in reading the original post, I just assumed he was talking not of building an opto-isolator, but in using whatever Radio Shack still stocked rather than tracking down the one that was commonly specified.

Michael

Reply to
Michael Black

Hello again, fellows.

John, the specified parts are no longer available. Radio Shack sells a "Matched Infrared Emitter and Phototransistor Detector" pair for $3.29, Catalog #: 276-142, and this is what I will be using. I found out firsthand about the IR pairs in my mouse about a year ago, when I dropped my mouse one too many times, and it finally broke. When I opened it up, to see if I could fix it, and I found two IR pairs, one for each axis. I never did fix that mouse, so I still have those components, and the main advantage I see in using them would be that the LED's are so tiny that there seems to be little doubt that I could drive them from my computer's JOYSTICK/MIDI port, which is where my box will be hooked up. The reason why I have decided to work with the larger components is simply because they will be easier to handle. :-)

Reply to
MikeMandaville

Hello, Ken.

Radio Shack no longer sells the TLP620

Reply to
MikeMandaville

Hello, Michael.

Ken was right in the sense that I am planning on building my own opto-isolator. Forrest Mims shows how to do this in his "Engineer's Mini-Notebook" entitled "Optoelectronics Circuits", which is no longer available. Mims suggests using heat shrink tubing, though I myself prefer several layers of electrical tape. I've made dozens of them before, and they always worked just fine.

Reply to
MikeMandaville

Hello, Glenn.

I confess to being awestruck by your organ project. I have thought about doing something similar myself, though I don't yet have the resources. I've had my music keyboard hooked up to my computer for about two weeks now, which is nice, because now when I play midi files, I use my computer as a music keyboard controller, and since my music keyboard is also hooked up to a little ten watt amplifier, I get a better sound than I used to when using my computer speakers. Even so, there is nothing like the sound of a genuine acoustical instrument, and your organ is really beautiful.

Reply to
MikeMandaville

Why would you do that? Radioshack sell a perfectly adequate TLP620, which will at least have known characteristics. Unless of course the object of your exercise is to learn about making optocouplers rather than MIDI systems.....

Cheers.

Ken

Reply to
Ken Taylor

You can get a 6N138 from Mouser or Digikey for < $1.00 one-off.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

Hello, Clifford.

Though there is little doubt in my mind that I could probably drive my homebrewed opto-isolator to more than twenty times the bits-per-second speed which is required by the MIDI specification, an opto-isolator is, of course, an analog device, and so it will be necessary for me to convert its analog output into a digital signal before that signal will then be "fast enough" for me to be able use it. Evidently, the Sharp PC-900 opto-isolator which is in the MIDI specification had its own on-chip Schmitt trigger to accomplish this purpose. The Schmitt trigger is a simple circuit which I will be able to build from locally purchased discrete components, and so there is no reason why my homebrewed opto-isolator should not be just as good as the one which is shown in the specification.

Reply to
MikeMandaville

Hello, Bob.

The main reason why I will be building my own opto-isolator, rather than sending away for one, is because I would rather not wait for shipping. You have said that the isolator has to operate on five milliamps. Can you give me a reference for this? I will build a driver, if I need to. I still don't see any problem. The Sharp 6N138 doesn't seem to have the right parameters. According to Digikey, it is supposed to operate on one-and-one-half volts. And none of the 6N138's that I have seen have a Schmitt trigger. Mine should be faster.

Reply to
MikeMandaville

which

That makes more sense. Sorry, a dim day. Still in New Year's mode (usually only lasts till October......)

Ken

Reply to
Ken Taylor

Digikey still has them for $0.72.

There are also many other units they sell that are equal or better.

--
John Popelish
Reply to
John Popelish

The reason the 6N138 was commonly preferred was that the cheaper alternatives were too slow to do MIDI. If you make your own, make sure it's fast enough.

Reply to
Clifford Heath

Well, after doing a little searching on the internet, I discovered that a two-lead phototransistor is not as fast as I thought it was. Evidently, it takes about ten microseconds to turn off. This still gives me plenty of margin. Just not as much as I thought I had.

Reply to
MikeMandaville

If I was building one, I'd use whatever opto-isolator I could easily get before I'd start messing with making one. That scrap computer power supply lying on the sidewalk should offer up an opto-isolator, as would that scrap modem that nobody wants because it's 1200baud. They may not be the best choice for the fast rate of MIDI, but I suspect they'd work better than someone making an opto-isolator.

Michael

Reply to
Michael Black

Where do you live that shipping is an issue? As one poster said, your going to need a lot of information about MIDI to be successful. A good starting point is the book, The MIDI Manual by Huber published by Sams. The five milliamp current loop is discussed in there and is in the MIDI spec. If you don't design to that no other MIDI device will be able to talk to you, not a good start. Also go to the MIDI web site, I don't have the URL, and down load information and specs. It's very useful. Yes, the 6N138 is not a schmitt trigger device, just a darington but it does have the current and speed required. If you use it be sure to bypass the second base with a 10K resistor to ground or it won't be fast enough. There is a lot to MIDI and you will need to have some way of processing the information, a micro processor or something. What is you plan? Actually the opto-isolator, though required, is almost incidental and is childs play compared to other MIDI issues, as you will find out. Good luck. Bob

Reply to
Bob Eldred

Ten microseconds is one-third of a bit time. I'd hardly call that plenty. Just go and buy a 6N138 and stop bugging us with your crazy notions.

Reply to
Clifford Heath

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