a new amiga midi interface...

[motosega]

i've making a midi interface for the Amiga, nothing revolutionary or spectacular, but cheap and easy to diy with parts that you can find anywhere.

an amiga midi interface is nothing special, it just needs to convert the 12v rs232 levels to 5v and put an opto isolator on the input.

there are several designs floating about online, some use hard to get opto isolators, random transistors that a guy had in his cupboard, etc.
but some are fairly dodgy and one even drives the midi output at 12v levels! (it probably works fine, but has the potential to destroy anything that doesn't have a properly isolated input)

so since i have to make two of these, instead of just doing what i normally do: "knock something up on a protoboard and forget how it works afterwards" i've decided that this is a good opportunity to learn kicad, get some boards made by one of these new cheap pcb manufacturers. and let the design files float around on the internet for future generations.


i'd appreciate it if any of the pro hardware people here could point out any terrible mistakes before its too late....


i decided to use a max232 for level conversion, it's the easiest way, and its cheap and available. i never used one before though so i'm using the example circuit direct from the datasheet.

are those two 1uf caps going to be ok on the 7805 ? or should i add an electrolytic too?
the midi part is fairly standard, the 6n138 opto isolator has served me well in other midi projects.

i decided to put the midi connector and the amiga serial port on pin headers, but maybe i could have both pin headers and footprints for pcb mount connectors.

[Daedalus]

Generally when using 78xx or 79xx regulators, I use a much larger electrolytic on the output, say 22uF. The datasheets generally have a small capacitor on the input, 220nF or so. But in reality, when the power rails are already relatively clean, pretty much anything will do. If you use one of the common pad spacings (say 2.54mm for through hole), you can always change them later if you find you have noise.

Also, I haven't checked the MAX232 datasheets in many years, but with a lot of logic you shouldn't leave inputs floating. It's worth checking that out, as I vaguely remember something about those chips latching to the rails in some situations.

[motosega]

Thanks,

I used the same 1uf caps that i needed for the max232 for the 7805, i should specify better caps but add a note to say that probably anything will do.

As for the floating pins on the max232, i'm pretty sure that i read in the datasheet that the inputs have internal pullups. i'll check.

my idea with this board is that it should be cheaper than a second hand midi interface from ebay(currently about 30€), and simple enough for a first soldering project. Any advice for idiot proofing the pcb design?

[Daedalus]

As I said, it's a long time since I checked the data sheets, so you could well be right about the floating pins.

I can't think of much in the way of advice, it's a relatively simple board so should be a good starter project as you say. Just the main stuff like putting a decent bit of space between parts and making sure the silkscreen is still visible after components have been fitted, so that orientation etc. can still be checked. They're all standard parts so the KiCAD footprints should do just fine. Also, make sure you use thermals if you're doing a ground plane so that the ground connections aren't too difficult to solder.

[Nosferax]

Is there any progress on this project? I'd be interested in building one of my own when it's ready.

[Sinphaltimus]

^^^DITTO^^^^ what Nosferax said.

[AwoLStill]

DITTO To what Nosferax and Sinphaltimus said,

The advice that Daedulus gave you to use KiCAD, combined with the fact that you can get professional quality PCBs made a low cost, should idiot-proof the PCB design.

If you decide to get some PCBs done, I would have one.

[pandy71]

Originally Posted by motosega

are those two 1uf caps going to be ok on the 7805 ? or should i add an electrolytic too?

You already provide to 7805 stabilized 12V so IMHO all caps you need are decoupling caps (to prevent 7805 self oscillation) but nowadays caps are cheap then if you not intent to produce mass multimillion scale converter you can safely add some additional caps.