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Concertina progress.
It's not pretty, but I now have:

A simple circuit set up on a breadboard to decode three output lines from the Arduino into one of 8 column select lines.

A piece of cardboard with four 1/4" holes drilled in it to form an approximate rectangle 0.4"x0.5" on centers. Each hole has a pushbutton switch mounted in it, two red, two black. The two red switches are each connected via a 1N4148 diode to a row line, as are the two black switches. The left red/black pair are connected to a column line, as are the right red/black pair. The four row and column lines have leads I can connect to the breadboard and hence to the Arduino.

Software on the Arduino which, given N rows and M (<=8) columns, polls NxM switches and reports over the serial line any changes.

So before I put everything away last night, I was able to look at something vaguely resembling a small portion of a Concertina button pad, push buttons on it, and see on my computer screen "Note 1 On, Note 2 On, Note 1 Off, Note 3 On, Note 2 Off, Note 3 Off" that corresponded in time to when I was actually pressing the buttons.

As a proof of concept, it shows that I can build a multi-press matrix keyboard and read it successfully and that the switches I'm trying will fit the spacing I want.

I am questioning the physical layout of the device, though.

The switches I have are nice for this task: They have somewhere around a 1/16" to an 1/8" of travel, they are silent, they require a decent, but not overly high amount of force to push, and they have no "break" in their action. They feel similar to actual concertina buttons. They panel-mount in 1/4" holes and will work on 2/5" centers.

But they don't seem to lend themselves to PCB-mount. The leads are flat, not round, and would require elongated holes in a PCB. They also have a hole big enough to feed a 22ga wire through. If panel-mounted, they don't need the PCB for physical support. I'm tempted to mount them to the "panel", and solder the rows and columns together in free-space. It would save considerably on PCB-expenses (I'm thinking of paying $5/sq.in, the button-pads are about 5.4sq.in each), but would look somewhat messy (where no one would see it on the finished work, unless I showed it off) and make me fearful of things moving and touching. All the negatives seem to be silly concerns.

I need to pick up a wire stripper.

It seems I can start on some physical layout. I can make full-size drawings of the button-pads, reversed, and use them as templates for drilling the mounting holes. I can also think about other physical layout issues: I was originally planning on putting headers onto the MIDI board and matching headers on the PCB I'm designing to hold the electronics. That way, I'd get a stack: the Arduino at the bottom, my board in the middle, and the MIDI board on top. However, I'm only using 7 out of 20 connections to the MIDI board and it's got 4 stand-off holes already. I could solder 22ga leads to the 7 connections I'm using and mount that board along-side the Arduino rather than on top of it. That would free up a lot of space on my board and perhaps simplify the layout. I'm also thinking of doing something similar with the headphone jack (the one I got does not appear to be easily board-mounted).

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A wire stripper won't work on rectangular wires. Frankly, I almost never manage to strip wires with a wire stripper without damaging the conductor. I've always had much better results cutting the insulation with a knife, razor blade, or even a pair of wire cutters, and then pulling the unwanted bit of insulation off with my teeth.

What are you going to enclose your concertina in? Is it going to look at least vaguely like the original "squeezebox"?

Fortunately, I'm not working with rectangular wires. I am currently trying to strip the 22ga hookup wire I'm using with dykes, but it's either really difficult to get the entire insulation cut (so that it will pull off without having to stretch and break the insulation) or I end up nicking the wire so much the wire itself breaks.

At this point, I'm thinking of a wooden hexagonal box, made by making 2 hexagonal (7" across the diagonal) end-panels of thin (1/8" max) wood with button-holes in it, putting 6 1/2in -square trim strips along the inside edge of each to hold 6 side-panels each about 3.5"x5" in size. One of the side-panels will have mounted (inside the box) the Arduino assembly, and an adjacent side-panel will be removable to gain access to the Arduino for reprogramming and battery replacement.

Go to an arts-and-crafts store such as Michael's or A.C. Moore. They have hexagonal boxes made out of thin, sturdy wood (sanded smooth but unfinished) in various sizes. You might want to buy two boxes, cut the bottom out, and use the two lids for a more authentic look.

That's a very interesting idea. I'll explore it. It sounds like access to the innards might be a bit tricky, but perhaps not more tricky than my plan. Also, cutting the bottom off might reduce the structural capability of the hex box.

The main thing I'm thinking of is the lids of the boxes, for their appearance. Just go take a look at what boxes are available in the crafts store - I'm sure you'll see ways to use them. (Although I think you're going to simply have to paint lines suggesting pleated leather on the sides...)

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