Wednesday, 14 January 2015

Rewiring a keyboard matrix

New year, new old projects. I'll go back to one of my past favorites, a keyboard. I'm rewiring an Apple keyboard into a different electrical matrix, removing all the ICs and stuff. There are some plans for this keyboard but let's focus on rewiring first.

I've lost the picture of the intact keyboard.
This Apple Extended keyboard is a good starting point because it has a switch for each individual key. If nothing else, tearing out all the keys leaves you with a valuable stash of switches.

I was thinking of keeping the arrow keys, but no, too big, too heavy.
After hacking the full keyboard to pieces (with a hacksaw), I'm left with a more limited "8-bit" keyboard. I left out the keypad, arrow keys and function keys, and still I'm left with a fairly heavy lump of keys.

It's possible to tear out individual keys, too.
Then, I cut the electrical connections on the circuit board with a Dremel wheel. I have previously done this with a carpet knife, which produces less dust and noise. But the Dremel saves time. 

Lisää kuvateksti
The above shows it's easy to re-solder the switches, but it is very time consuming. Here I have to hope there are no connections on the other side of the board, but from what I can see it is very unlikely.

I re-wired the switches according to a ZX Spectrum keyboard matrix, using tiny pieces of wires. I'm not going to connect this keyboard to a Spectrum (unless for testing), but it's nice to have some "standard" to work on. 

Halfway there...
Of course, the 5x8 matrix is not sufficient for connecting all the keys, it will have to wait.

I got tired soldering the matrix with small bits of wire, and with the other direction I tried an alternative approach (i.e. an even more horrible kludge). The red wires are longer, and only parts of the wire are exposed where needed. I did this by brutally melting through the plastic coating on the wire, so it's not certain the connections are very good.

Almost there... note that some keys like Space and Enter are further away than they would be on the Speccy.

And there I have it, an old Apple keyboard re-wired into a ZX Spectrum keyboard matrix, although without any lines leading out... yet. So there's still a significant amount of work to do.

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