adm3a numpad recreation (and bonus usb numpad)

Having an adm3a is cool, but getting the options installed is very tempting. Everybody gets the lower case font upgrade, but how may people have the numpad? I have seen pictures of exactly two in existence, the first is the picture from wikipedia (reproduced below) and the second is from the manual. In the era of 3d printing it should be possible to make an equivalent with modern keyboard components, but that wouldn’t feel quite right. It so happens I stumbled upon an authentic stackpole numpad mechanism on electronics goldmine, or all electronics, or one of those surplus sites. I grabbed a couple because I didn’t know when I’d ever run across another one.

For this application I didn’t really need a PCB, since there’s no components in the numpad, just switches wired up in a matrix. The same matrix as on the keyboard. That’s right, this numpad didn’t expand the keyboard matrix, it lies parallel to it. The terminal has no idea if a key is being hit on the keyboard or the numpad, meaning all modifier keys wok the same with the numpad. Modern keyboards send different scancodes, but this one acts like if you hit shift-num5 you’ll get the ‘%’ sign. Go ahead and try it on your keyboard and I’ll bet you don’t.

The connector on the board for the numpad is J6, see how it has pin numbers that are a subset of all available rows and columns?

With this setup how will I go about making an expansion connector on my PCB? do I pin things out to an IDC header? make it easy to wire up a crimped ribbon cable?

No, why would I do that? I only need to make one of these. Well, I made two and gave one away, but you get my point. The PCB isn’t really for this purpose anyway, I decided that if I had some spares of these mechanism that I could make a QMK capable numpad out of one. That just means learning some config file magic that I’ve already forgotten and soldering up a PCB full of anti-ghosting diodes. In the end I think it came out pretty well.

Now that’s a little different than the numpad above. It turns out my mechanism has an extra set of keys, and I’m not convinced the real one didn’t have the same extra set of keys hidden under there. Take a look at all that empty space above the top row, other than style reasons it might be covering up some extra switches.

In any case I have 2 variations of the top shell and two variations of the bottom. With and without the extra row, and with and without a cutout for usb type c.

I think it works rather well with a USB port out the back. The ribbon cable I ran through a slit I carved where the clamshell case meets. The bottom is fairly sparse, and I took the opportunity to stick on some rubber feet.

All the part files and stl files are up on my github, thanks so much to Brandon for modeling them for me, one day I’ll re-learn solidworks enough to do things myself. I also have my PCB design, but I doubt anyone will come across more keyswitch mechanisms. The QMK numpad config files may be more useful to people. If someone does come across more mechanisms I think I have one spare PCB.