Keyboard Mod: Salvaging Keypad

I left my cheapo keyboard in the car, and the ABS plastic warped, contorting it like a potato chip. After some key sticking ugliness with my regular (great cheap) keyboard, I started using a BTC board that feels like a laptop keyboard. It's nice, but, lacks a keypad. I thought, maybe, I could use the keypad on the warped keyboard.

The keyboard is wired as a kind of grid, with a dimension of 14 by 12 wires, and each key is a switch between two wires. A squarish grid is used because it maximises the number of switches for the number of wires. Each key press is detected by seeing which two wires are connected. This detection is done by "scanning" the grid, testing each pair of wires by putting a voltage on the wire in one axis, and seeing where the voltage appears along the other axis. The physical keyboard isn't 14 x 12; it's 6 by around 23 keys... but the underlying wiring is the grid circuit adapted to fit under the keys.


This "illustration" shows how a grid might be mapped to the physical keys. The actual keyboard is more complex.

The mod consisted of taking the clamshell apart, then mapping out the wiring of the keys on the keypad. You take it all apart, and then use a continuity tester to see which terminals map to which keys.

Most of the pins were on the "right" side of the circuit. Two pins, 13 and 14, were not. Thus, if I could fix up the wiring for 13 and 14, I could just cut the keyboard circuitry with a pair of scissors, and isolate the keypad. That's what I did (more or less).

Reconnecting pins 13 and 14 was weird. I used the traces on the (now) scrap "left" side of the circuit as "wire." The circuit traces are nice, because the they are flat, and made of the same material. Connecting the circuit involves simply positioning the electrodes to touch, and then taping it down. To isolate the "wire", you just cut away the excess, unneeded circuit. There was a strip of circuit along the top that mapped perfectly to the traces for the keypad. The traces I needed to wire came in two sizes -- tightly spaced narrow lines, and larger, looser lines. A little hunting around found a bit of circuit that had the thin traces in one area, and the thick ones in another. The strip was nice and long, so I cut it out. Then, I "wired up" one side, with some tape, and the other side in the same way. The strip had to be twisted a little, but that wasn't a big deal. The ends were held down with tape.

I cut the clamshell to remove the keypad, cleaned it up, then, put it all back together. The keys were a little sticky, so I took them off (pry them with a credit card) and sprayed a little WD40 on the backs. This was a bad idea, and the keys got even stickier. The problem wasn't lubrication, but the warping of the plastic.

I plugged it back in, and it worked. The only problem was that the keys stuck, so the pad was basically useless.

Total time was around 2 hours from start to finish.