I have several of the 3M Lead Check tests and used one on the exposed copper layer of a PCB.
As expected there was no reaction. Which is good since I've been carrying this Feegle XS around with me for a while now.
A patina has formed on the copper surface. I replaced the steel screws with some brass ones, the outline of the old screws is still visible.
Like most rubber dome keyboards, there are no anti-ghosting diodes. Instead, the matrix is arranged in a way that it is unlikely to be able to press any combination of keys that would cause ghosting, or blocking.
The matrix on the Raspberry Pi 400 keyboard is 8x16. 24 total io pins used. The connector is 26 wide, 2 are not used (H and J in my numbering).
This is for the ANSI layout keyboard. Someone with the ISO version will need to compare.
The order of the pins on the bottom of the PCB are not in the same order as the ribbon connector.
My original labeling using A through Z on the ribbon connector.
This matrix/keymapping has not yet been verified. I have tested the matrix here.
The keyboard of the Raspberry Pi 400 is connected to a dedicated MCU that uses one of the 4 ports of the built in USB hub.
D+ and D- USB data lines are circled. They go through the vias to the other side of the board and run across to the VLI USB chip. The large test pad is VBUS.
On the other side of the board the D+ and D- lines connect to the VLI chip.
On the bottom of the PCB under keyboard ribbon connector the matrix pins are broken out. If you could reprogram the Holtek controller you could connect a custom keyboard matrix. Unfortunately this is a OTP (one time programmable) device, so unless you can work with the existing matrix the Holtek is not very useful.
The easiest way to replace the keyboard would be to cut the traces between the USB hub and the Holtek chip. Then connect the D+ and D- to a separate controller.
The keyboard has a power key (Fn+F10). This functions similarly to the modification I did on the Flirc case. A custom replacement keyboard could use an io pin to trigger the power on/off the same way.
A tinier Feegle. This is the same design as the Feegle but using 4.5x4.5mm tactile switches instead of 6x6mm. The only functional difference is the lack of a RESET button. There is only a USBASP button. Holding the USBASP button while plugging in the USB cable will enter bootloader mode.
Originally planned to use the same through hole PTC fuse I use on most V-USB boards, but with it overlapping the resistors. This turned out to be too thick and would require 6mm spacers. To keep to the 4mm spacer thickness I used a surface mount PTC fuse.
Uses the same blue LEDs for the V-USB circuit and miniature 16MHz crystal.
The switches are tiny. 6mm height for the keys and 5mm for the USBASP button. I used the top plate to hold the switches in place while soldering. It would be impossible to keep them all aligned without the plate holding them in place. Since the base of the switches are shorter than most of the other components I soldered them on first, so the plate could rest against the switches firmly.
Wookie for scale. 63x46.5mm. The holes in the top plate are 2.5mm for the switches. 3mm holes for the LEDs. Flat laptop style M2 screws.
With 4mm M2 spacers the total thickness is about 9mm. I found some aluminum M3 spacers on Digikey that look much better than the transparent nylon ones and fit right over the M2 brass spacer.
I sanded one the top plates to expose the copper layer.
6mm switch on the left, 4.5mm on the right. 4.5mm through hole switches are not very common. Surface mount versions are easy to find. I found these at lcsc.com. They are available in a few different heights and in two different actuation weights (160g/250g).
The 4.5mm switch is only 3mm tall at the base. The mini USB connector and ATmega328 are both about 4mm tall so the board can't be made any thinner.