Want to convert a keyboard to QMK with N-key-rollover: ? diodes essential, ? which switches frontplate-mountable

[dreieck]

Ahoj,

Introduction:

I have got a mechanical keyboard (not really special) whose haptics I like, but it even does not register some 3-key-combinations (so in total has 2-key-rollover. Some three-key-combinations are registered, but an by me often used `<CTRL>`+`<SHIFT>`+`<k>` is not). So I like to convert it to an open source QMK firmware supported keyboard because I want to have at least 4-key-rollover in practice (and while doing so, then why not all-key-rollover).

I doubt, if the keyboard matrix has no diodes (it has none), I cannot convert it to an all-key-rollover keyboard without changing the keyboard's matrix, right?

The keys are clicky, the electrical contact is not made within the mechanical switches itself but within a foil below the clicky switches: The switches have a stem that presses down some rubber dome that connects contacts within a foil (or changes their capacity -- I haven't checked if the keypresses are registered capacitive or resistive).


Question 1:

So am I right that it means that, since the keyboard matrix in the foil is without diodes, I cannot just with another controller get all-key-rollover, but I have to completely re-make the keyboard matrix?, or is it somehow possible to get all-key-rollover with just changing the electronics PCB?


Question 2:

If I remake the keyboard matrix I also would change the switches, because I cannot really make such a foil-type on my own. Since I am unable to make a PCB, I would like to stick the switches in the front plate holes and have them beeing fixed there and do hand-wiring. What type of switches (Kailh, Cherry MX, ...) I have to select so that they can be mounted that way?


Photographs and keyboard type:

Below you find photographs. The label says "SilverCrest SGK 3 A1" and "IAN 306133", the manual can be founde here. The LED light disperser plate inside has printed "RPJ0805D-832" followed by two asiatic characters.

[Findecanor]

It looks like your keyboard has a regular membrane contact mechanism. Each "switch" works by having two metallised circles: one on the top and and one on the bottom membrane that get pressed together to connect and conduct electricity.

People sometimes ask if they could modify their 2-KRO mechanical keyboard with discrete switches and PCB into NKRO: That is not possible without doing a lot of work to the circuits on the PCB to add diodes, and then to replace the controller.
The PCB mods are not just about soldering on diodes: Most often, solder pads for switches in a keyboard matrix are not connected to just one trace: they often have one trace going in and one going out. That means that if you cut both traces so as to isolate the pad and add a diode, then you'd also have to add a wire to repair the connection between the two traces that you have cut.

However, your keyboard has a membrane: those are difficult to solder onto, and there is practically no room for any diodes, so that would be even more difficult.

One possible mod (that I have only brainstormed about, and never seen anyone do) might be to replace the bottom membrane with a custom-made double-sided PCB. Then the membrane's contacts would make contact with the circular contacts on the PCB, and the PCB could have surface-mounted diodes on the bottom side for N-key rollover. But that would not be straightforward either: a PCB is slightly thicker than a membrane, so you would probably have to do some precision mods to the keyboard's case (if it would fit at all).

There are a couple commercial membrane keyboards out there that do have a NKRO matrix (but not necessarily NKRO on the interface). One example is the Razer Ornata: in that one, the matrix is a on a pair of membranes and a PCB: There is a single trace from each switch to the PCB, on which there are diodes.

[dreieck]

Ahoj,

thanks, you answered question 1 for me.

Just to get more clear -- I think I phrased it in my question 2 but it seems I made it not clear nough:

That is not possible without doing a lot of work to the circuits on the PCB to add diodes, and then to replace the controller.
The PCB mods are not just about soldering on diodes
[...]
However, your keyboard has a membrane: those are difficult to solder onto, and there is practically no room for any diodes, so that would be even more difficult.
[...]
One possible mod (that I have only brainstormed about, and never seen anyone do) might be to replace the bottom membrane with a custom-made double-sided PCB. [...]

I would rip out the membrane completely, not use a PCB other than the controller's PCB (the "controller board" was what I was referring to as "PCB"), would like to stick the keys through the front plate and do handwiring.

So that brings me again back to question 2: Which switches are needed that they can be mounted in the holes of the front plate? (The current "mechanical" part of the switches is also mounted that way.) Do any standard Cherry MX and compatible switches work, or do they need to be special switches, or not at all because it needs a special front plate? I know that they are handwired self-made keyboards around, and the keys in those must be fixated in some other way than by soldering to a PCB.

That would meen reusing the housing (and frontplate), including stabilisers and keycaps, and making the rest new.

Regards!

[mmm]

Question 2:

If I remake the keyboard matrix I also would change the switches, because I cannot really make such a foil-type on my own. Since I am unable to make a PCB, I would like to stick the switches in the front plate holes and have them beeing fixed there and do hand-wiring. What type of switches (Kailh, Cherry MX, ...) I have to select so that they can be mounted that way?

I am not quite sure whether the plate supports other existing switches. You'll have to remove a switch and then check against the dimensions of a Cherry MX switch cutout. Both Kailh and Cherry makes MX-style switches, which have the same PCB and plate footprint. Measuring is the option here (unless someone has already done this, for the switch in the keyboard currently) - or trying to insert an MX switch with the plate removed from the assembly.

The stabs are probably fine, I would guess.

It takes quite some work handwiring a big board, but it is definitely possible. Depending on the fit, they might be a bit loose, the switches, but that can be fixed with glue

But best way is trying, taking a loose MX switch if you have one laying around, and seeing whether you can re-assembly the keyboard with only the plate and nothing else, and whether there is enough space below the plate to fit the switches. The plate might also become a bit too unstable without the rest of the components supporting, but a bit of creative engineering can possibly fix that.

Good luck!

[paperWasp]

I just wonder if a LIDL supermarket keyboard is worth the effort?
Maybe to test the modification process and then use something better to start with?