Use of capacitive touchpad for keyboard projects

[pw]

Hi all
I joined the forum just to get feedback on an idea that might make it a lot quicker and easier to build custom mechanical keyboards. (I'm not into keyboards per se, but I would like to get an ergodox or similar at a reasonable price, hence my recent interest).

Anyhow, the idea is to use a simple pcb overlay held against a capacitive touchpad to register key presses. The pcb would be a simple array of copper circles, each connected to one of the keyboard switches. The other side of all the switches would be connected to (eg) some mass of copper, like an unetched pcb, to ensure a large change of capacitance with each keypress. Some quick experiments with the Apple magic touchpad and the FingerMgmt utility suggest to me that these touchpads are responsive enough, and have a high enough resolution, that the idea could work.

Of course driver software would be required to translate the key presses to characters.

The advantages of this approach (if it works) would include:
Simpler construction - no diodes, a single wire from each key back to the overlay.
No microcontroller, and no cables if you use a bluetooth touchpad.
No need to wire any particular key to any particular pad on the overlay, as a training sequence would be necessary in any case to register each key with the driver software.

If this much works ok, it might even be possible to design the overlay in such a way that the trackpad could still be used as a normal trackpad, by building a thin, stick-on overlay from eg Pyralux, dividing each pad into sectors, and using separate pcb tracks to each sector to ensure that there was not a gross loss of resolution for finger gestures.

What do you think? It may have already been tried, as I said Im not an enthusiast per se.

[Parak]

Sorry, can't say that I'm completely following, but wouldn't you at the very least need a touchpad as large as a keyboard? Combining multiple would be iffy, too. You'd also need custom switches that actuate (meaning come within correct sensing range of the touchpad) halfway into travel, which is a massive challenge in itself.

Capacitive sensing in keyboards has been done before - for example, my two favorite switches, capacitive buckling spring and beam spring were capacitive based. However, their method of functionality is different from what you describe, of course. An actual matrix on a pcb is utilized, with sense and strobe lines, to minimize pin count.

I'd say for most intents and purposes however, the means of detecting a keypress is a relatively minor consideration, all things considered. Far more importantly, the switch must feel good, and everything else falls under secondary considerations (of varying priority). This is also why typing on a flat surface is very unfun.

[dorkvader]

welcome to geekhack. There was a keyboard that did this once. Apple appropriated them and all the innovative parts of their touchpad were pioneered by it.

Ah that's it fingerworks KB
https://en.wikipedia.org/wiki/FingerWorks

http://ergocanada.com/products/keyboards/fingerworks_lp.html

https://geekhack.org/index.php?topic=34870.0

[pw]

Hi

Looks like Ive not been clear.

Im not thinking of reinventing capacitive switches, or using tocupad directly as a keyboard (which has been done for numerical keypads for example). Im suggesting only to use the touchpad as a sensing device.

The switches would be mechanical, and arranged separately in their own keyboard. The key switches are connected (by wires, then pcb tracks) to a matrix of small circles of copper on a planar pcb, which is simply held over the touchpad, or stuck to the touchpad. The keyboard can be any size you like, the only limitation is the resolution of the touchpad, limiting how many distinct key signals can be registered over the space of the touchpad. I think the Apple magic pad could accommodate well over 100, because its size is bigger than 10cm square, and its resolution is much better than 1cm.

Pressing one of the mechanical keys would have the same effect as a finger touching the pad in the location of the circle of copper on the pcb to which it is connected.

[Findecanor]

By "PCB of copper circles", you mean one that is flexible like the upper membrane in a membrane switch?
And this sits on top of a off-the-shelf touchpad with the keyboard logic in software on the host?

Well, custom PCBs with capacitative sensing do not have to be expensive to manufacture. Members of the community have already made their own replacement controllers, and PCBs for the IBM Model F and Beamspring for instance. These allow them to be used with modern computers that speak only USB, more programmability, and with the IBM Model F XT project - new layouts using old components.

Something that I do think would be more interesting could be a replacement keyboard for the Microsoft Surface 3 Surface 2.
AFAIK, Microsoft has not released any specs to their proprietary keyboard port, so to make your own folding keyboard you would either have to reverse-engineer it., or ...
build something that sits on top of the "Touch Keyboard", and works like you describe it.

Edit: MS Surface 3 does not have a touch keyboard option.

[dorkvader]

By "PCB of copper circles", you mean one that is flexible like the upper membrane in a membrane switch?
And this sits on top of a off-the-shelf touchpad with the keyboard logic in software on the host?

I think he means like foam/foil capacitive switche,s but instead of over capacitive contacts on a PCB they are over a capacitive contacts on a large touchpad-like device.

So it'd be more like that one KB where you could rearrange the keys wherever you like (they thought it'd be good for gaming) than the fingerworks.

as far as semi-touchpad capacitive like mechanical keyboards. I believe a say a synaptics patent on that.

https://encrypted.google.com/patents/US8735755
http://www.synaptics.com/en/thintouch.php

[Findecanor]

So it'd be more like that one KB where you could rearrange the keys wherever you like (they thought it'd be good for gaming) than the fingerworks.

I think something like that exists, which has individual key modules that are magnetic on a larger sensor pad. Originally not made for typing but as a digitized pad for CAD work that would allow you to trigger special functions from the keys, and the key modules were quite expensive.
I'm sorry I forgot the name for it and do not have any saved picture ...

[Oobly]

If you're wiring up a whole matrix of mechanical keys anyway, it's better, cheaper and easier to just wire them up to a Teensy as a controller. It's not much extra work to wire up the diodes.

Interesting concept, though. Reminded me of the ErgoDex DX1: http://www.ergocanada.com/products/other/ergodex_dx1_input_system.html

[pw]

Ergodex looks interesting, thanks - I thought about attaching keys to the touchpad with suction cups, but its not big enough. The ergodex only provides 25 keys, so it looks like they are not trying to compete with keyboards for typing.

[Findecanor]

Interesting concept, though. Reminded me of the ErgoDex DX1: http://www.ergocanada.com/products/other/ergodex_dx1_input_system.html

That is the one I was referring to but couldn't remember the name of. Thanks.