As you may know this is not the first keyboard with displays in the key caps and I'd like to use this post to draw some comparisons with my project. LC Board / E3 Keys

Maybe the great grandfather of all these keyboards is the LC Board, you can see the commercial version live here: https://www.youtube.com/watch?v=3U20m6KjVrc The first prototype was created in 1984 in Germany and got produced as commercial version from 1987 on by "Hohe Elektronik". It featured 34 programmable keys with monochrome LCD dot matrices:

(https://deskthority.net/download/file.php?id=7771)

(https://deskthority.net/download/file.php?id=7772)

According to https://deskthority.net/viewtopic.php?f=62&t=5726 it was invented by Reinhard Engstler who still owns a company that produces and sells keys with LCD displays (even RGB versions)!

However these switches are not the usual keyboard switches, but a custom version with a size of 24.5 mm by 23.5 mm and seem to be used in control panels for industrial / mixing applications:

(https://www.e3-keys.com/products.html)

You can find out more on https://www.e3-keys.com, they even have some more extensive history post there: https://www.e3-keys.com/news.html#news1

Art.Lebedev Optimus Maximus keyboard

Maybe the best known keyboard with displays:

The 'Optimus Maximus', still advertised on the manufacturer's website https://www.artlebedev.com/optimus/maximus/ However, it is out of stock since almost a decade and the initial price tag was around 1600 USD according to https://en.wikipedia.org/wiki/Optimus_Maximus_keyboard

Most critics you can find online are around the typing experience: People used to get tired after 30 min of typing as the keys are so stiff. Other people mentioned a high pitched sound coming from the keyboard or the separate power supply you needed to operate it. Independent of it's flaws I would say this one was very ambitious the most influential keyboard with displays.

There were variation like the 'Optimus Popularis' which used a single big screen under the keys and the key caps are just transparent to let you see through the underlying segment of the display, so the key caps are suspended on the side.

(https://www.artlebedev.com/optimus/popularis/)

According to this review, the typing experience is better than expected, however it still had a price tag of 1000 USD and the layout is maybe not a fit for everyone.

Additionally there was a 6 key macro pad called 'Mini Six' for around 400 USD. IMHO a bit too much for 6 keys!

Nemeio

One of the more recent attempts to create a keyboard with programmable displays, it uses the same approach like the 'Optimus Popularis' with a singe screen at the bottom and transparent key caps on top, but it uses e-Ink instead of a full RGB display: https://www.nemeio.com/

Funded via a campaign on kickstarter and indiegogo, this keyboard was initially planned to be released by the end of 2019. The current schedule says 'End of 2022'. As it is not yet release we have no review yet and I do wonder about the typing experience of those keys. Of course the fit of the layout is another open question.

Private Project by James Brown

This is a project very similar to the PolyKeyboard by fellow Twitterer James Brown who uses the same displays but with shorter flex cables that are soldered on a flex PCB which is then once again soldered to the main PCB of the keyboard. I was surprised how fast (compared to my project) James came up with working solutions.

The main difference to the PolyKeyboard is that the displays are controlled via I2C instead of SPI and the flex cable doesn't go through the RGB slit of the key switch but just through the PCB via a slot in front of the key switch:

https://twitter.com/ancient_james/status/1525645446923702273?s=20&t=AAAGqh2W81XA7f4Jm9mOxg

I have to admit that this design leaves much freedom of choice for the key switches. 

https://twitter.com/ancient_james/status/1538808294118424578?s=20&t=nmTf1NkwTr6LqAmxBh_jOQ

Also the resin cast keycaps look pretty nice.

Sonder Design

https://en.wikipedia.org/wiki/Sonder_Design

Another single screen e-ink keyboard that according to Wikipedia got bought by Foxconn and the disappeared. The Guardian also covered that keyboard including a YouTube video of the prototype where you can see the e-ink screen in the background refreshing:

E-inkey Dynamic Keyboard

Not sure if this project ever got beyond the design phase as there are only renderings and the indiegogo campaign which wasn't a success. However, it was the only e-ink keyboard that considered to have the displays in the key caps:

There are some more, but only in form stream decks:

Infinitton

Once again the famous technique to use a single RGB screen in the back, this time for a smaller format, founded via kickstarter: https://www.kickstarter.com/projects/2086416348/infinitton-a-smart-touch-screen-keyboard?ref=discovery&term=keyboard%20screen

Elgato Stream Deck XL

I'm not sure how it works under the hood but it also looks like a single RGB display with transparent keycaps. One of the few 'Display Keyboards' that are still produced: https://www.elgato.com/en/stream-deck-xl

Finally the PolyKeyboard! (now PolyKybd)

What can I say about the PolyKeyboard? Well, one day, hopefully, it will be a kit! Which can be assembled by anyone ;)

From the beginning, I stayed away from solutions that would not work for kits.

• For instance resin cast key caps:  These look beautiful, but I cannot even do casts for 10 keyboards. That would be too labor intense. Instead I'm using relegendable key caps which are commercially available in different sizes, the only customized part here is a 3D-printed stem which is doable with my printers (or with some on demand print farms).
• Also soldering the flex cable of the displays to the PCB would be a maintenance burden if a display ever stops working. Instead I'm using a socket to plug the flex cable in.
• No custom key switches! Use the key switches of your choice: At least to some degree, as the flex cable needs to fit though the RGB slit of the key switch. I published a compatibility chart in an earlier post and think we can maybe get even smaller flex cables so that there are more compatible switches out of the box.

It is open source / open hardware, already by today (okay, I know, I have to organize it a bit more) so your are welcome to join in! The firmware is made with QMK, which means that there is no limit to the customization possibilities. The current dev-kit, which I try to put together, is a split keyboard, but I'm also thinking about a 70%ish staggered layout. The only customized component I need, is the display with an extended flex cable, as the current length is not sufficient to plug it into the socket (and I don't want you to extend the flex cable yourself, it is possible and I do that for my prototypes, but it is very painful!!). I just need to reach the minimum order quantity so that my Chinese manufacturer is willing to customize the attached flex cable. For that I need some budget upfront... currently thinking about my possibilities.

I hope you enjoyed my short overview. Maybe you are aware of another project I didn't mention? Please let me know.

Edit: People informed me that there was also such a keyboard by Microsoft, the so called "Adaptive Keyboard" and there is still some information left on Microsoft's website: https://www.microsoft.com/applied-sciences/projects/displaycover

And also here: https://www.engadget.com/2010-08-12-microsoft-adaptive-keyboard-prototype-debuts-at-center-of-uist-s.html

It looks like there were multiple iterations, some with e-ink displays, others with RGB displays and a touchscreen above the F key row:

And now there is also the Flux Keyboard:

https://fluxkeyboard.com/