deskthority

I recently met a man named Aaron who had discovered a cache of terminals from the 1980's. More on that to come. Among them were a few made by the Teletype Corporation subsidiary of AT&T. This terminal used 122 key style keyboard that I had not seen before. At first glance I wasn't expecting anything exciting but on closer inspection, the switch mechanism turned out to be quite interesting.
The keyboard uses a foam and foil switch that is truly clicky and quite tactile. Both the click and tactility is provided by the separation of a metal collar on the slider from a magnet retained by the switch housing. The resulting feel has sharp tactility, very unexpected for a foam and foil switch.


The keyboard is documented in the wiki AT&T Teletype 56K 460 ACW

Here is a video of the switch: The keycap attaches to a plunger that presses on a spring. The spring presses on the slider which has a ferrous collar. The ferrous collar is attracted to a magnet retained by the switch housing. When the keycap is pressed, the spring compresses and force on the slider increases until it exceeds the attraction between the magnet in the housing and the collar on the slider. The slider then breaks free and releases the spring tension which forces the foam and foil pad in contact with the PCB.

Here are few pictures of the switch and mechanism:

Are you able to do a switch weighting test with coins?

Very interesting switch. The use od a magnet could create a very tactile, non clicky switch, though it would probably cost a lot to make such a switch too. (1 ring magnet per switch)

That's... amazing. Super awesome find, nice write-up too!

Now I gotta find one xD .

I gotta keep an eye out for these AT&T boards now, thanks!

Magnetic trapdoor switches are something I’ve wondered about for ages. Seems like something we should be able to make, with magnetised ferrous rings, inside plastic housings… once affordable and durable 3D printing takes off. You know the recipe: MX mount and footprint. Just picture getting two dimensions of choice: spring weight *and* magnet strength. Tasty.

Muirium wrote: Magnetic trapdoor switches are something I’ve wondered about for ages. Seems like something we should be able to make, with magnetised ferrous rings, inside plastic housings… once affordable and durable 3D printing takes off. You know the recipe: MX mount and footprint. Just picture getting two dimensions of choice: spring weight *and* magnet strength. Tasty.

I was thinking the same thing. The key feel on these is interesting and there does seem to be some hysteresis. It is reminiscent of a plate spring or buckling spring switch but feels a bit sluggish. The feel is complicated by the mushy foam landing.

This switch uses older ceramic magnets, so I imagine the magnetic components could be reduced in size using modern rare earth magnets.

Engicoder wrote: I was thinking the same thing. The key feel on these is interesting and there does seem to be some hysteresis. It is reminiscent of a plate spring or buckling spring switch but feels a bit sluggish. The feel is complicated by the mushy foam landing.

Can't the foam be replaced with something like Silicone or Sorbothane?

Those materials can last far longer than any Foam plus the Sorbothane, will out live nearly everything else on this planet .

Easiest thing to use is this. Strong adhesive. Hard urethane foam so no mushy feeling. Nothing corrosive and lasts. Standard 2mm thickness that can stack. Even pre-cut into basically key sized pieces.

Even Sorbothane is a urethane.





The feel is a hard but not-totally-solid bottom. That is, it feels like you hit a hard stop but if you really lean into it, you can force it down further. Great replacement for this kind of thing.


Would love to pick up one of these boards myself and try it.

I had a board like this pass through a while back with a different layout. See http://coronthica.com/by-uuid/2ee42964- ... 917a6b94d/ .