The key switches found in this Cherry Solid State Capacitive keyboard are one of the rare key switch sets that employ foam and foil technology over a capacitive internal PCB board. You will see that the foam and foil pads have deteriorated over the years and are no longer usable but it is an interesting discovery nonetheless.
This keyboard is shaped in the form of an XT clone board from Cherry. However, the key caps on this particular keyboard are those used for a terminal of some kind ( I haven’t determined which one yet) but documentation indicates that it is also re-configurable for compatibility with an IBM XT computer. The data sheet for a similar model of keyboard is on Deskthority at the following link: w/images/3/33/BFN3-8354.pdf
What makes this keyboard a little more interesting is that it has key switches that appear to be similar in key stem and housing to a Cherry key switch shown in an early 1980 patent. That patent is shown here: wiki/Capacitive_keyboard_%28patent_4458293%29
The figures in the diagrams within that patent show a key switch with an embedded conical spring that would press against an underlying capacitive PCB plate covered with a dielectric layer. This is different from the key switches on this keyboard because the spring is on the external part of the key stem and the bottom of the key switch has a foam and foil layer as shown below. You can actually see the well for the external springs on those patent diagrams.
Also to note: the patent has the following text so I believe this is the patent for this key switch:
The construction is very similar and I would dare to say that the parts are the same except for the springs and the extra foam and foil pads. Unfortunately, I’m not sure if any examples of the key switch that is described in this patent has surfaced lately so I can’t compare them.It should be understood that the invention is not limited to the use of capacitive elements such as springs or to capacitive elements which touch an insulated capacitive plate as shown in FIG. 5. Thus, for example, a foam pad with an electrically conducting film may be employed as a movable capacitive element without departing from the invention. An additional spring could also be provided to bias the pushbutton 21 upwardly.
Moreover, the foam pad, spring or other movable capacitive element could be supported in spaced relation above an underlying capacitive plate during the rest state of the switch and could be moved adjacent to but spaced from the plate to define a switch activation capacitance when the switch is pressed.
Below are detailed pictures of this version of the Cherry solid-state capacitive keyboard. I call these Cherry "shamrock" key switches as opposed to the other keyboard that I call Cherry "split Shamrock".
- Cherry KFN3-8351 - keyboard
- Cherry KFN3-8351 - keyboard.JPG (330.61 KiB) Viewed 2184 times
- Cherry KFN3-8351 - keyboard back
- Cherry KFN3-8351 - keyboard back.JPG (225.13 KiB) Viewed 2184 times
- Cherry KFN3-8351 - cover removed
- Cherry KFN3-8351 - cover removed.JPG (326.87 KiB) Viewed 2184 times
- Cherry KFN3-8351 - springs in place side view
- Cherry KFN3-8351 - springs in place side view.JPG (279.08 KiB) Viewed 2184 times
- Cherry KFN3-8351 - capacitive PCB contact side
- Cherry KFN3-8351 - capacitive PCB contact side.JPG (323.54 KiB) Viewed 2184 times
- Cherry KFN3-8351 - capacitive PCB back side
- Cherry KFN3-8351 - capacitive PCB back side.JPG (300.25 KiB) Viewed 2184 times
- Cherry KFN3-8351 - controller with date.JPG
- Cherry KFN3-8351 - controller with date.JPG (320.94 KiB) Viewed 2184 times
- Cherry KFN3-8351 - PCB markings
- Cherry KFN3-8351 - PCB markings.JPG (349.21 KiB) Viewed 2184 times
- Cherry KFN3-8351 - PCB label close-up
- Cherry KFN3-8351 - PCB label close-up.JPG (280.93 KiB) Viewed 2184 times
- Cherry KFN3-8351 - key switch assembly key stem side
- Cherry KFN3-8351 - key switch assembly key stem side.JPG (186.97 KiB) Viewed 2184 times
- Cherry KFN3-8351 - key switch assembly foam and foil side
- Cherry KFN3-8351 - key switch assembly foam and foil side.JPG (288.27 KiB) Viewed 2184 times
- Cherry KFN3-8351 - key switch assembly profile
- Cherry KFN3-8351 - key switch assembly profile .JPG (197.87 KiB) Viewed 2184 times
