In article ,
DaveC wrote:

Underlying the key mechanisms is a double-sided film with concentric circuit
pads (for each key) on either side of the film. The key cap pushes down a
little inverted cone of what looks like silicone rubber to touch the film.

This isn't a contact switch; the top of the film has just one pad, as does
the bottom of the film; no electrical connection is being made.

Are you absolutely certain about that? It sounds to me like you're
describing a classic keyboard construction style.

The "AppleDesign" keyboard is one example - Three layers of clear
plastic (Mylar? Something else?). On the "inside" of the two outer
layers, conductors and contact pads are printed. The two outer sheets
are held apart by a third sheet of slightly stiffer plastic with holes
punched in it at the points where contact is expected to be made.
Hitting a key presses an inverted rubber cone like what you describe
onto the upper sheet, pressing the upper and lower layers together
through the hole at that locaiton in the center layer, completing a
circuit from the "top sheet" to the "bottom sheet".

On semi-casual visual inspection, the entire key matrix appears to be a
single sheet of plastic with printed circuit traces and contact pads
criss-crossing every which way. However, closer examination reveals it
to be a "sandwich" of three sheets with the contact pads very clearly
existing on the facing sides of two sheets, with a third "holey" sheet
between them to keep contact from happening anywhere except the desired
places.

If I were a betting man, I'd lay money that you've got exactly the same
concept going on with the keyboard you're looking at. It may be the most
common type of keyboard construction there is these days, short of an
array of individually packaged switches.

The connector to the motherboard is a 40-pin flexible mylar cable.

40 conductors sounds just about right for the type I'm speaking of. In
the AppleDesign, those 40 conductors - 20 from each layer of the
sandwich - get fed to a chip that takes care of converting each key-hit
into the serial datastream used by the four wire Apple Desktop Bus.

What brand of machine are we speaking of here, anyway?

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