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Laptop keyboard - how does it work?

D

DaveC

Jan 1, 1970
0
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.

Is this hall effect? I can't see anything on the end of the cone, unless
there's something impregnated in it.

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

What technology is used in this kind of keyboard? With 40-pins going
off-board, I presume all matrix processing is done on the motherboard?

Google didn't turn up any in-depth descriptions of keyboard technology.

Thanks,
--
Please, no "Go Google this" replies. I wouldn't
ask a question here if I hadn't done that already.

DaveC
[email protected]
This is an invalid return address
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L

Lord Garth

Jan 1, 1970
0
DaveC said:
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.

Is this hall effect? I can't see anything on the end of the cone, unless
there's something impregnated in it.

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

What technology is used in this kind of keyboard? With 40-pins going
off-board, I presume all matrix processing is done on the motherboard?

Google didn't turn up any in-depth descriptions of keyboard technology.

Dave, these are capacitance switches. Read here:

http://www.discovercircuits.com/C/capacitance-sw.htm

You can type by just touching the membrane lightly...
 
B

Bill Jeffrey

Jan 1, 1970
0
Not Hall effect, since there is no magnet involved. Most likely
something about (or some part of) the rubber is conductive - silicon
rubber can be made conductive. Perhaps the degree of conduction changes
when the rubber is squished.

It is also possible that it is capacitive sensing - the layers form a
capacitor, whose value changes when the rubber is deformed to bring the
conductors closer together. Sounds pretty elaborate, though.

Bill
 
D

Don Bruder

Jan 1, 1970
0
DaveC said:
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?
 
D

DaveC

Jan 1, 1970
0
What brand of machine are we speaking of here, anyway?

Apple Macintosh PowerBook G3.
--
Please, no "Go Google this" replies. I wouldn't
ask a question here if I hadn't done that already.

DaveC
[email protected]
This is an invalid return address
Please reply in the news group
 
E

E. Rosten

Jan 1, 1970
0
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 sure? Those pads are often conductive (check with a resistance
meter). A common construction is for the conductive pad (often not all
that consuctive) to push down on a pattern of traces like this:

____________________
| | | |
| | | | | | | | |
| | | | | | | | |
| | | | | | | | |
| | | | | | | | |
_____|___|___|___|___|

Some keyborads (for instance olver versions of the Microsoft Natural
keyboards (I don't know about new ones)) have 3 sheets of plastic bonded
together. The top and bottom sheet have traces printed on, and the central
sheet has holes, but otherwise keeps the other two sheets apart. The keys
push the two outer sheets together (where there is a hole), making a
contact.


Is this hall effect? I can't see anything on the end of the cone, unless
there's something impregnated in it.

Very unlikely.
What technology is used in this kind of keyboard? With 40-pins going
off-board, I presume all matrix processing is done on the motherboard?

Very likely.

If you're interested, I suggest getting an old keyboard (since you can get
new ones for less than 10UKP, I expect you can get an old one for next to
nothing, and probably for free if you try hard enough) and pulling it
apart to see how it works. Many modern seem to use traces printed on
plastic with rubber domes to act as springs. Some old keyboards (like the
BBC computer) had about 70 individual switches soldered down to a board.


-Ed

--
(You can't go wrong with psycho-rats.) (er258)(@)(eng.cam)(.ac.uk)

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for /s 15 d f pop 240 420 m 0 1 3 { 4 2 1 r sub -1 r show } for showpage
 
D

Don Bruder

Jan 1, 1970
0
DaveC said:
Apple Macintosh PowerBook G3.

Then I'd bet my last nickel your keyboard is operating exactly as I
described, using the "three sheets of plastic sandwich". Apple tends
strongly toward the "Find a design that works, then stick with it until
something else changes raadically enough to force an alteration to that
design" philosophy when it comes to "other than the motherboard" stuff.
 
R

Rich Grise

Jan 1, 1970
0
Some keyborads (for instance olver versions of the Microsoft Natural
keyboards (I don't know about new ones)) have 3 sheets of plastic bonded
together. The top and bottom sheet have traces printed on, and the central
sheet has holes, but otherwise keeps the other two sheets apart. The keys
push the two outer sheets together (where there is a hole), making a
contact. ....
If you're interested, I suggest getting an old keyboard (since you can get
new ones for less than 10UKP, I expect you can get an old one for next to
nothing, and probably for free if you try hard enough) and pulling it
apart to see how it works. Many modern seem to use traces printed on
plastic with rubber domes to act as springs. Some old keyboards (like the
BBC computer) had about 70 individual switches soldered down to a board.
I've seen several that look like this:
http://neodruid.net/KeyZilla/index.html#Keyboard

Cheers!
Rich
 
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