Laptop keyboard - how does it work?
 

im not sure which design you have, some are three layers of plastic the
middle one containing holes when key pushed it makes the outer two layers
connect, what i think you have is the type that is in alot of things these
days.
my only knowledge of hall effect is in automobiles and if im right i think
there is a sensor that picks up on the magnetism, but yours i think has
the interwoven copper tracings on the board/film and the little rubber pad
has some sort of black disk underneath it, when the disk touches the
coppertracings it completes the circut, how it works im not sure, but
there is no processor needed to detect the signal, you could feed an led
lamp of off it and it would receive a weakend current through it, maybe
that pad is some sort of conductor

The ones I have played with are just that - a 3 layer setup. The middle has
holes and the top & bottom conduct when pressed together. Carbon type
traces are the conduction medium.

WT

"AshTray700" wrote in message
lkaboutelectronicequipment.com...
im not sure which design you have, some are three layers of plastic the
middle one containing holes when key pushed it makes the outer two layers
connect, what i think you have is the type that is in alot of things these
days.
my only knowledge of hall effect is in automobiles and if im right i think
there is a sensor that picks up on the magnetism, but yours i think has
the interwoven copper tracings on the board/film and the little rubber pad
has some sort of black disk underneath it, when the disk touches the
coppertracings it completes the circut, how it works im not sure, but
there is no processor needed to detect the signal, you could feed an led
lamp of off it and it would receive a weakend current through it, maybe
that pad is some sort of conductor

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
---------------------------
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.

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,

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?

--
Don Bruder - - New Email policy in effect as of Feb. 21, 2004.
Short form: I'm trashing EVERY E-mail that doesn't contain a password in the
subject unless it comes from a "whitelisted" (pre-approved by me) address.
See http://www.sonic.net/~dakidd/main/contact.html for full details.

Hi!

You should have Googled it. :-) Sorry, couldn't resist, but the answers are
out there.

The technology used is similar to a lot of desktop computer keyboards.
Little rubber domes push down on a "layered" set of contact points. When the
layers touch, the circuit is completed and the result is sent to the
keyboard controller somewhere in the computer.

There are also other ways of doing this...IBM used capacitive switching in
their model M keyboards from so many years ago. I've also seen foil pads
that were pushed down onto a circuit board to make keypresses.

You can also fine the "dome construction" method as well. This design works
similarly to a remote control where similar pads with a small "spot" of
conductive material is on the bottom. Sometimes this material is integrated
into the dome itself. This is probably the keyboard type you have.

William

On Wed, 06 Apr 2005 19:03:41 +0000, 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 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.) )

/d{def}def/f{/Times findfont s scalefont setfont}d/s{10}d/r{roll}d f 5/m
{moveto}d -1 r 230 350 m 0 1 179{1 index show 88 rotate 4 mul 0 rmoveto}
for /s 15 d f pop 240 420 m 0 1 3 { 4 2 1 r sub -1 r show } for showpage

In article ,
DaveC wrote:

On Thu, 7 Apr 2005 14:20:53 -0700, Don Bruder wrote
(in article ):

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

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.

--
Don Bruder - - New Email policy in effect as of Feb. 21, 2004.
Short form: I'm trashing EVERY E-mail that doesn't contain a password in the
subject unless it comes from a "whitelisted" (pre-approved by me) address.
See http://www.sonic.net/~dakidd/main/contact.html for full details.

On Fri, 08 Apr 2005 15:09:15 +0100, E. Rosten wrote:
On Wed, 06 Apr 2005 19:03:41 +0000, 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.

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

"AshTray700" wrote in message
lkaboutelectronicequipment.com...
im not sure which design you have, some are three layers of plastic the
middle one containing holes when key pushed it makes the outer two layers
connect, what i think you have is the type that is in alot of things these
days.
my only knowledge of hall effect is in automobiles and if im right i think
there is a sensor that picks up on the magnetism, but yours i think has
the interwoven copper tracings on the board/film and the little rubber pad
has some sort of black disk underneath it, when the disk touches the
coppertracings it completes the circut, how it works im not sure, but
there is no processor needed to detect the signal, you could feed an led
lamp of off it and it would receive a weakend current through it, maybe
that pad is some sort of conductor


Hall Effect sensors are used all over, as you suspect they respond to a
magnetic field. I've seen keyboard switches that use them but I have yet to
actually encounter a computer keyboard with that design. They are commonly
used as the ignition pickup as well.