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

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:

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.

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

Apple Macintosh PowerBook G3.

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DaveC snipped-for-privacy@privacy.net This is an invalid return address Please reply in the news group

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.

Very unlikely.

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

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{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

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.

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...

I've seen several that look like this:

Cheers! Rich

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.