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Laptop keyboard - how does it work?

Discussion in 'Electronic Basics' started by DaveC, Apr 6, 2005.

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  1. DaveC

    DaveC Guest

    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

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  2. Lord Garth

    Lord Garth Guest

    Dave, these are capacitance switches. Read here:

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

    You can type by just touching the membrane lightly...
     
  3. Bill Jeffrey

    Bill Jeffrey Guest

    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
     
  4. Don Bruder

    Don Bruder Guest

    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?
     
  5. DaveC

    DaveC Guest

    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

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  6. E. Rosten

    E. Rosten Guest

    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

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

    /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
     
  7. Don Bruder

    Don Bruder Guest

    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.
     
  8. Rich Grise

    Rich Grise Guest

    I've seen several that look like this:
    http://neodruid.net/KeyZilla/index.html#Keyboard

    Cheers!
    Rich
     
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