Laptop keyboard - how does it work?
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Posted by DaveC on April 6, 2005, 10:03 pm
Please log in for more thread options 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 me@privacy.net This is an invalid return address Please reply in the news group | ||||||||||||||||
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Posted by Lord Garth on April 6, 2005, 11:06 pm
Please log in for more thread options > 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... | ||||||||||||||||
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Posted by Bill Jeffrey on April 7, 2005, 4:55 pm
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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, | ||||||||||||||||
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Posted by Don Bruder on April 8, 2005, 12:20 am
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> 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 - dakidd@sonic.net - 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.
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Posted by DaveC on April 8, 2005, 11:08 am
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On Thu, 7 Apr 2005 14:20:53 -0700, Don Bruder wrote > 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 me@privacy.net This is an invalid return address Please reply in the news group | ||||||||||||||||
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