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How do touch pads work?

What is the technology behind touch pads on laptops?  How do they work?  Are they heat sensing or pressure sensitive?  I can use my finger, but not a pencil eraser.

Thanks.
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pseudocyber
Asked:
pseudocyber
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CallandorCommented:
I would say they're pressure activated.  They're also smart enough to know what a finger motion does.  See http://www.kdscomputers.com/notebooks/faqs/faqs_touchpad.asp

New kind of touchpad: http://www.pcmag.com/article2/0,4149,1356414,00.asp
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HippyWarlockCommented:
the touch pad acts like a video camera, recording the objects touching its surface. An embedded microprocessor then applies an algorithmic process to convert those touches into commands understood by the computer

techie type:
http://www.paia.com/touchsw.htm

Peace
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middenfaceCommented:
im not sure about presure being the only source for input info, my touchpad doesnt work if you use non conducting materials such as a pen top, eraser, gloved hand etc etc. Perhaps there is a conductivity element involved, i.e. skin secretions alow conduction thus ungloved fingers give input while gloved do not....... what do you think?
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middenfaceCommented:
just experemented with a snickers wrapper wrapped around my index finger and it works, however rubber examination gloves do not... weird!!! perhaps my conduction hypothesis is a bit off ;-))
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pseudocyberAuthor Commented:
The pencil with an eraser eliminates the "pressure activated".  It also eliminates the "video camera" idea.  The conduction idea sounds like it has merit though.
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pseudocyberAuthor Commented:
I found this at the Patent Office,

Quote:  "1. A touchpad for providing data input to a computer system by tracking movement of an object in contact with the touchpad, and which includes a flexible touch-sensitive surface that conforms to arcuate surfaces such that the flexible touch-sensitive surface is non-planar, said touchpad comprising:

at least two flexible and non-conductive sheets having at least three touchpad sensing electrodes disposed thereon, wherein the at least two flexible and non-conductive sheets overlap such that the at least three touchpad sensing electrodes form a grid which defines a touch-sensitive area of the touchpad, and wherein the at least two flexible and non-conductive sheets can bend to thereby conform to an arcuate surface such that the flexible touch-sensitive surface is not in a single plane, wherein movement of an object is detected and followed across the arcuate surface;

a printed circuit board; and

touchpad control circuitry which is mounted on the printed circuit board, wherein the touchpad control circuitry is electrically coupled to the touchpad sensing electrodes so as to receive sensing information therefrom in order to detect contact of the object with the touchpad, track movement of the object across the touchpad, and removal of the object from the touchpad to thereby perform such a function as cursor control for the computer system, and wherein the touchpad control circuitry generates a plurality of signals that correspond to data input from the touchpad. "

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=/netahtml/search-bool.html&r=39&f=G&l=50&co1=AND&d=ptxt&s1=touchpad&OS=touchpad&RS=touchpad

Going to give points to middenface for his/her idea about the conductivity.  Thanks! :)
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middenfaceCommented:
Im  thinking all you would need is a matrix of + and - terminals then passing your finger over them will trigger closed circuits whereever it touches
+-+-+-+-+-+-+-+-
-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-
-+-+-+-+-+-+-+-+

can anyone get their touchpads under a microscope, ive tried x10 magnification with a handlens but cant see diddly squat. perhaps a Binocular microscope will pick em up at x20-x40 but I dont have one in my lab, only electron microscopes and MRI but i think these would be overkill
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middenfaceCommented:
posted too late dagnabit,, gotta refresh bfore post!!!!
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middenfaceCommented:
Thanks V much
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