How Does a Touchpad Work?

by Shawn M. Tomlinson


When laptop computers first arrived on the market, most of them had a rollerball, basically an inverted mouse, to control movement on the screen. Mice had a system of directional rollers inside that were in contact with the main rollerball underneath. which told the sensors what direction to move the cursor. The trackball reversed this by having the ball exposed, with a button or two where your thumb would rest. The problem was that the ball wasn't that accurate. Something had to be done. The touchpad was created by George E. Gerpheide in 1988. Computer designers at Apple Computer decided to use the design and licensed it from Gerpheide. The first touchpad appeared on the Macintosh Powerbook 190 in 1995.


A touchpad allows users to drag their fingers along in the direction they want the mouse pointer to go. It does this by sensing the pressure of the finger along a vertical and horizontal electrode grid just beneath the surface "skin." AC travels through the electrodes to the circuits. These circuits check the capacitance of both grid layers against each other. This determines where the pointer is and where it should be moved to. It's sort of an electronic version of triangulation in that the touchpad has to take your three-dimensional movements, figure out where your finger is and where it's going, and then translate this into two-dimensional movement of the pointer on the screen.


Since the original touchpad was introduced to the public by Apple, many innovations have been implemented. The touchpad has gotten more sensitive over the years, but until recently, placing more than one finger on the touchpad at a time has confused the circuits and prevented the pointer from moving. Apple introduced a new touchpad that allows the user to click right on it, plus gives the user the ability to scroll quickly, rotate or zoom photos or use three fingers to flip through a library.

About the Author

Shawn M. Tomlinson has been a newspaper and magazine writer for more than 28 years. He has written for a variety of publications, from "MacWEEK" and "Macintosh-Aided Design" to "Boys' Life," "Antique Week" and numerous websites. He attended several colleges, majoring in English, writing and theater, and has taught college classes about writing.

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