I have always found laptop touch pads to be annoying and unmanageable devices. Which is why I always use a mouse instead. However, my whole view on the topic was toppled this morning when I went back to review for the AP exam and found myself reading about capacitors. I rediscovered the many many uses of the simple concept of a capacitor with two conducting plates and equal but opposite charge. Not only is it used in cameras and keyboards, it is also the basis behind the touch pads that I have now began to appreciate.
A touchpad is made up of a series of conductors arranged in in two layers. The upper layer uses vertical electrode strips while the lower layer uses horizontal strips. A thin insulation separates each layer, and the layers are connected to an integrated circuit. The electrodes are charged with AC, and the integrated circuit constantly measures the capacitance between the two layers. When someone touches the pad, the capacitance between the electrodes changes since the current wants to flow to your finger to complete the circuit. (and the dielectric properties of a finger are very different from the air). Thus, the charge builds up at the point of the grid under your finger. By measuring this change in capacitance, the circuit knows exactly where the pad was touched. As a finger moves over the touchpad, the changes in location are translated into cursor movements. The pad can be sensitive to other commands as well including taps and scrollbar in certain regions.
I would have never guessed in a million years the simple and ingenious physics behind the dull-looking touch pad (pictured below). Amazing how innovative people can be. And here I must reluctantly admit that physics is useful after all.
Pad looks something like this inside (probably not as big though…)
Here is mine (unfortunately i couldn’t find the camera, I suspect that my mom took it to Mongolia, and I don’t thing the old ones can be plugged into the comp…so i no have original pictures, sorry doc…)
