How Do Touchpads Work?

By Angie Briggs

Introduction

A touchpad, sometimes referred to as a trackpad, is a common feature on laptops. It is also included on many electronics, such as PDAs, handheld consoles, and portable media players. On most desktops, the mouse is the primary pointing device, but most laptops use a touchpad instead. Its surface tracks the motions and pressure of a finger (or stylus, depending on what device you are using) and correlates it to the position on-screen. The touchpad was invented by George E. Gerpheide in 1988. In 1994, Apple Computer became the first company to use touchpads when it offered them in its Powerbook laptops.

Capacitance and Conductance

There are two common technologies that enable touchpads to work: capacitance and conductance. Capacitance is the most common method and enables the touchpad to hold an electrical charge. It creates two arrays, one vertical and one horizontal, and when your finger touches a spot on the touchpad, it locates the coordinates of that location. Think of it like a map. You enter two coordinates (one latitude, one longitude) that will lock down a location. The pad sensors then convert the location from the touchpad to the location on the screen. Conductance is a less common technology and enables the touchpad to have two surfaces with small insulators in between them. When you touch the top surface it connects to the bottom surface, and the location of pressure is determined by sensors and translates the location in a similar way as with capacitance.

Functions

Touchpads use relative motion, similar to a computer mouse. Relative motion enables you to move your finger or stylus across the surface and a cursor will move on the screen. There are usually two buttons on the bottom that use the same function as the buttons on a mouse for left and right clicking. On most touchpads, tapping it will also have the same result as clicking the left button. Most newer touchpads also have a function for scrolling, just as a middle mouse button does. There is a location on the touchpad called a hot spot that enables this function. Hot spots sometimes include other functions like media playback. They are sometimes referred to as click wheels.