Everyone's used a touch screen monitor, but do you know how they work? We'll take you behind the screens.
The touch screen is the simplest, most intuitive computer interface around. Touch screen monitors are used
for hand-held computers, PDAs, industrial equipment, point of sale equipment, lab equipment, public information kiosks, and more. They are also very popular among young children, people with disabilities, artists, and gamers.
Touch screens may be simple, but the technology behind them is not. In fact, there are at least four touch-screen technologies in use today. These include resistive, capacitive, infrared, and sound wave acoustic (SAW).
Some work well on heavily trafficked devices such as ATM machines, while others are better suited to laboratory settings. Let's touch upon how the various flavors of touch screens work.
Resistive touch screens are the oldest and most widespread of the four technologies. They're common in LCD screens such as those on Palm computing devices. You've probably used one at your bank's ATM.
One reason they're de rigeur with public space devices like ATMs is that a gloved hand can operate them. If you happen to be banking in Buffalo in the middle of the winter, you can appreciate this feature.
Resistive touch screens respond to the pressure of a finger, a fingernail, or a stylus. A protective sheet of polyester or Mylar is the outermost layer of the techno-sandwich that is a resistive touch screen. Beneath the protective layer lies a sheet of conductive material through which electric current constantly flows. Beneath the conductive layer lies a layer of separator dots. Beneath the separator dots is a resistive layer. In the absence of a touch, the separator dots prevent
the conductive layer from making contact with the resistive layer. All these layers sit atop a layer of glass or plastic, under which a CRT or LCD display resides.
When your finger presses the outer protective layer it pushes the outermost conductive sheet into the resistive sheet. The resistive sheet alters the uniform flow of electricity in the conductive layer.
A touch screen controller detects the conductive layer's departure from uniform current and pinpoints the touch
event, interpreting it as a vertical/horizontal coordinate on the screen (x- and y-axes).
An analog-to-digital converter changes the electrical signal into binary data so a computer's CPU can understand it. Finally, software translates the coordinate into an input command such as "withdraw funds" or "check balance."
A capacitive touch screen features a capacitive (charge storing) layer either on top of a sheet of glass or sandwiched between two sheets of glass. The capacitive layer is typically composed of indium tin oxide.
Electrodes are connected to oscillator circuits that sit in each corner of the screen. The electrodes convey a
consistent amount of voltage across the capacitive layer. Changes in current cause the oscillator circuits to vary their electrical frequencies.
When you touch your finger or another conductive object to the screen, current is drawn to it. This causes the four oscillator circuits to vary in frequency depending on where you touched the screen. The frequencies that are produced can be translated into x- and y-coordinates, thus pinpointing the touch event.
An analog-to-digital converter changes the electrical information into digital information that a computer's CPU can understand. Software reads the screen coordinate as a particular command, as with resistive touch screens.