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How Touch Screens Work       BY GURU (DAVE SMITH )


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

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."



Capacitive Touch Screens

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.