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How do touchscreens work?

|February 8 2011 |


Touch-screen devices have become a rage offlate, especially so after the iPhone phenoomenon. There are various kinds of touch screens with various degree of sensitivity. The three major classes are

  • Resistive

  • Capacitive

  • Surface acoustic wave


The resistive system consists of a normal glass panel that is covered with 2 conductive layers separated by a resistive layer.An electrical current runs through the two conducting layers while the monitor is operational. When a user touches the screen, the two layers make contact in that exact spot and the current here changes. The change in the electrical field is noted and the point of contact is calculated by the processor. Once the coordinates are known, a special driver translates the touch into signals that the operating system can understand. Thus when a user touches the screen, the device responds correspondingly.

 



In the capacitive system, a layer that stores electrical charge is placed on the glass panel of the monitor. When a user touches the monitor with his or her finger, some of the charge is transferred to the user, so the charge on the capacitive layer decreases. This decrease is measured and the processor calculates exactly where the touch event took place and then relays that information to the touch-screen driver software. One advantage that the capacitive system has over the resistive system is that it transmits almost 90 percent of the light from the monitor, whereas the resistive system only transmits about 75 percent. This gives the capacitive system a much clearer picture than the resistive system.

The third class of touchscreens are used mainly on a computer peripheral only. On the monitor of a surface acoustic wave system, two transducers (a device that can both transmit and receive signals) are placed along the x and y axes(vertical and Horizontal axes) of the monitor's glass plate. Also placed on the glass are reflectors -- they reflect an electrical signal sent from one transducer to the other. The receiving transducer is able to tell if the wave has been disturbed by a touch event at any instant, and can locate it accordingly. The wave setup has no metallic layers on the screen, allowing for 100-percent light throughput and perfect image clarity. This makes the surface acoustic wave system best for displaying detailed graphics (both other systems have significant degradation in clarity).

A major area of difference betwenen all these classes is the stimulus used. A resistive system registers a touch as long as the two layers make contact, which means that it doesn't matter if you touch it with your finger or a rubber ball. A capacitive system, on the other hand, must have a conductive input, usually your finger (human body is a conductor), in order to register a touch. The surface acoustic wave system works much like the resistive system, allowing a touch with almost any object -- except hard and small objects like a pen tip a knife or a stone.

Here is an interesting watch....... Replacing the touch panel on an iPhone..

So i hope this sheds some light on the touch screens. Next time you use a TS mobile, watch out for the class of touch screen they are..... Cheers

PS : Find detailed notes on touchscreens here :
http://en.wikipedia.org/wiki/Touchscreens#Capacitive
http://www.explainthatstuff.com/touchscreens.html

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