QUESTION CORNER

G. KRISHNAMURTHY

Mobile touch screen

How do mobile touch screens work?

RAM POOJAN CHAURASIA

Sultanpur, Uttar Pradesh

Mobile phones may use two types of input devices. In regular mobile phones, a keypad type of device is used, which is mounted separately from the screen of the cellphone. Whereas in touch screen cellphones, a touch screen is a cellphone display screen that also acts as an input device. The touch screens are sensitive to pressure; a user interacts with the mobile applications by touching pictures or words on the screen.

Most mobile phone keyboards are basic in that they use a tactile surface you are accustomed to touching, and underneath is a basic rubber peg (black dot) which travels some depth until it encounters resistance in the form of the actual keyboard surface which is sometimes called a ‘bubble board.’

This is basically a semi-circle of aluminium shaped in the form of a dome and provides that springing effect of key and feedback on your finger when you press down and the button regains its at-rest shape and normal position.

Touch screen technologies used in mobile phones include resistive, capacitive and surface-wave based system.

The resistive system consists of a normal glass panel that is covered with conductive and resistive metallic layers. These two layers are held apart by spacers, and a scratch resistant layer is placed on top of the whole setup. An electrical current runs through the two layers while the monitor is operational.

When a user touches the screen, the two layers make contact exactly at that spot. The change in the electrical field is noted and the coordinates of the point of contact are calculated by the processor. Once the coordinates are known, a special driver translates the touch into something that the operating system can understand, much as a computer mouse driver translates the movements of a mouse into a click or a drag.

The change in the electrical current is registered as a touch event and sent to the controller for processing.

In the capacitive system, a layer of an electroconductive material (most often indium tin-oxide) that stores electrical charge is placed on the glass panel of the monitor. When a user touches the monitor with his finger, some of the charge is transferred to the user, so the charge on the capacitive layer decreases. This decrease is measured in circuits located at each corner of the monitor.

The computer calculates, from the relative differences in charge at each corner, exactly where the touch event took place and then relays that information to the touch screen driver software. Resistive touch screen panels are generally more affordable but offer only 75 per cent clarity and the layer can be damaged by sharp objects.

One advantage of the capacitive system over the resistive system is that it transmits almost 92 per cent of the light emitted from the monitor, whereas the resistive system transmits only about 75 per cent. This gives the capacitive system a much clearer picture than the resistive system. Also, the capacitive system has a very long life (about 225 million clicks).