Whether you own an iPhone, an Android phones such as the Samsung Galaxy S, or a RIM phone such as the Blackberry Torch, one of the most important items that makes your smartphone smart is its touchscreen, so lets take a look at how they work.
The two main touchscreen technologies are capacitive and resistive. The first to be introduced was the capacitive and this was first used on computer screens from around 1975. With a capacitive touchscreen there is an insulating layer (glass) which has been coated with a transparent conductor. Very few materials are both transparent and electrically conductive, and the best one known is indium tin oxide (ITO). Our bodies also conduct electricity so when we touch the glass screen, we complete the formation of a capacitor (that is an insulator sandwiched between two conductors) ITO and our finger. This causes a change in capacitance and the position where that change occurs is readily detected.
In fact there are several different types of capacitive touchscreen which are more sophisticated than the fundamental model described above, but they are all based on similar concepts.
Resistive touchscreens have a number of layers but those that make it work are two very thin electrically conductive layers which are separated by a very small gap. The layers are in fact a series of horizontal lines (on one sheet) and vertical lines (on the other sheet). To these layers are applied different electrical potentials (voltages). When you touch the screen with your finger you physically deform the screen very slightly but sufficiently for the separated layers to touch and make contact at the place of maximum pressure. This causes an electrical current to flow through the circuit you have created. The position of the contact is readily computed by sensing the potentials of the lines.
Modern smartphones are able to detect simultaneously more than one touch. These screens are called multi-touch screens. The iPhone is multi-touch and uses capacitive technology. Multi-touch screens are used for gesture recognition and typical gestures are swiping the screen, pinching together the thumb and a finger, and separating the fingers. These are used for actions such as zooming the displayed image.
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