Touch Screen Mobile Phones - How Do They Work?

Since the launch of the iPhone and iPod touch, the market has seen an explosion in the number of mobile phones available with touch screen technology. Instead of having buttons or tiny pivot controls, touch screen mobiles are operated completely by touching icons on the screen. Usually, the fingertip is used to operate the phone, but it is also possible on some phones to use a type of stick or stylus. As well as their use in mobile phones, touch screens are popping up all over the place – in doctor’s surgery waiting rooms, at airports and rail stations, in shops and even in museums, in interactive exhibits.

Types of Touch Screen Technology

Different touch screens work in different ways. Older types use resistive technology, which means that tiny resistors on the surface of the coating on the screen respond to pressure. This can be from a finger or stylus but this type of screen is fairly delicate and doesn’t allow a really clear image to be presented, so resistive touch screens are rarely used in mobiles phones. More common are touch screens with capacitive technology. These are much tougher and also allow a higher resolution image but they can only be operated by a naked finger, as the controls respond to a change in electric current on the surface of the screen, and the source of touch must be grounded to Earth for this to work.

Computer touch screen devices may also operate with either infra red technology or surface acoustic technology, but these are not sensitive enough to be useful in mobile phones.

What is a Touch Screen Made Of?

A typical mobile touch screen has four different layers. At the bottom is the layer that contains the glue to stick the screen to the mobile phone body. Above that is a layer of glass that is clear but coated in a thin layer of metal that is highly conductive. After a spacing layer, the top layer, which is the interface with the user, is made from a tough and resilient polyester, with a metal layer on the bottom, which is also very good at conducting electricity.

When the surface of the mobile screen is touched by your finger, the electrical current that is passing through the two conductive layers changes. This change in current causes a change in the voltage difference across the layers, which triggers changes in the chips in the main body of the phone.

Sensitivity and Touch Screen Mobiles

The technology has improved in the last couple of years but the early touch screen phones did have problems with sensitivity. Because of the change in electrical conductance in the screen as the finger hovers just above the surface, some users had problems launching applications without meaning to. Today’s models have solved that problem to a great extent and many include ‘training’ software so that the screen ‘learns’ to respond more accurately to the touch of a regular used. It is probably also the case that the user gets used to the phone by using it frequently too, so better results are obtained with practice.

Problems Using Touch Screens in Cold Weather

One funny problem encountered by people with touch screen phones is their inability to get the phone to respond if they have gloves on. The need for a naked finger means that gloved fingers produce a very hit and miss response when used on a mobile touch screen. It is now possible to buy specially developed gloves with pads on the finger tips that are designed specifically for use with an iphone. Other users, particularly skiers, have suggested putting some conductive wire inside gloves to enhance grounding to Earth. Bizarrely, using a piece of meat such as a sausage to touch the mobile screen has also been tried and does apparently work...