How do touch screens work?

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“We, nowadays swipe everything with our fingers”- these are the words that overheard two friends talking to each other a few days ago. This story got me thinking that many of us do not know what is going on behind these glossy screens. There are many touch screen technologies like resistive, acoustic, surface capacitive, projected capacitive, optical imaging and infrared but most commonly used touch screen technologies are resistive and capacitive.

The resistive touch screen literally resists our touch and if we press hard enough we can feel the screen bends slightly, this is what makes it work. There are two layers in resistive touch screen including the conductive layer and the resistive layer which are separated by tiny dots called ‘spacers’ to keep the two layers apart until we touch it.

The electric current runs through the conductive layer all the time.

When we touch the resistive layer, it comes in contact with the conductive layer, thus the electric current changes at that point which the software recognises and the function corresponding to that point is carried out.

Resistive screens are durable and consistent but they are harder to read because of multiple layers which reflect more ambient light. Ambient light is the surrounding light reflected back from the screen making the text on that very screen harder to read. Also they can only handle one touch at a time ruling out two finger zoom like that in an iPhone.

Resistive touch is used in restaurants, ATM’s, factories and hospitals because of low cost and high resistance to liquids and contaminants.

Unlike resistive touch screens, capacitive touch screens do not use the pressure of our finger to change the flow of electrons instead they work with anything that holds an electric charge including human skin.

Capacitive touch screens are constructed using materials like copper and indium tin oxide which store electric charges in an electrostatic grid of tiny wires each smaller than a human hair.

Capacitive touch screen consists of a glass substrate, conductive layer, a protective coating, a controller and electrodes at the corners. The electrodes apply a low voltage to the conductive layer creating a uniform electrostatic field.

When finger touches the screen, a tiny electrical charge is transferred to the finger to complete the circuit creating a voltage drop at that point on the screen (This is why capacitive touch screen does not work when we wear gloves if it is not fitted with conductive thread).

The location of this voltage drop is recorded by the controller and the software processes this voltage drop and orders the ensuing action. This is how a capacitive touch screen works. Capacitive touch screens are used in smartphones, tablets, PDAs etc. iPhone was one of the first consumer-focussed smartphones to use a capacitive touch screen.

The writer is a freelancer and educator and can be mailed at [email protected]

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