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DariuszSankowski/Pixabay.com

How Does a Touchscreen Actually Work?

Here's why your smartphone craves skin contact.

SIGNE DEAN
9 OCT 2015
 

These days we don't think twice before tapping and swiping the ubiquitous touchscreens around us. For toddlers, it seems almost innate, and hey, even Teletubbies have touchscreens built into their blobby stomachs now. Chances are you tapped a link to access this article. But have you ever stopped to think about how a touchscreen actually works? We've got some answers.

 

Firstly, the touchscreen you use at the local ATM and the one on your smartphone are completely different types of technology. Screens that you tap at the ATM or the automated supermarket checkout are called 'resistive' touchscreens - they consist of two thin, flexible layers just barely spaced apart, with an electric current running in between. The actual LCD display of the device is located behind these layers.

When you press your finger on a resistive screen, the top, flexible layer touches the bottom one, and the interruption in the electrical current is noted by the device, and it calculates the precise location of the point of contact. Depending on what button is located under your finger, the software registers the coordinates and performs the command.

Resistive screens are a relatively cheap technology that's been around since the 1970s. Because they respond to pressure, you can activate them with bare fingers, gloves, or styluses (remember the PalmPilot?), but you can't swipe or use multi-touch gestures, because the electric current encased within can only successfully register one point of contact at a time. Additionally, the plastic used doesn't have the same clarity as glass, which is why ATMs and some in-flight entertainment screens have that characteristic haziness.

The glass-covered smartphone you carry in your pocket uses something different - it's called a 'capacitive' touchscreen. The name, unsurprisingly, comes from the term 'capacitor' - an electronic component that can temporarily store an electric charge (not to be confused with a flux capacitor). A capacitor is built out of two conductive layers separated by an insulator, and this principle is employed in making your iPhone respond.

If you hold your smartphone in bright sunlight at just the right angle, you may be able to notice a grid of dots underneath the glass surface. This is an array of horizontal and vertical transparent conductive wires that form capacitors at the crossings, with tiny electrical currents running through. These transparent wires are typically made out of indium tin oxide, and are located on the opposite sides of a sheet of glass, which acts as the insulator and is laminated to the top layer you touch.

Human skin conducts electricity, which is how a capacitive touchscreen responds - as you're writing a text, your finger decreases the charge at the intersection of the grid where the capacitor is located, and the microprocessor calculates which contact points were activated. This info is then relayed to the software, which in turn performs what you wanted - or, more likely, triggers yet another typo.

Unlike resistive touchscreens, capacitive touchscreens don't respond to pressure at all. Even though sometimes it feels like pressing harder did the trick, it's probably because squashing your finger on the glass increased the surface area of the touch, helping the processor register contact. If you have 'fat fingers' and touch too many areas of the screen at once, it gets confused and doesn’t respond at all. 

So a need for electrical conductivity is why you can't operate a smartphone with gloves or a plastic stylus, which are both essentially insulators. But as anyone who lives in a cold-enough climate can confirm, if you're in a pinch, swiping the screen with your nose will work just as well as a finger.

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