Question: How does touchscreen technology work?

Aaron Boardley answered on 18 Nov 2015:

There are two main types of touchscreen: capacitive and resistive. Longer explanations below – skip to the bottom for a short answer!

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*Capacitive* touchscreens are the sort you will find on a mobile phone. A small voltage is applied across the screen to create an electric field. When something that conducts electricity – such as your finger – touches the screen, this disrupts the shape of the electric field. The fact that charge can flow through your finger means that that area of the screen is less able to store the charge.

The change of this ability to store charge (the ‘capacitance’) is measured from the four corners of the screen. By working out the change in capacitance compared to each of the four corners it can pinpoint your location on the screen, as the distance to each corner will be proportional to the change in the capacitance between where you are touching and the corner. A bit like if you were to push a physical ‘dent’ in the screen: the slope between this dent and the corner of the screen would depend on how far away from the corner it is, so by measuring this slope you could work out the location of the dent.

Because this relies on electricity being conducted by young finger, it means touchscreens don’t really work with gloves!

*Resistive* touchscreens have two conducting sheets separated by a layer of resistive material – so electricity can only flow when they are pressed together. The first layer has a voltage across it that will be different in different places. When you press the sheets together, the second sheet picks up this voltage. Because the voltage is different in different places, all it needs to do is then look up the voltage it received and it will know which area on the first sheet it corresponds to. Because this technology just involved pressing sheets together it can be done with anything, such as a pen, a stylus, or a gloved hand.

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Sorry if that got a bit technical. The key thing is that by pressing on a screen you either disrupt an electric field (capacitive) or form a circuit (resistive). By measuring exactly how much the field is disrupted, or what is flowing through the circuit, the device and work out where it was touched.