How the touch screen works

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Alejandra Rangel
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How the touch screen of smartphones works

Let's see, first of all, how the touch screen of smartphones works analyzing the operating principles of the technology that underlies the interaction that takes place between the user and the screen of his device, which is generally of the capacitive or, much more rarely, resistive.

How the capacitive touch screen works


The vast majority of smartphones currently in circulation (almost all) integrate one capacitive touch screen. Unlike the resistive touch screen (which I will talk about in detail later), this technology allows you to interact with the various elements on the screen by touching (even very delicate) on the touch-sensitive surface using your fingers or, in any case, tools. that are capable of conducting electricity (e.g. capacitive gloves or capacitive pens).


The name of this technology, in fact, is due to the fact that it is based on the use of electrical devices called capacitors o capacitors which detect the temperature level in the interaction. What allows all of this? The credit goes to a flow of electrons that is generated and passed on the surface of the display, so that, when a conductor of electricity (such as a fingertip or a capacitive pen) touches it, the flow undergoes a distortions, which makes it possible to locate the exact point that has been touched by the user.

For this to happen, a very thin layer of is applied to the outside of the display glass metal oxide. To the four corners of the panelinstead, a light one is applied tension which spreads evenly over the entire surface of the display due to the metal oxide.



This makes the electron flow is distorted when the screen is touched with one or more fingers (in this case we speak of schermi multi-touch, i.e. able to recognize multiple touches), allowing the matrix of film capacitors positioned on the panel under the glass surface to "read" the variation in surface capacity that has occurred, identifying in a rather precise manner the exact point where the touch occurred.

How the resistive touch screen works

The operating principle of the touch screen resistivo, a technology now absent on smartphones but still used in the industrial field, for example for the creation of screens of various types of machinery and systems, is clearly different from that of the capacitive touch screen.

In resistive screens, in fact, the screen is made up of various layers or “sheets”, two of which are covered with a conductive material. To go into more detail, resistive touch screens are composed of a so-called upper layer PET (PolyEthylene Terephthalate), which is a thin patina or film made from polyethylene: being flexible it allows you to exert pressure on the display. The second layer, called BG (Bottom Glass), is nothing more than a relatively thick layer of glass that allows you to perceive the pressure where it occurred.

Below the PET and above the BG there are two layers of resistive substance obtained from a so-called metallic compound ITO (Indium Tin Oxide). When the PET is pressed, the pressure is felt by the first ITO layer rejoining the second, which interacts with the BG.


When pressure is applied to a specific point on the touch sensitive surface, it is “read” by the screen and converted into digital data, processed by the device's processor. To understand the exact point where the touch occurred, coordinates are used X e Y.


This is made possible by the fact that PET and BG are connected via 4 sensors (called in jargon bus) to the processor and each of these corresponds to a Pin: respectively Pin 1 e Pin 3 for PET e Pin 2 e Pin 4 for the BG. When pressure is applied to the screen, two phases occur: one in which the coordinate is calculated X and the other where that is calculated Y.

When the user exerts pressure on a point on the screen, the two touch sensitive layers come into contact and a particular electronic circuit is created. voltage divider, which causes a different voltage to be created depending on where the screen is touched. By measuring this difference, the device is able to determine the exact point where the touch actually occurred. Clearly, the operations just described take place in a very short time, in the order of milliseconds, without the user noticing the entire process that has just occurred.

Compared to the capacitive touch screen, the resistive type has a much lower touch sensitivity, a lower level of precision, a shorter resistance over time, a worse visibility and the impossibility of detecting more touches. These are some of the main reasons that led to the decline of this technology, which is now considered obsolete.


How touch screen monitor works

In case you were wondering, the touch screen technology is not exclusive to smartphones and tablets only, as even the monitor computers can be sensitive to touch. The technology behind the touch screens of PCs is almost the same as that of mobile devices, so I suggest you read the previous chapters to learn more about the subject.


In fact, even computer monitors are equipped with digitizers (usually capacitive), which allow you to recognize the touches made by the user on the screen. Those passive, they simply recognize the point on the screen touched by the user, while those active, offer a higher degree of accuracy, are sensitive to multiple pressure levels and are recommended for those who want to take notes on the screen (perhaps using a touch pen), as they already recognize pressure levels.

In case you do not have a computer equipped with a touch screen monitor, know that there are devices capable of transforming "standard" displays into touch displays. How do they work? Usually these devices consist of one bar that you have to magnetically place at the base of the monitor (or on one of the other sides, according to the instructions provided by the manufacturer) and connect via USB at the PC.

Their technology usually consists in emitting an invisible field of light on the surface of the screen: when the user touches a point of the same, the light field is interrupted and the device "reads" the movements that are made with the fingers converting them into actions, just as if the screen were an authentic touch screen.

On the market there are various types and prices: if you decide to buy one, make sure it is suitable for the size of the monitor on which you want to install it and that it is compatible with the operating system installed on your computer.

How touch screen pen works

Touch screens can also be used penne as an input tool. The choice of pen varies according to the type of device on which you want to use it. On capacitive touch screens it is possible to interact with the elements on the display using passive capacitive pens, which include at one of their ends a thin sheath made of an electrically conductive material. Their strong point is that they cost very little.

Clearly this kind of solution simply allows you to interact with the elements on the screen and offers a non-optimal level of precision and smoothness. For this reason, if you want to use a touch pen to take notes, draw or perform other precision tasks (especially if you are using a tablet or graphics tablet), it is practically essential to use a model of it. active pen, to be connected via Bluetooth to your device.

On the market there are various types and prices: keep in mind that the higher-end models allow you to use multiple pressure levels, simulating in a rather realistic way “real” pens, pencils and brushes. They are usually associated with the device through a connection procedure through which the pen is "paired" with the device, allowing it to function correctly. I also point out that these pens often have a interchangeable tip, which must be replaced when it wears out. In addition, the Bluetooth pens integrate a internal battery and, therefore, to function they must be recharged regularly.

I would also like to point out that not all touch screen pens are compatible with all displays. Based on the choices made by the various manufacturers of these devices, some pens may be compatible with some operating systems or even only with some device models (an example is Apple Pencil, compatible only with some iPad models).

As regards the writing quality of the pens on the touch screen, what was said in the previous chapter is valid: the quality of the stroke varies according to the type of digitizer present on your device. Those passive, are able to recognize the position of the pen in a more or less precise way, but are not able to "read" the pressure exerted, its inclination, the speed of the stroke and so on.

Screens equipped with active digitizers, on the other hand, it offers a much higher writing precision, with a sensitivity to multiple levels of pressure, making writing on the screen fluid, almost comparable to that on paper. In addition, they allow you to write on the screen by placing the palm of your hand on it, without running the risk that involuntary touches interfere with the pen (a function that takes the name of palm rejection).

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What to do if the touch screen does not work

The screen of your smartphone seems to be unable to respond to commands and you would like to know what to do if the touch screen does not work? It depends on the origin of the problem: if the malfunctions are related to software problems, you can try to solve them by performing the restarting the device by turning it off and on again.

If the device does not respond to commands and you are unable to turn it off and on again using the "standard" restart procedure, try perform a forced restart.

  • Android - press and hold the button for about ten seconds Power or, if that doesn't work, press and hold key combinations Power+Volume – or Power+Home, for about ten seconds.
  • iPhone 6s, iPhone 6s Plus and earlier - press and hold the buttons for about ten seconds Power Homeuntil the display turns black and the Apple logo reappears.
  • iPhone 7 e 7 iPhone Plus - press and hold the buttons for about ten seconds Power Volume (-)until the display turns off and the Apple logo reappears.
  • iPhone 8, 8 PlusiPhone X and later models - press and quickly release the key Volume +, repeat the operation with the key Volume (-) and then hold the right side button until the display turns off and the Apple logo reappears.

For more information on the procedures I have just described to you, consult the guide I have just linked to you, where I have provided you with more detailed instructions on how to restart a locked phone.

If you continue to experience touch screen malfunctions on your device, I recommend that you contact the manufacturer's customer care because these may be due to hardware problems.

How the touch screen works

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