How Does a Touchscreen Work?

Smartphones have become an integral part of modern life. With their reserves of information and apps to do anything you desire, they have become our beloved personal assistants. Initially, smartphones started out as humble mobile phones – devices that let you talk with a person on a similar device. Although humble, these were revolutionary, but it was only a matter of time before the world realized they had the potential to be so much more. With the advancement of technology, they were soon upgraded with better features and more usability.

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One of the revolutionary advancements was the major upgrade to the Input System. In the vast majority of modern phones, physical buttons have been replaced with touchscreens, which are far more efficient and practical. We already make use of touchscreens almost everywhere, aside from smartphones, such as in elevators, ATM machines, cash counters, etc.

downloadThe only problem is that we don’t understand much of what goes on behind that ‘black mirror’. Our knowledge about touchscreens isn’t any more developed than a toddler – we’re fascinated by them and rarely ask questions… until now.

There are basically two types of touchscreens:

Perimeter-Based Touchscreen

These touchscreens have emitters embedded in the phones that emit waves. The interference in the wave pattern is registered as an input. There is no need for actual contact in order to detect an input; even hovering works. The waves can be infrared light waves or ultrasonic sound waves, which are similar in concept, but differ in effectiveness and accuracy. The wave setup has no metallic layers on the screen, allowing for 100% light through output and perfect image clarity.

Overlay-Based Touchscreens

These touchscreens have sensors embedded under the glass layer, so actual contact with the screen is necessary for registering input. Overlay-based touchscreens are more popular because of their low cost and higher durability. There are two types of Overlay-based touchscreens that are used in your phones and other handheld devices.

Resistive Touchscreens

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Resistive Touchscreens

The resistive system works as a result of the interaction between two layers – the resistive layer and the conductive layer. The resistive layer is at the top, followed by the conductive layer, then the glass protector, and finally the display screen. The resistive layer is separated from the conductive layer by small spherical spacers. When you press the resistive layer, it actually bends and touches the conductive layer. The conductive layer then sends a current originating from this point of contact, while the processor in the phone uses this current to figure out the location of the point. Resistive screens are very durable and accurate, but not too efficient. These are used in places where accuracy is a priority over speed of use, such as in an ATM or cash-counter.

Capacitive Touch Screens

The capacitive system, on the other hand, does not have flexible screens, but instead utilizes the conductive nature of our skin. These touchscreens consist of a matrix of electrical circuits arranged on two similar, but perpendicular films that are thinner than a human hair. These layers have low-voltage current flowing through them, which gets transferred to our fingertips upon touch. The voltage drop due to this loss of charge is detected by four electrodes located at the four corners of your phone. Using the voltage drop data, the processor finds out the exact location of the input.

With the use of four electrodes and two conducting layers, unlike in resistive systems, it is possible to register slide input, as well as simultaneous multiple touches. The capacitive system transmits 90% of the light from the monitor, whereas the resistive system only transmits 75%. Due to this basic difference, capacitive surfaces reflect less ambient light, making it easier to see the screen.

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Ambient light reflection can make it difficult to read

Another area in which the systems differ is in what registers as a touch event. A resistive system registers a touch as long as the two layers make contact, which means that it doesn’t matter if you touch the screen with your finger or a rubber ball. A capacitive system, on the other hand, must have a conductive input, usually your finger, in order to successfully register a touch.

All of these touchscreen technologies can also be integrated on top of a non-touch-based system, like an ordinary LCD that is converted into an Open Frame Touch Monitor.

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Although this touchscreen technology was invented in the 1960s, high-end phones still use the basic concept developed back then. Some would say that we need an upgrade to newer tech.

Disney Research is currently developing a touchscreen technology called ‘TeslaTouch’. This aims to provide the user with hap-tic feedback and lets the touchscreen interact with the user. Voltage on the screen due to the conducting finger could be oscillated to control the friction between the screen and the finger. This could provide the user with the feeling of texture, or the user could find heavy files more difficult to drag than lighter ones! The future is going to be quite an amazing place!

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TeslaTouch texture feedback. Credits:Disney Research

References:

  1. Wikipedia
  2. How Stuff Works
  3. Explain That Stuff
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About the Author:

Harsh Gupta graduated from IIT Bombay, India with a Bachelors degree in Chemical Engineering. His pedantic and ‘know-it-all’ nature made it impossible for him not to spread knowledge about (hopefully) interesting topics. He likes movies, music and does not shy away from talking and writing about that too.

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