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How do Touch screens work?  

How do Touch screens work?  

By: Branch Education What enabled smart phones to dominate as a technology- to be so prolific and critical to our lives that I would rather lose my car keys or wallet than my Smartphone. The answer to this question isn’t a simple one liner,  but rather it's a combination of answers, app development, wireless internet, carrier networks, Steve Jobs’ brilliant marketing, though I would argue the most important contributor was the seamless combination of different functions and technologies into a single package.

In this blog we’re going to explore the most distinctive feature of the Smartphone’s merging of technologies- the touch screen display. There are three technologies in the touch screen display these technologies are combined all on top of each other. When you felt and used a Smartphone for the first time, you just knew that you were holding something revolutionary – something different from all previous phones. These… were not new technologies.

Many devices used a tactile interface, and color displays had already been a standard for most phones. Even toughened glass had been discovered in the 1800s. But the innovative element was combining them seamlessly. One layer on top of another like magic. Ok, so let’s get into the layers of a touch screen display. On the top, we have the protective glass.

A lot of us have had a screen shatter but think about how many times you’ve dropped it and it hasn’t. That’s because a Smartphone’s glass is over 5x stronger than normal glass. And, before the first iPhone showed up in 2007 the standard for cell phone screens was plastic and although plastic doesn’t shatter, it is very easily scratched. If the screen were covered in plastic, it wouldn’t last a week sitting in your pocket next to your keys before having dozens of scratches all over it. So, what makes toughened glass so much stronger? A Smartphone’s glass is an aluminosilicate glass that is toughened by soaking it in a bath of molten potassium nitrate.

These causes the sodium atoms in the glass migrate out, and much larger potassium atoms take their place. Because the potassium atoms are much larger, they generate a substantial compressive force on the surface of the glass. Here’s a quick analogy: imagine filling the backseat of a car with 3 average sized people. They fit snugly but if you push them, they're still able to move. Now replace those 3 people with 3 football linebackers.

Those linebackers are just flat out stuck- unable to move. It would take much more force to move those linebackers from their seats. This is the fundamental concept behind what makes toughened glass special, the atoms are compressed so it would take more force for the glass to break.

Below the toughened glass is a projected capacitive touch screen that senses the presence and location of conductive materials, such as your finger tip. This touch screen is composed of two transparent diamond grid pattern sprinted on polyester with an optically clear insulator in the middle. The diamond grid pattern is printed with a transparent, material called Indium Tin Oxide or ITO which acts as a conductor. Let’s take a closer look on how it works.

Say we build up a bunch of electrons on this blue diamond, however because there is an insulator in the way, the electrons cannot move. The electrons generate a negative electric field which causes a bunch of positive charges to build up on the yellow diamond. This is called a capacitor. Now, when we move a conductive material such as the tip of your finger close to this capacitor it disrupts the electric field which changes the amount of positive charges that build up on the yellow diamond.

The change in positive charges caused by this disruption on the yellow diamond is measured, and the processor registers this as a touch. The location of the touch is detected by scanning the charges or voltage along the blue diamond rows, while actively measuring each yellow diamond column. Note that each row of blue diamonds is connected together; also each column of yellow diamonds is connected. This setup makes a grid of blue columns and yellow rows. Just to clarify again, all of these components are made with transparent materials. Measuring each point requires too much circuitry, so we only measure each column.

The charge or voltage gets sent to each row in quick succession, so the processor can register multiple touches at once below that are a display which uses LCD or OLED technology. While the LCD and the OLED display both produce high quality images, in this episode we are going to focus on the OLED technology as it is the standard in most new Smartphone. OLED stands for Organic light emitting diode. This high-resolution OLED display is what generates the high-quality images that we see whenever we look at our Smartphone. This is a crazy intricate grid!  Current 2018 high-end phones can have over 3.3 million pixels. That means there are 10 million microscopic individually controlled dimmable red green and blue lights in the palm of your hand. Take a moment to think about the engineering level required to control let alone design and manufacture that many microscopic lights! OLED displays are composed of a massive grid of individual pixels and each pixel is composed of a red green and blue sub pixel.

Each sub pixel’s light intensity is controlled by a small thin film transistor that acts as a dimmer switch. There are many layered structures in each sub pixel, however explaining the function of each layer will have to be saved for a future episode. Photons are produced in the sub pixel by electrons that are driven from the negative to the positive terminal. When they pass through this middle layer here, called the emissive layer, photons are emitted through a release of energy.

 

The compounds wont to structure the emissive layer determines the colour of the sunshine emitted, and therefore the intensity of this light depends on what percentage electrons undergo. This explanation is greatly simplified but the research, engineering and science behind OLEDs is extensive.

Actually the 2014 Nobel Prize in Physics was awarded to three researchers for his or her discovery of efficient blue light emitting diodes! So, let’s summarize: on rock bottom is an OLED display composed of a ten million itty bitty little colored lights. On top of that's a transparent projected capacitive touchscreen which will sense one or multiple finger touches at a time. And on top of that's strengthened glass that protects your screen from scratches and most falls.

Now you too are a touch screen expert! If you've got any questions, post them to the comments below. Subscribe and tell your friends or family about something you learned.

 

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