How Touchscreens Work

I've always been fascinated by touch screens. You probably were too, if you're a gen-Z and up and you witnessed Steve Jobs unveil the iPhone. Or if you're one of my gen-Z brethren, you probably saw some kind of touchscreen in your school.

Most of you probably weren't too curious how they worked. But I was. And I ended up dedicating my life studying Wikipedia for hours, getting degrees in math and science, practicing my skills... All to tell you that it's just wires.

The Variety of Touchscreens

Steve Jobs unveiling the iPhone is the thing that we think about when we think about touchscreens. But they've been around for QUITE a while. Since the 70s, at least. We're going to talk about the technologies used from simplest to most complicated.

Optical Touchscreens

The first touchscreens used light beams to figure out where your finger was... you can think of this set up a bit like a garage door sensor. There's a light on one side of the screen and a light detector (photodiode) on the other side. You line up a bunch of these in a grid on the top and the bottom... and then BAM. You have a device that can figure out where something was put in front of the screen... Like your finger. But these were bulky, and you could trigger a touch without even touching the screen... which isn't so fun to use.

Resistive Touchscreens

The next one I'd like to talk about are resistive touch screens. This is the kind I first had experience with, though a giant projector screen called a SMART board. You probably use these at ATMs and other things like that... they typically feel mushy.

Context

What makes this one more complicated than optical touchscreens is the addition of time and transparent conductors into the equation. Remember that a computer can do things billions of times faster than you can... including measuring voltages on wires. Most touchscreens can scan the screen about 200 times per second, and each scan is a whole process with very short steps.

Second, let me first remind you that transparent, printable, electrical conductors exist, and they're used by resistive touchscreens. This is going to be important for you to understand, so you don't think "wHERe aRE thE wIReS?"

Every resistive touchscreen has at least two layers: a flexible one, closer to your finger, and a rigid one, right above the screen. You push on the flexible layer to register a touch.

Digital Resistive Touchscreens

You can think of digital resistive touchscreens as having a grid of lines on each of they layers - one layer has lines going one way, the other has them going 90 degrees to the first layer. The lines on the base are typically driving lines and the screen dumps electrical current into them, and the lines on the flexible layer are sensing lines, which means each one is connected to something like an ohm meter.

The screen will rapidly turn on and off each driving line sequentially. When you touch your finger to the surface of the screen, you make contact between the flexible and rigid layer and connect a driving and sensing line together. When the driving line directly under your finger turns on, the screen will read the sensing lines and determine that you touched the screen. And it knows where you touched it, because it knows which driving line was on when it saw the touch, and which sensing line picked up an increase in current. The intersection of those two lines must be where you touched the screen.

This technique can work for multitouch, but usually instead of a grid created by lines, the screen will instead be divided into cells.

Analog Resistive Touchscreens

However, digital resisitve screens are expensive. Enter analog resistive screens. Now there is no grid. Just a single uniform layer of conductor across the layers.

But how does that work? The key is that the conductor on the layers is also a resistor, which means that if you apply a voltage to one side of the layer and siphon it off at the other side, you create a uniform voltage gradient across the layer, so the voltage is different at every point and can be measured.

The arrangement of this screen is to have a driving wire and a sensing wire on opposite ends of the screen, on each layer. They're still rotated by 90 degrees.

When you turn on the driving wire on the layer closest to the screen, you create that voltage gradient. When you touch the layers together, you'll send voltage up to the outer layer, which is then measured by the outer layer's sensing line. The controller determines if the voltage sense exceeds the threshold, and if it does, it stores it.

Then, the controller switches things up. Now voltage is applied to the outside layer, and the inside layer starts sensing. Then, based on the voltages detected, it does some math to figure out where in the voltage gradients you touched.

It's pretty genius in theory. No complicated grid to print onto plastic, which means less money... But it does mean frequent calibration, no multitouch, and... well, I guess this is an all-resistive touchscreen flaw... It just feels cheap.

Capacitive Touch Screens.

Steve Jobs complained about an iPhone prototype because it had a plastic screen. I don't know if the prototype had a resisitve screen, and Apple certainly didn't invent capacitive screens, but either way, Jobs insisted that the plastic had to go. But how do you make a mobile touch screen when you can't use plastic to make a resistive screen?

Capacitors

Capacitors are a little... and I do mean little... like batteries. They hold electrical energy. But while batteries use chemicals to store energy, capacitors literally hold electrons in an insulator between two conductors... essentially, it's static electricity. Capacitors generally hold it for a short time and are pretty leaky.

In theory, you kind of become a capacitor when you build up static electricity. Your socks (or shoes) are the insulator, and the ground and your skin are the conductors. When you touch something connected to the ground, you usually yelp... but you are also rapidly discharge that electricity that you stored by being part of a capacitor.

Capacitive touch screens exploit the fact that your skin is a conductor. When you touch the screen, you create a tiny capacitor with some invisible electrodes underneath the surface of the glass. The screen detects that capacitor and registers a touch. But that detection gets a little crazy.

Detecting Capacitance Changes

There are a few ways to detect capacitance along a wire (which is basically what the sensors are under the screen). I'm going to cover two popular ways: drainage and radio frequency changes.

The simplest way to detect a capacitor is to give a wire a certain amount of energy, and detect how long it takes for that energy to fall down to zero volts. In practice, this isn't as common because it adds another delay to figuring out where a screen is touched.

The more common way is to apply alternating current to the wire. There are two tell-tale signs of capacitance here: if there is a capacitor along the wire, then the phase (timing) of the signal will change. Capacitors are greedy and taake time to fill up with energy. This adds a delay, and there are ciruits that can pick that up.

Additionally, any capacitors along the wire will oppose voltage changes. If the voltage is constantly changing, like it is with alternating current, the capacitor will essentially modify the frequency of the output.

Mutual Capacitance Screens

Mutual Capacitance screens are made of a grid of wires, like digital resistive ones. One layer contains driving lines, with A/C current, while the other contains sensing lines. They're separated by an insulator again, and every second, they switch on one of the rows while detecting capacitance on all the columns.

When nothing is touching the screen, the active line in the driving line layer creates a capacitor with the sensing lines. The phone knows what the capacitance is normally.

When you put your finger on the screen, you modify the capacitance at a particular point, and when the screen turn on the row under your finger, the column underneath will detect an abnormal capacitance. The screen will then register a touch there.

The timing aspect allows the screen to register multiple touches accurately. Even if you have your finger along the same driving line, two distinct sensing lines will still pick up abnormal capacitance.

Self Capacitance Screens

These work a lot like mutual capacitance screens, but think of each row and column as acting indepently. They're always detecting capacitance, but with the phone itself as the other capacitor; not another row. Again, when you touch the screen, it registers an abnormal capacitance on the screen at a certain row and column.

However, these suck at multitouch, and most touchscreens are mutual capacitance screens that act as self capacitance only when the screen is off.

Summary

Most screens out in the world today are capacitance screens... like the one on your phone.

• Most touchscreens include a layer of driving lines and sensing lines.

• Turning on one driving line at a time allows you to sense the position of the touch on the X axis, and sensing on all columns at the same time on the Y axis. Turning on one line at a time also allows you to do multitouch.

• Capacitance is detected by changes in frequency of an oscillator or attenuation of an AC signal.

The Significance of this Article Being Posted Now

This is the first article I've originally written on this website since 2021. The Andy Explains Tech vision has lived on for 5 years... well... until I accidentally fried my server and then re-established it on my personal page... but yeah.. you get it. So my writing skills might be rusty, my grammar worn after 5 long years of corporate jobs and college classes, and my explanations slightly AI-ified... But I'm going to do my best.

This incarnation is going to have higher quality content. Moving away from the legacy PHP CMS I wrote will allow me to add videos and more interactive content too.