You see them practically everywhere nowadays; those hand held devices that you use by touching or sliding your fingers around. They’re on iPads and iPods smart-phones and blackberries and countless other gadgets. But how do they work? How do they know where you’ve touched them or how hard, or if you’re trying to drag something across a screen?
The answer is, they use a technology called a capacitance system. What they do is have a transparent layer of material between the images you see and the plastic surface that is literally alive with electricity. It is kept in a constant state of charge; so much so that when you press your finger, or multiple fingers against the screen, a tiny amount of that electrical charge is transferred from the screen to your finger. The charge is so small you can’t feel it obviously, but it can be measured, and that’s the key to understanding how this technology.
The devices also have sensors, generally located just below each corner of the screen that measure the change in electrical charge across the middle layer. Because there are four sensors, a microprocessor or electronic “brain” behind the screen can use the information that is constantly being sent to it from all four sensors to figure out within a hundredth of an inch just exactly where you touched the screen. And as if that wasn’t enough, the processor can also measure both the quickness and duration of the touch to figure out when you’re using the screen to make choices or to select things. It’s all rather clever, but no that original, touch screens have been around for decades, it’s just now that micro-circuitry has caught up to it that allows for touch screens to be used in such small devices.
There’s a little more to the story here though, because micro-electronics can’t do it all on its own, there has to be something more. That thing is the software that is written and embedded on chips in the device that interprets the signals that are received and does the things that seem smart to us, like make a choice on a screen or allow us to resize things we’re looking at, or drag them across the screen.
And the screen itself, is generally little more than a whole lot of very tiny Light Emitting Diodes (LEDs) that are also controlled by circuitry that is driven by software.
So, that’s how it’s done. The real magic is in the micro-circuitry, but that’s an article for another time.