Touchscreens are everywhere.
Not just in smartphones, but in supermarkets, ATMs, and even airplane seats.
And you may have noticed that not all touchscreens are the same.
The old-school touchscreens can be pretty tough to use -- sometimes it feels more like
a push-really-hard-screen instead of a touchscreen.
On the other hand, certain smartphones and computer monitors are really responsive to
many different touch patterns.
There are lots of different technologies out there, but they’re all trying to achieve
the same goal: sending precise electrical signals from specific locations on the screen.
One of the most widely used types is the resistive touchscreen, where you have to physically
push and bend the screen to make it work.
Resistive touchscreens are made of two separate layers:
The top layer is made from a flexible and transparent material, such as polyethylene,
which is a common plastic used to make things like soda bottles.
And the bottom layer is made of something more rigid, like a sheet of glass.
To make the screen work, both of these layers are thinly coated with some sort of metal
compound that conducts electricity, like indium tin oxide -- which is commonly used because
it’s transparent.
These layers are also separated by tiny insulating dots, which /don’t/ conduct electricity,
called spacers.
They keep the screens apart to make sure there aren’t any false touch signals.
When the screen is on, a small voltage is applied across the screen in both the horizontal
and vertical directions.
As soon as you push down on the flexible screen with anything, like your finger or a stylus,
it connects the two layers together.
This changes the voltage, and a small processor connected to the screen can calculate exactly
where you pressed in X and Y coordinates.
These resistive touchscreens are pretty affordable and durable.
So, they’re useful for things like credit card readers in grocery stores, where you
need to capture touch data of a messy signature -- over and over again.
But they can be a little frustrating to use if you don’t push hard enough.
Plus, they normally can’t understand multiple-touches at the same time – so they’re no good
for two-finger zoom or more complex tasks.
That’s why these days, most smartphones rely on capacitive touchscreens, where your
finger becomes a key part of the electronics.
There are different kinds of capacitive touchscreens, and they can vary from device to device.
But one basic design is a sheet of glass containing a grid of hair-thin lines of a conductive
metal, like indium tin oxide.
The grid lines in one direction are called the driving lines, which provide a constant
electric current.
And the lines in the other direction are called the sensing lines, which detect this electric
current.
At every point where the sensing lines and the driving lines cross, there will be a specific
electrostatic field, which is registered as neutral by the processor in your smartphone
or computer.
But that all changes when something conductive comes along and touches it -- like your finger.
See, the human body has a natural capacitance, which means our bodies can conduct electric
current, and can store electric charge.
So when your finger touches the screen, the charge in the screen is drawn around that
point, distorting that electrostatic field.
The electricity doesn’t actually /flow/ through your finger.
Basically, the electrostatic field feels the effects of your electric charge and redistributes
itself accordingly.
Even really small changes are detected by the processor, which can then interpret the
patterns you’re making – whether it’s a tap or a slide.
Because the lines of the grid are so thin, capacitive touchscreens are super accurate,
and some versions can process multiple touches at a time.
But they won’t work if you have gloves on -- because the cloth isn’t conductive, unless
your gloves have those special fingertips with metal fibers inside.
Plus, something like sweat can affect how electricity is conducted across the screen,
because it’s full of salts.
It’s all about the materials that can affect the electrostatic field generated inside your
screen.
So next time you’re texting on a smartphone or scrolling through internet forums on a
tablet, just remember: you’re actually a part of the electronics making it work.
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