ELI5- How does your phone know when you touch the screen?
In: Technology
There’s a couple different types of touch screens.
One screen senses the electric current. Your body conducts electricity, so your phone senses the current and reacts accordingly. It’s why you can’t use another object like your fingernail or a stylus on something like an iPhone.
There are some screens where your touch interrupts the flow of electric current. These are typically the screens that allow non-human use such has a stylus.
Most older touch screens use what’s called a resistive touch. There’s basically two layers and when you apply pressure to the top layer it causes it to bend and physically make contact with the second layer. The second layer always has electricity flowing through it and so when you push the layers together the top resistive layer changes the resistance in the circuit and causes a voltage drop at the point of contact. Your phone defects that change and converts it to a coordinate. Resistive touch is nice because it’s cheap and works with anything that can touch it, but it’s got two main problems. The first is that it can only pick up one touch at a time, which just isn’t enough for a lot of features like two finger zoom. The second is that you have to physically bend the outer screen, which distorts the picture beneath it. Especially on a smaller screen like a phone, that’s not good because it makes it harder to see what you’re doing.
Newer phones use capacitive touch. These have a really tiny mesh of wires that are too small to see and have small currents flowing through them. When you touch a point on the screen you basically create what’s called a capacitor which is two conductive materials (the wire and your finger) spaced out by a non conductive material (the glass). This changes the capacitance in the circuit which creates a measurable voltage change that sensors can detect. Capacitive touch is more complicated to make, but it’s more precise, can recognize multiple touches, doesn’t require physically bending the outer layer, and can work with things like force touch. The downside is it only works with things that are conductive, like your finger or special styluses.
There are other kinds of touch screens that measure things like how the light or sound changes when you touch them or even measure the vibration in the device to try and figure out how they were touched, but you don’t really see those in phones and they’re in general not as good.
What you see as the surface of your phone screen is not just a simple piece of plastic or glass. It is actually a transparent sensor that can detect when something touches the surface and can tell which point on the surface of the sensor is touched.
This sensor sends the touch information to the processor in the phone and the software running decides what to do with the information.
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