How does the iPhone touch screen work?

Have you ever wondered how your iPhone can understand the touch from your finger and interpret it into controlling your phone yet can’t do the same if you are wearing gloves?

To figure this out we need to understand how your iPhone screen works.

Most people look at the screen and assume it’s just a layer of glass on top of a LCD (Liquid Crystal Display), but actually on top of the glass screen are a series of technical layers .

The materials used in these layers which include indium tin oxide are not actually transparent (or see through), but they are so thin (nanoscale), that your eyes can’t see them and so it looks like the screen is just made out of one piece of glass.  Typically you can’t see anything that is thinner than 0.1mm (100 microns or 100000 nanometers) with your naked eye and so all the technology sitting on top of the glass layer is invisible.

The diagram below shows all of the layers that sit on top of the glass part of the iPhone to give it the interactive touch screen properties.

The technology that the screen uses is called mutual capacitance, which involves two layers, a driving layer and a sensing layer arranged in a grid pattern.

The grid pattern enables a coordinate system to be put in place, so that the location of each individual capacitor is known.

The driving lines provide the current, the sensing lines detect the current and in between the two is an insulating layer which stops the charge spreading from one coordinate to another.

When the screen is not being touched, the driving lines provide a constant current that is stable and registers as a neutral charge state.

The human body has a natural capacitance, which means we are all electrically conductive and can store electric charge.

You are probably only aware of this conductivity when you’ve experienced a static shock on a cold, dry winters day, caused by releasing a build up of charge from your body.  However the charge that you naturally have is what allows you to control your iPhone.

When you touch the screen of your iPhone with your finger the sensing capacitance layer interacts with your natural body capacitance and this changes the electrical charge at that interaction point.  The change in charge is registered as electrical impulses and the iPhone’s processor interprets the size, shape and location of the touch.

The software within the iPhone is designed to interpret the touch patterns and differentiate between a single touch, a sliding touch, or a pinch touch – all of which us iPhone users are experts at depending on whether we want to type, zoom or flick through facebook photo’s.

So why doesn’t your iPhone work when you are wearing your winter gloves? Well gloves are typically made from non-conductive materials, which are materials that do not easily allow electric charges to flow.  Wool is an insulator of both heat and electricity which is good for keeping your hands warm but not for allowing your natural electric charge to transfer from your fingers to your iPhone screen.  The solution?  You can now buy special gloves that have conductive tips, or you can do what I do in the winter and use your nose 🙂

 

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