How Do Touchscreens Work?

Touchscreens are everywhere, and they’re a big part of people’s daily lives. Every smartphone on the market has one, and they’re popping up in cars and appliances now too. How do they actually work, though?

There are quire a few touchscreen technologies out there, but two are more common than the rest. One is becoming somewhat of a legacy technology while the other has become the single most dominant implementation.

iPad Touchscreen

Resistive Touchscreens

Resistive touchscreens were the first major way touchscreens were made. Most earlier mainstream touchscreen devices used resistive touchscreens, and chances are, if you have a single-touch screen, it still does.

Construction

Resistive Touchscreen

Resistive touchscreens are made of three layers. The bottom layer is a piece of glass with a grid of conductive film. Then, there’s a very thin gap of air. On top is plastic film that also has a clear grid of conductive material. Wires leading from the glass layer run to a microcontroller which can interpret interaction with the screen and feed that information to the device itself.

How It Works

When you touch the screen, you’re pressing the plastic film into the glass. The conductive grids on each surface meet and complete a circuit. Different positions on the grid produce different voltages. Those voltages are then passed to the screen’s controller which uses the voltage to interpret the position on the screen that was touched and pass that along to the device.

Downsides

Resistive touchscreens are analog. They rely on measuring changes in voltage. These screens also require “moving part.” The physical position of the conductive layers matters, and they can drift over time, resulting in inaccuracies and re-calibration.

Resistive screens tend to be less responsive and less durable because of their their construction too.

Capacitive Touchscreens

Capacitive touchscreens are the answer to their resistive predecessors. These are the current frontrunners in the touchscreen world. With capacitive touchscreen came multitouch screens.

Capacitive touchscreens have a few other names, in case you encounter them. People also call them projected capacitance, pro-cap, or p-cap screens.

Construction

Capacitive Touchscreen

Capacitive touchscreens have similar parts to resistive screens, but they have a few key differences. They have a thin glass base with a conductive grid. In the middle, there’s an ultra thin layer of non-conductive material, usually glass. Then, on the outside, is another rigid conductive layer with a grid of conductors. Of course, there are also wires running off of the base with a controller that connects to the device.

How It Works

Capacitive touchscreens work like capacitors. They store a charge. That charge is minimal, though. When your finger comes into contact with the top conductive layer, it competes a circuit and the charge discharges into your finger. That same connection allows to the charge to arc into the bottom layer and be measured there too.

The controller can use the conductors and their positioning as well as the magnitude of the electrical activity to measure your interaction with the screen. Because these touchscreens can measure the activity of each capacitor separately, they can interpret multiple touches at the same time.

Downsides

Capacitive touchscreens have much more minimal drawbacks, but they’re still there. First off, they can be influenced by electromagnetic interference. If there is a strong enough electromagnetic field generated by another electronic device or even a component of the same device, the screen may read erroneous input.

Because these screens read all of their capacitors individually, they can receive too much input. When your face or palm hits your phone’s screen, it gets slammed with a load of input data. That phone then has to determine whether it should try to act on it all or discard it. That requires additional system resources.

Closing

Touch screens are convenient, and they’re a part of just about everyone’s daily lives. Though they might seem like magic, there are some fairly basic electronic principles at play.