Touchscreens

Touchscreen in action!

(By, Kevin Le)

It is hard to believe that less than a decade ago, touchscreens were rare in our society.  Nowadays, almost everyone in first world nations has a device with a touchscreen – be it a tablet or a phone.  Yet, there is a lack of understanding in today’s world as to how exactly touchscreens work.  There are three different types of touchscreens: resistive, capacitive, and surface acoustic wave.  Although many believe touchscreens work by sensing heat in some way, it actually works similar to the way computer mice work.

Resistive System:

The resistive touchscreen consists of a normal glass layer that is covered with a conductive layer and a resistive layers, which are held about by spacers.  And a third scratch-resistant layer is placed on the top of it all.  When the touchscreen device is on, an electrical current runs between the conductive and resistive layer.  When a user touches the screen, they apply a slight amount of pressure which pushes the two layers together.  In turn the electrical field is altered and the coordinates of the point of contact are taken into account.  When the coordinates are determined, a special driver interprets the information into a way the operating system can understand, much like a computer mouse driver interprets the mouse’s movement.  However a resistive system tends to have a low clarity, and only emits about 75% of light on the monitor – but resistive systems are the cheapest of the three systems, on the other hand.

Capacitive System:

In a capacitive touchscreen, a layer that stores electrical charge is placed on the glass panel of the monitor.  Because of how electrons move, when a user touches the layer, some of the electricity is transferred to the user, so the capacitive layer decreases.  Circuits at each corner of the monitor measure this point of contact, and a computer calculates the movement of contact in relation to the calibration of the four circuits.  Unlike a resistive system, a capacitive system can emit 90% of the monitor’s light.

Surface Acoustic Wave System:

In a surface acoustic wave touchscreen, two transducers (one receiving, one sending) are placed along the x- and y- axes of the monitor’s glass plate – much like how a track-ball mouse is set up with x- and y- rollers.  Reflectors which sends electrical signals between each transducer are also placed on the monitor.  The receiving end of the transducer is easily able to determine touch movements nearly instantaneously by measuring the disturbance of wave frequencies being sent by the other transducer.  Because there are no metallic layers in surface acoustic wave systems, they emit 100% of the monitor’s light and allow for perfect image clarity.  But detailed graphics come up with a price, and surface acoustic wave systems are the most expensive of the three systems.

Sometimes people may discover that they can use a touchscreen with their bare hands, but that nothing happens when they are wearing gloves.  This is so because each type of touchscreen system requires different stimuli to register touch events.  The resistive system works as long the conductive and resistive layer make contact, so just about anything can be used with a resistive system.  The conductive system requires a conductive input, so a finger will work, but a piece of rubber will not.  The surface acoustic wave system works similarly to the resistive system, except hard and small objects like the tip of a pen will not work on surface acoustic wave systems.  In the end, for something so dominating and revolutionary in today’s society, touchscreens are surprisingly simple and even bare several likenesses to the humble computer mouse.

Posted in How Things Work