What keyboards do
Let’s first achieve an understanding of how regular keyboards work. Every key is a switch. The switch itself is made of two conductive pieces of plastic; one is the key itself and the other is built into the keyboard. When a key is pressed, the two plastic surfaces touch and complete the circuit. This allows a current to flow. Every key allows a different current to flow so the computer doesn’t mistake one key for the other. A piece of rubber in between the plastic pieces bounces the key back to its original position, allowing the process to start again.
Types of touchscreens
Resistive touch screens work in the same way as typical keyboards. When you press down on the screen, you are pressing down on a layer of plastic. The two layers of conductive plastic underneath the surface layer and on top of the glass are separated by insulators and spacer dots. Pressing down on the surface causes these two surfaces to touch and complete a circuit. A chip inside the glass reads the coordinates of where you pressed and interprets it. It can only read one point at a time, making special functions like two-finger zooming impossible for resistive touch screens.
There are two main types of capacitive touch screens: projected and surface. Both operate under the same principle, but they differ in execution.
Underneath the layer of plastic and above the glass lie two networks of electrodes. These electrodes generate an electric field. When you press on the screen, the charge in the atoms of your finger alter the electric field and change the currents in multiple electrodes at once. A chip reads the ratios between these currents and interprets them. This is how touch screen phones work.
The screen itself is constructed as a capacitor: there are top and bottom conductive layers with an insulating layer in between them. Voltage is applied at the four corners of the screen to generate a uniform electric field. When you touch the screen, electricity transfers to your body and causes a voltage drop in the point of contact. If you are wearing gloves, however, the fabric acts as an insulator and the screen cannot transfer electricity to your finger. A chip reads the coordinates of this voltage drop and interprets it.
Infrared (Frustrated Total Internal Reflection)
Why aren’t touch screens bigger? Because as the screen get larger, the electricity used to sense touch get in the way of, well, sensing touch. The electricity essentially interferes with itself. Infrared screens, though, can be much larger. Panasonic has even developed a 103 inch (diagonal) infrared touch screen! (Click here for a video on it).
These screens work by utilizing numerous infrared LED lights distributed around the screen. The light is shone at the perfect angle to cause something called total internal reflection. At most angles, light would just pass straight through the screen, but at the right angle, it’ll reflect back into it! The word “frustrated” in FTIR refers to the user interrupting the reflection. When you place your finger on the screen, it interrupts the reflection and the light scatters off of your finger. The cameras on the back of the screen read this scattered light and interpret it.
Surface Acoustic Wave
These screens use ultrasonic sound waves. The sound waves are produced by the transducers, then reflected, and finally absorbed by receivers. When you touch the screen, you are interrupting the sound waves and absorbing some of the sound’s energy. This lowers the amplitude of the sound. Sensors detect this and the device interprets it.
Inductive screens generate a weak magnetic field and require a specially made stylus to operate. The stylus changes the magnetic field at its specific location, using induction to produce a current which the device can interpret. Most tablet PCs and drawing tablets use inductive touch screens.