Capacitive vs Resistive Screens
There are many types of touch screens, we will be focusing on capacitive and resistive screens. Resistive screens are cheaper, and require more pressure or the use of a stylus to work, while capacitive screens are more expensive and are generally very responsive. Examples of Capacitive screens are phones, while resistive screens are used in ATMs.
Generally, resistive screens have two layers of material separated by a thin layer of air, with the top layer (the one that the stylus comes into contact with) being a flexible poly-carbonate layer (such as PET film), and the bottom layer being a rigid material (such as glass). These two layers are lined with a conducting material, with a gap of air separating these layers. Below the rigid material, sensors are placed to detect a change in voltage. When users press their finger against the screen, there is a change in the resistance, as the conducting materials come into contact, hence the sensors detect this change and calculate the coordinates of this point.
On the other hand, capacitive screens make use of the fact that our fingers are electrical conductors. They have a layer of capacitive material to hold an electrical charge, and touching the screen actually changes the amount of charge at a specific point of contact, as our skin conducts electricity at a rate different to that of glass or other materials used in the screen. When you touch a screen, your finger disrupts the electric field, changing the electrical capacitance (amount of charge that can be stored in an object per unit charge in electrical portion) of that portion of the screen. The system then detects the location of that touch.
However, for iPhones, being able to detect one touch alone is not enough. The iPhone has a multi-touch user interface that allows users to touch multiple points on the screen simultaneously and perform various actions. This means that the iPhone has a different system. Besides the capacitive system mentioned above, the iPhone’s capacitors are arranged according to a coordinate system. Every point on the grid then generates its own signal when touched and relays that signal to the phone’s processor, allowing the phone to determine the multiple touches and movements.
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