How Does a Touchscreen Work?
How a screen feels your finger: capacitive and resistive screens
Summary
Phone touchscreens are capacitive: under the glass there is a grid of transparent conductors, and each crossing acts like a tiny capacitor. Your finger, which conducts electricity, lowers the capacitance of the crossings where it touches a little; the screen measures this drop to find where the finger is. Resistive screens, used in older devices, work by pressure: when you press, two layers touch and the voltage measured gives the contact point.
In depth
In mutual-capacitance (projected capacitive) screens, vertical transmitter (TX) and horizontal receiver (RX) electrodes are made of a transparent conductor such as indium tin oxide. The controller drives the transmitters in turn hundreds of times a second and measures the signal on each receiver; because the finger pulls part of the electric field towards itself, the capacitance of the crossings under it drops. Every local peak above a threshold counts as a touch, and the weighted average of the surrounding nodes gives the position far more precisely than the grid spacing; since every crossing is measured separately, several fingers are seen apart. In a resistive screen two resistive layers are kept apart by spacer dots; when a voltage is put across one layer it becomes a voltage divider, and the voltage read by the other layer is proportional to the position of the contact.
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