How can a flat touch button sense your finger?

Some buttons do not move when pressed. Beneath their glass or plastic cover is a conducting pad connected to sensing electronics. The pad can hold separated electric charge relative to its surroundings. The amount held for a given voltage is described by capacitance.

A finger near the pad changes the electric field and adds coupling through the person’s body to the surroundings. In this self-capacitance design, the apparent capacitance increases. The cover keeps the finger from directly contacting the pad, but an electric field can still act across it. The sensor detects a change rather than needing a metal switch to close.

Suppose the untouched pad has an invented capacitance of 10 picofarads. Pico means one trillionth; picofarads and picocoulombs describe very small capacitance and charge. At 3 volts, the rule Q = CV gives a charge scale of 30 picocoulombs. Adding 1 picofarad changes that scale to 33 picocoulombs. Real electronics detect such changes through charging and sampling circuits, compare them with a calibrated baseline and reject noise. Many touchscreens use a different, mutual-capacitance electrode arrangement; this explanation concerns one self-capacitance pad.