Mutual capacitance is now the more common projected capacitive approach and makes use of the fact that most conductive objects are able to hold a charge if they are very close together. If another conductive object, in this case, a finger, bridges the gap, the charge field is interrupted and detected by the microcontroller.
The resistive touch screen consists of a glass layer with a conductive coating on top and a polyester top sheet with a conductive coating on the bottom. The conductive surfaces are held apart by “spacer dots”, usually glass beads that are silk-screened onto the coated glass. On a 5-wire resistive design (the most commonly used kind of resistive screen in large format POS applications), a voltage is applied to the 4 corners of the glass layer. When a person presses on the top sheet, its conductive side comes in contact with the conductive side of the glass, effectively closing a circuit (this is called pressure sensing). The voltage at the point of contact is read from a wire connected to the top sheet.
The surface capacitive touch screen has a conductive coating on the front surface with wires connected to each corner. A small voltage is applied to each of these 4 corners. The operation relies on the capacitance of the human body. When a person touches the screen, a small current flows to the point of touch, causing a voltage drop which is sensed at the 4 corners.