Since the human body is a conductor, when a human finger approaches an electrode embedded in a Projected Capacitive Input (PCI or PCAP) touch screen, the capacitance between that finger and the electrode will be changed. This makes it possible for the controller to locate the touch position through a capacitance change in the trace structure. Theoretically, the force necessary to operate projected capacitive touch screens is zero because the changes of capacitance can be detected when a conductor is close enough to its surface. In addition, the surface material for PCAP touch screens is glass, which is highly scratch-resistant, durable, and reliable in harsh environments.

The operational theory of a PCAP touch screen begins with two patterned Indium Tin Oxide (ITO) layers under a glass substrate cover which create an X-axis and Y-axis electric field. These electric fields project above the glass surface between adjacent ITO traces. When a finger approaches the glass surface, a new balance in the electric field will be established between the finger and the corresponding X-axis and Y-axis. The controller IC will locate the ITO traces exhibiting capacitance changes to pinpoint the location of finger touch accurately.

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