Standard keypads allow users to press keys or enter codes to control equipment and machinery. They use several basic switching technologies. Direct membrane keypads consist of a membrane painted with conductive ink, a spacer, and a substrate. Holes punched in the spacer are arranged so that pressing a key deflects the contacts on the membrane to complete the circuit on the substrate. Indirect full-travel membrane keypads mount keys above three layers of punched, flexible polymer sheets that are sandwiched together. Circuits are silk-screened with a conductive silver ink on both the top and bottom membrane layers. A middle spacer separates the two circuits and holes punched in the top spacer are arranged so that pressing a key deflects the top circuit to connect with the bottom circuit. Carbon pill keypads contain protruding, pill-shaped covers that are molded over the top of each key. When a key is pressed, the conductive pill actuates a pad trace. The average life for a carbon pill exceeds 5 million actuations. The contact resistance is usually less than 200 W.