Industrial 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.
Industrial keypads mount on the front or rear of panels and are made from a variety of plastic, thermoplastic, and metallic materials. There are four standard keypad sizes: 1 x 4, 3 x 4, 4 x 4 and 5 x 4. For each keypad size, the first number indicates the number of columns and the second indicates the number of buttons. For example, 1 x 4 keypads have one column and four rows of buttons. 3 x 4 keypads are telephone-style devices with three columns and four rows of buttons. 4 x 4 keypads have four columns and four by rows of buttons. 5 x 4 keypads have five columns and four rows of buttons. Electrical switch specifications for standard keypads include maximum current rating, maximum alternating current (AC) voltage rating, and maximum direct current (DC) current rating. Standard keypads with X-Y outputs are matrix-style devices. Devices with a single pole or common bus outputs require the same number of pins as buttons.
Industrial keypads are often backlit and include light emitting diodes (LED) to indicate process status, system functions, machine interlocks, and alarm conditions. Embossed switches have graphics or alphanumerical characters that are raised from switch surface. Switches, panels and keypads that are designed for outdoor use are water resistant or waterproof and can withstand both ultraviolet (UV) light and variations in temperature. Industrial keypads that are used in medical and electronics applications provide protection from electromagnetic interference (EMI), radio frequency interference (RFI), and electrostatic discharge (ESD). ESD protection may be accomplished with static-dissipative or conductive elements in the switch material, or through special electronics grounding.
