Membrane keyboard

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Membrane keyboard as used on the East German Robotron Z1013.

A membrane keyboard is a computer keyboard whose "keys" are not separate, moving parts, as with the majority of other keyboards, but rather are pressure pads that have only outlines and symbols printed on a flat, flexible surface. Very little, if any, tactile feedback is felt when using such a keyboard, and error-free blind typing is difficult.

Membrane keyboards, which work by electrical contact between the keyboard surface and the underlying circuits when keytop areas are pressed, were used with some early 1980s home computers, and have been much used in consumer electronics devices. The keyboards are very inexpensive to mass-produce, and are more resistant against dirt and liquids than most other keyboards, but due to the low or non-existent amount of tactile feedback provided, most people have difficulty typing with them, especially when large numbers of characters need to be typed. Chiclet keyboards were a slight improvement, at least allowing individual keys to be felt to some extent.

Aside from early hobbyist/kit/home computers and some video game consoles, membrane-based QWERTY keyboards are used in some industrial computer systems, and are also found as portable, even "rollable-collapsible" designs for PDAs and other pocket computing devices. Smaller, specialised membrane keyboards, typically numeric-and-a-few-control-keys only, have been used in access control systems (for buildings and restricted areas), simple handheld calculators, domestic remote control keypads, microwave ovens, and other similar devices where the amount of typing is relatively small or infrequent, such as cell phones.

Modern PC keyboards are essentially a membrane keyboard mechanism covered with an array of dome switches which give positive tactile feedback.

How it works[edit]

As can be seen from the diagram below, the membrane keyboard basically consists of three layers; two of these are membrane layers containing conductive traces. The center layer is a "spacer" containing holes wherever a "key" exists. It keeps the other two layers apart.

Cross-section diagram of a typical membrane keyboard. The thickness of the bottom three layers has been exaggerated for clarity; in reality, they are not much thicker than pieces of paper or thin cardstock.

Under normal conditions, the switch (key) is open, because current cannot cross the non-conductive gap between the traces on the bottom layer. However, when the top layer is pressed down (with a finger), it makes contact with the bottom layer. The conductive traces on the underside of the top layer can then bridge the gap, allowing current to flow. The switch is now "closed", and the parent device registers a keypress.

Typical applications include;

  • Industrial controls
  • Access control systems
  • Medical equipment
  • Telecommunications apparatus
  • Telephone systems
  • Household appliances
  • Security systems

Source, APEM, Membrane switch panels

Internal layers of a modern full-travel computer keyboard; bottom contact layer, spacer layer with holes, top contact layer, then elastomer top layer to provide restoring force to keytops.

List of keyboards with membrane interfaces[edit]

QWERTY layouts unless otherwise specified

See also[edit]

References[edit]

External links[edit]