Membrane keyboard

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 can be 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.

The expression is also sometimes used in connection with modern PC keyboards which utilise dome switch technology. Although the lower layers in some dome-switch keyboards are essentially the same as the membrane keyboard, the dome-switch keyboard includes additional upper layers (rubber domes and solid key-caps).

How it works

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.

List of computers with membrane keyboards

QWERTY layouts unless otherwise specified

• Atari 400 (full-travel replacement keyboards much like those that came with the Atari 800 were available)
• Cambridge Z88 (arguably a mix between a membrane and chiclet keyboard)
• Elektronika BK-0010, in early versions (it was a Russian home computer, hence the JCUKEN layout)
• Magnavox Odyssey², also known as the Philips Videopac G7000 video game console
• OLPC_XO-1, an inexpensive laptop computer intended to be distributed to children in developing countries around the world
• RCA COSMAC VIP, a do-it-yourself (DIY) kit computer with 16-key hex keypad
• Robotron Z1013, Eastern-German home computer (alphabetical layout)
• SEGA SC-3000, a computer version of an early SEGA game console
• Science of Cambridge Microcomputer Kit 14 (MK14); the initial version had a 20-key extended hex keypad
• Sinclair ZX80, a pioneering British home computer that was available as a DIY kit
• Sinclair ZX81 (similar to the preceding entry)
• Sinclair ZX Spectrum+
• Synertek SYM-1, a sibling of the MOS/CBM KIM-1 with a 29-key extended hex keypad
• Timex Sinclair 1000 (a U.S. version of the ZX81)

See also

• Chiclet keyboard
• Keyboard technology