Membrane Keypads
A membrane keypad or keyboard is one where the keys are not separate, moving parts. Instead, they have outlines and symbols printed on a flat, flexible surface. Membrane keypads function by electrical contact between the keyboard surface and the underlying circuits when key areas are pressed. Two major benefits of membrane keypads are that they are very inexpensive to mass produce and are more resistant against dirt and liquids than most other keypads.
In general, membrane keypads consist of three layers. The top and bottom layers contain conductive traces. The center layer, called a spacer, is non conductive and contains holes wherever a key exists. Its function is to keep the other two layers separate. The key or switch is open because current cannot cross the non-conductive gap. However, when the top layer is pressed down causing it to make contact with the bottom layer, the circuit closes. The conductive traces on the underside of the top layer meet those of the bottom layer, allowing current to flow. This is when a membrane keypad registers a keystroke.
Membrane keyboards and keypads have a variety of applications including, PDAs, pocket computing devices, simple handheld calculators, access control systems, remote controls, and similar devices. Typically, they are used in applications where the amount of typing is relatively small or infrequent because most people have difficulty typing with them due to the low or non-existent amount of tactile feedback they provide.