Learning Resources

Keyboard Technologies

Keyboards use a variety of switch technologies. It is interesting to note that we generally like to have some audible and tactile response to our typing on a keyboard. We want to hear the keys "click" as we type, and we want the keys to feel firm and spring back quickly as we press them. Let's take a look at these different technologies:

• Rubber dome mechanical
• Membrane mechanical
• Capacitive non-mechanical
• Metal contact mechanical
• Foam element mechanical

Probably the most popular switch technology in use today is rubber dome. In these keyboards, each key sits over a small, flexible rubber dome with a hard carbon center. When the key is pressed, a plunger on the bottom of the key pushes down against the dome. This causes the carbon center to push down also, until it presses against a hard flat surface beneath the key matrix. As long as the key is held,. the carbon center completes the circuit for that portion of the matrix. When the key is released, the rubber dome springs back to its original shape, forcing the key back to its at-rest position. Rubber dome switch keyboards are not expensive, have pretty good tactile response and are fairly resistant to spills and corrosion because of the rubber layer covering the key matrix.

Another type of Switch Technology is Membrane Technology. Membrane switches are very similar in operation to rubber dome keyboards. A membrane keyboard does not have separate keys though. Instead, it has a single rubber sheet with bulges for each key. You' may have seen membrane switches on many devices designed for heavy industrial use or extreme conditions. Because they offer almost no tactile response and can be somewhat difficult to manipulate, these keyboards are seldom found on normal computer systems.

Capacitive switches are considered to mechanical because they do not simply COI circuit like the other keyboard technologies. current in these switches constantly flows all parts of the key matrix. Each key is sprin: and has a tiny plate attached to the botto plunger. When a key is pressed, this plate i very close to another plate just below it. A plates are brought closer, it affects the a current flowing through the matrix at that r processor detects this change and interpr' key press for that location. Capacitive keyboards are expensive, but do not Sl corrosion and have a longer life than keyboard. Also, they do not have prob bounce since the two surfaces never come contact.

Metal contact and foam element ke1 not as common as they used to be. Me switches have a spring-loaded key with a st on the bottom of the plunger. When pressed, the metal strip connects the two circuit signalling the pressing of key to th When the key is released it bounces back ( attached to it. The foam element switd of the same design as metal switches, but piece of spongy foam between the bo plunger and the metal strip, providing tactile response. Both technologies havt response, make satisfactory audible "cli less expensive to produce. The probler contacts tend to wear out or corrode f, keyboards that use other technologies. no barrier that prevents dust or liquid in direct contact with the circuitry of the key matrix.