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The keyboard concept—a letter for every pushbutton—is almost ancient, dating back to the days of the first typewriter. The design seems fixed in stone. Actually, it's the product of something even more immovable—human inertia. The basic layout and function of the keyboard has changed little since the last half of the 19th century, and the mold for the computer refinement was first cast in 1987. No matter what computer you buy today, you're almost certain to get a keyboard that follows the now industry-standard design. That's good because you can confront any computer and start working in seconds. But that's also bad when those seconds stretch to hours of uninterrupted typing.

All keyboards have the same function: detecting the keys pressed down by your fingers and relaying that information to your computer. Even though two keyboards may look identical, they may differ considerably in the manner in which they detect the motion of your fingers. The technology used for this process—how the keyboard works electrically—can affect the sturdiness and longevity of the keyboard. Although all operate in effect as switches by altering the flow of electricity in some way, the way those changes are detected has evolved into an elaborate mechanism. The way that technology is put into action affects how pleasant your typing experience is.

Nearly every technology for detecting the change in flow of electricity has been adapted to keyboards at one time or another. The engineer's goal has been to find a sensing mechanism that combines accuracy—detecting only the desired keystroke and ignoring errant electrical signals—with long life (you don't want a keyboard that works for six words) and with the right "feel," the personal touch. In past years, keyboard designers found promise in complex and exotic technologies such as Hall-effect switches, special semiconductors that react to magnetic field changes. The lure was the wonder of magnetism—nothing needs to touch to make the detection. A lack of contact promised a freedom from wear, a keyboard with endless life.

In the long run, however, the quest for the immortal keyboard proved misguided. Key boards rated for tens of millions of keypresses met premature ends with a splash from a cup of coffee. In the end, manufacturers opted for the simplest and least expensive, hard-contact technology, as close to a plain switch as you can get without a wall plate. The chief alternative, capacitive technology, led the pack at the start. It was more reliable, longer lived, more complicated—and for a long while, more popular. It was also more expensive. In today's dollars, the keyboard of the first computer cost more than an entire current computer system. Something had to give.


The direct approach in keyboards involves using switches to alter the flow of electricity. The switches in the keyboard do exactly what all switches are supposed to do—open and close an electrical circuit to stop or start the flow of electricity. Using switches requires simpler (although not trivial) circuitry to detect each keystroke, although most switch-based computer keyboards still incorporate a microprocessor to assign scan codes and serialize the data for transmission to the system unit.

Design simplicity and corresponding low cost have made switch-based keyboards today's top choice for computers. These keyboards either use novel technology to solve the major problem of switches—a short life—or just ignore it. Cost has become the dominant factor in the design and manufacture of keyboards. In the tradeoff between price and life, the switch-based design is the winner.

Three switch-based keyboard designs have been used in computers: mechanical switches, rubber domes, and membrane switches.

Mechanical Switches

Mechanical switches use the traditional switch mechanism, precious metal contacts forced together. In the discrete switch design, the switch under each keyboard station is an independent unit that can be individually replaced. Alternatively, the entire keyboard can be fabricated as one assembly. Although the former might lend itself to easier repair, the minimum labor charge for computer repair often is higher than the cost of a replacement keyboard.

The contact in a mechanical switch keyboard can do double-duty, chaperoning the electrical flow and positioning the keycaps. Keyboard contacts can operate as springs to push the keycaps back up after they have been pressed. Although this design is compelling because it minimizes the parts needed to make a keyboard, it is not suited to computer-quality keyboards. The return force is difficult to control, and the contact material is apt to suffer from fatigue and break. Consequently, most mechanical switch keyboards incorporate springs to push the keycaps back into place as well as other parts to give the keyboard the right feel and sound.

Although several manufacturers have built keyboards with mechanical switches, the design has fallen from favor. Probably less than two percent of all keyboards use discrete switches.

Rubber Dome

Rubber dome keyboards combine the contact and positioning mechanisms into a single piece. A puckered sheet of elastomer—a stretchy, rubber-like synthetic—is molded to put a dimple or dome under each keycap, the dome bulging upward. Pressing on the key pushes the dome down. Inside the dome is a tab of carbon or other conductive material that serves as one of the keyboard contacts. When the dome goes down, the tab presses against another contact and completes the circuit. Release the key, and the elastomer dome pops back to its original position, pushing the keycap back with it.

The rubber dome design initially won favor on notebook computers, where its resistance to environmental stress (a euphemism for "spilled coffee") was a major strength. The design also is inexpensive to manufacture. The switches for the entire keyboard can be molded as one piece about as easily as making a waffle. The design was readily adapted to desktop keyboards and provides the foundation for many inexpensive, lightweight models.

Properly designed, a rubber dome keyboard has an excellent feel—the give of the individual domes can be tailored to enable you to sense exactly when the switch makes contact. A poor design, however, makes each keypress feel rubbery and uncertain. Moreover, some elastomers have a tendency to become stiff with age, and as a result, some keys can become recalcitrant.

Membrane Keyboards

Membrane keyboards are similar to rubber domes except they use thin plastic sheets—the membrane—printed with conductive traces rather than elastomer sheets. Most designs use a three-sheet sandwich—top and bottom sheets with printed contacts and a central insulating sheet with holes at each key position to hold the contacts apart. The top sheet is dimpled, and pressing down on the dimple pushes the top contact down to meet the bottom contact. The dimple snaps from one position to another, giving distinct tactile feedback that indicates when you've pressed a key.

The membrane design often is used for keypads to control calculators and printers because of its low cost and trouble-free life. The materials making contact can be sealed inside the plastic, impervious to harsh environments.

By itself, the membrane design makes a poor computer keyboard because its contacts require only slight travel to actuate. However, an auxiliary key mechanism can tailor the feel (and key travel) of a membrane keyboard and make typing on it indistinguishable from working with a keyboard based on another technology.


The biggest problem with all contact keyboards is the contacts themselves. Although switches work well for room lights, they are fickle when it comes to the minuscule voltages and currents used by computer systems. At a microscopic (and microtemporal) level, making a contact is an entire series of events—tiny currents start and stop flowing until finally good contact is made and the juice really gets going. The brief little pulses that precede making final contact aren't apparent when you throw a light switch, but a computer can detect each little pulse as a separate keystroke. Keyboards have special circuits called debouncers that make your keypresses unambiguous. However, as contacts age, they oxidize. Oxidation puts a less-conductive layer of oxide on the contacts and makes your keypresses even less reliable—to the point they cannot be unambiguously detected.

Early in the history of the computer, manufacturers scorned contact keyboards because they knew traditional contact materials were fated to short lives, not the tens of millions of keystrokes they expect from keyboards. Modern keyboards use ceramics and exotic metals that, resistant to oxidation, can achieve the required life.

The most popular of the non-contact designs is the capacitive keyboard. Capacitance is essentially a stored charge of static electricity. Capacitors store electricity as opposite static charges in one or more pairs of conductive plates separated by a nonconductive material. The opposite charges create an attractive field between one another, and the insulating gap prevents the charges from coming together and canceling out one another. The closer the two charged plates are, the stronger the field and the more energy that can be stored. Moving the plates in relation to one another changes their capacity for storing a charge, which in turn can generate a flow of electricity to fill up the increased capacity or drain off the excess charge as the capacity decreases. These minute electrical flows are detected by the circuitry of a capacitive keyboard. The small, somewhat gradual changes of capacity are amplified and altered so that they resemble the quick flick of a switch.

The first computer keyboards and some of the more robust designs of the past all used capacitive technology. The best of these designs could work reliably for 100 million keystrokes compared to 10 to 15 million for most contact designs. Although that extended life may be commendable, most people never typed even 10 million keystrokes during the lifetime of their computers. They usually move on to another computer well before the keyboard wears out. The added cost of the capacitive design consequently yields no additional useful life, and the design has fallen from favor. The last capacitive keyboard from a major manufacturer rolled out of the Key Tronic Corporation factory in early 1999.

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