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Capacitive Keyswitches

All of the other keyswitch technologies described in this section are contact designs. They all work using different ways of causing physical contact to establish a circuit and register a keypress. There is one keyboard technology, however, that detects keystrokes without using any form of contact at all.

The capacitive keyswitch design makes use of a design characteristic of capacitors to determine when a key has been pressed. A capacitor is an electronic component that is comprised (at least conceptually) of a pair of parallel metal plates. When an electric field is applied to the plates, a charge is stored there. You can read more about capacitors on this page discussing basic electrical components.

Without getting into too much unnecessary detail, the capacitance of a capacitor or circuit can be measured, and is dictated in part by the amount of space between the two plates of the capacitor. This means that if you change the amount of space between the plates, you change the capacitance of the circuit... I am sure you can see where this is leading...

Capacitive keyswitches are structurally similar to foam and foil switches. A plunger with a spring wrapped around it has a plate attached to the bottom. A second plate is fixed in position below the first one. As the keycap is pressed, the plunger moves down and the space between the plates is reduced. A special circuit detects the change in capacitance that occurs as a result, and thus senses the keypress. (Some designs reverse the mechanics: when you press the key it separates the plates instead of moving them closer together. All that matters is that the distance changes, one way or the other.)

You might think that this is a superior way of sensing a keypress, and that this must be a new and revolutionary design. In fact, it's one of the oldest designs. It was conceived as a solution for the problems experienced with contact keyboards during the 1970s and prior decades, when contact keyswitch reliability was often poor. This is of course the most important advantage of the capacitive technology: no physical contact is made between the sensing elements. This results in very long lifespan--typically tens of millions of keystrokes. Also, issues with dirt, contamination and corrosion are almost unheard of, because there's no need for a circuit to be closed to make a keystroke. As a result, the annoyances associated with these problems, such as "bounce" (multiple keystrokes for a single keypress) or unrecognized keystrokes, are very rare.

Of course, there's no free lunch, and there are drawbacks to the capacitive design as well. The first is that since there is no contact, there would be no tactile feedback at all with such a mechanism unless it is specifically added in to the design. This is in fact what keyboards using the capacitive approach do: they add a mechanical or electromechanical device to create a tangible "click" when each key is pressed. This results in capacitive keyboards having very noticeable tactile feedback: some would even say it's too noticeable (especially those accustomed a more modern, typical rubber dome contact keyboard).

The other major issue with these keyboards is that they are expensive. This should not come as a surprise given everything I have just described. They are typically found only in very high-end or specialty applications today. You can actually find them more often in very old "true blue" IBM keyboards from the early 1980s.

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