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Publisher19.1 Keyboard Switch Types
Underneath each key is a switch. When
the key is pressed, the switch closes. Three types of switches are
used in keyboards:
These keyboards use standard spring-loaded momentary-on switches,
most of which are made by Alps Electric. Pressing a key compresses a
spring and causes a plunger on the bottom of the key to make physical
contact to close the connection. When the key is released, the spring
forces it back into rest position. These keyboards provide the clacky
feel typical of the original IBM Selectric and PC keyboards, are
quite durable, and are usually relatively expensive ($75 to $125).
These keyboards are unique in that pressing a key does not make
electrical contact to complete the circuit. Instead, movement of the
plunger on the bottom of a key alters the state of a capacitive
circuit, which the keyboard controller recognizes as a key press.
Keyboards with capacitive switches provide clacky feedback, are even
more durable than mechanical keyboards, and are quite expensive ($100
to $200). As far as we know, only IBM and Lexmark have produced such
keyboards.
Most current keyboards use membrane switches. Unlike mechanical and
capacitive keyboards, which use discrete physical switches for each
key and are correspondingly expensive to produce, a membrane keyboard
combines all key switches into one unit comprising three membrane
layers. The bottom layer has printed conductive traces that
correspond to the individual key switches. The middle layer is a
spacer, with holes that expose each underlying switch. The top layer
is an array of rubber domes, against each of which the bottom of a
key impinges. When a key is pressed, it forces the conductive bottom
of the rubber dome through the spacing layer and into contact with
the switch traces on the bottom layer, completing the circuit. When
the key is released, the rubber dome forces it back into rest
position. Early membrane keyboards were known for mushy feel and lack
of tactile feedback. Current production models are better in that
respect, so much so that it is often difficult to tell by feel alone
whether you are using a mechanical keyboard or a modern membrane
keyboard. Membrane keyboards are also inexpensive ($15 to $50), and
nearly as durable and reliable as the best of the mechanical and
capacitive keyboards.
Switch type as it related to durability was an important factor when
keyboards cost $200. With high-quality membrane keyboards now selling
for $25 or so, that distinction is much less important.
