Editing Keyboard terminology (section)

==Switches==
Broadly speaking, switches can be discrete (separate units) or they may be one indivisible assembly.

The term "switch" is somewhat misleading. In most cases, discrete switches really do switch electric current on and off, but they can also use forms of electromagnetic sensing. See [[keystroke sensing]] for more detailed information on the various types of switch, including metal contact, capacitive, and Hall effect.

===Discrete switches===
Keyboards are all different, but the most common arrangement of discrete switches is [[plate mount]]: the switches snap into slots in a mounting plate, before being soldered to a PCB.

[[File:Keyswitch illustration.svg|centre]]

Typically, switches are self-contained units with solder terminals on the bottom. A plunger called a '''[[slider]]''' provides the vertical motion (supported by a '''[[return spring]]'''), and the keycap fits onto the slider. The term "stem" or "keystem" can refer either to the protrusion that holds the slider or, with [[Cherry MX]] switches in particular, the entire slider. Ridges in the upper shell stop the switch falling through the slot in the mounting plate, and retention clips at the sides of the switch press against the plate from below.

[[File:Switch parts, external.svg|centre]]

===Rubber dome over membrane===
The vast majority of consumer keyboards manufactured today now are '''[[rubber dome over membrane]]'''. Silicone rubber domes, normally in the form of a sheet of domes, provide the spring force that pushes the keys back up when released, replacing the metal helical springs in more expensive keyboards. The electrical switching is provided by three sheets of thin plastic, called [[membrane]]s. Thin keyboards, such as those found in laptops, use '''[[scissor switch]]es''', which are simply a variety of rubber dome over membrane where scissor mechanisms support the keycaps in order to reduce the overall height of the keyboard.

[[File:Rubber dome over membrane.svg|centre]]

===Switch configurations===
Normal switches are called '''momentary''', as they typically are only closed (connected) for a moment, and will open (disconnect) when you let go. Normal switches will connect a single circuit pathway, or provide only one of some other form of sensing; such a switch is called '''single pole single throw normally open''' (SPST NO). NO is also called '''form A''', and NC is also called '''form B'''.

'''[[Alternate action]]''' switches (also '''latching action''' or '''push-push''') will stay down when pressed; to release them, they must be pressed a second time. Such switches were used for [[lock key]]s in the 70s and early 80s, after which they went out of fashion, with Apple being the last prominent manufacturer to use them, up to the [[Apple Extended Keyboard II]].

'''[[Double action]]''' or '''two-stage''' switches connect two circuit paths in sequence, with the second connection made after the key is pressed further. These are more likely to be found in electric typewriters.

Redundancy can be provided with a second pair of terminals. Here, a single switch with a single plunger connects two separate circuit paths at once; this is called '''double pole single throw''' (DPST). These are not generally found in keyboards, but [[Micro Switch SD Series]] switches are known for having redundant sense lines. In some cases, switches clearly support four terminals, but are seldom if ever seen with the second pair of holes populated, with the available factory configurations left as a mystery. As time passes, and more specifications are acquired, the nature of switches become clear. Some are found to have four terminals for DPST, and some are found to have four terminals for two-stage action.

Some switches have three terminals to provide '''changeover''' action (current is diverted from one circuit to another), or four terminals where one pair of contacts is normally open and the other is normally closed. The latter will function as the former if the input terminals are connected at the PCB. Changeover is also described as '''form C'''.

The increased affordability of microprocessor-driven equipment has removed the necessity of more complex switch configurations for keyboards, as the software takes care of the machine state, but other (non-computer) equipment may also find a use for switches other than SPST NO.