Editing Keystroke sensing (section)

==Conductive==
The majority of computer keyboards work on the principle of switching electric circuits on and off. To register a keystroke, a circuit pathway is connected. The keyboard can be modelled as a grid ([[keyboard matrix|matrix]]) of simple push-button switches:

[[File:Keyboard circuit diagram -- conductive.svg|centre]]

While this is simple in principle, there are various ways to achieve this aim. Further, in reality this simplistic arrangement is not practical. Conductive keyboards permit current to flow the wrong way through the circuit, leading to [[ghosting]]: when certain combinations of keys are held, the computer believes that additional keys are being held. Conductive keyboards typically use filtering logic to detect and block keystrokes before this happens, or employ diodes at each key to prevent the incorrect current flow. A few keyboards, such as the [[BTC 51X9 series]], do take the simplistic approach and suffer from ghosting.

===Metal contact===
{{Main|Metal contact switch}}

Behind membrane keyboards, the second most common switch type is metal contact. Within the switch are two pieces of metal that are pressed together to conduct current. In most cases, these contacts are also the switch terminals, but in cases such as [[Futaba linear switch]]es, the terminals are separate parts. Generally, there is one stationary contact made of thicker, inflexible metal, and one movable contact made of a springy metal such as phosphor bronze. Depending on the design, the slider either presses the contacts together (typical in [[Alps clone]] switches) or separates them until it is depressed (such as with [[Cherry MX]] switches). In some cases, such as with [[Hi-Tek Series 725]], both terminals are flexible, and the contacts may be identical, as they are the [[PED keyswitch]], to reduce the number of different parts required.

The design of the movable contact varies greatly; it can be a [[leaf spring]] (as with Cherry MX), a foil sheet (as with [[Alps SKCL/SKCM series]]), and a wire spring, as with [[MEI T-5 series]].

<gallery widths=250 heights=187>
File:Monterey K110 Hua-Jie -- switches.jpg                 | [[Hua-Jie AK series]] switches, with one dismantled to reveal the contacts
File:Alps SKC -- long switchplate parts, top.jpg           | Fully disassembled Alps [[switchplate]] with foil contact
File:Cherry MX -- switch contacts.jpg                      | [[Cherry MX]] contacts, which are also the switch terminals
File:Mitsumi miniature mechanical MYT -- contacts.jpg      | [[Mitsumi miniature mechanical]] contacts, held open by the slider
File:NMB Hi-Tek - Internals Front.jpg                      | Hi-Tek Series 725 dual movable contacts, in their closed position as the slider has been removed
File:RS 337-217 -- contacts, from side.jpg                 | Sprung wire movable contact in a MEI T-5 series switch
File:Futaba complicated linear contact assembly opened.jpg | Sealed contact assembly from a [[Futaba complicated linear]] switch
</gallery>

See: [[:Category:Metal contact switches|List of metal contact switches]]

====Metal leaf====
Metal leaf is a subset of metal contact switches where the movable contact is a leaf spring. While this covers most switch types, there are many exceptions, some of which are described above.

===Membrane===
{{Main|Membrane keyboard}}

Membrane keyboards contain thin plastic sheets (ca. 0.06–0.09 mm thick) with exposed electrical traces. Typical designs use either one or three membrane sheets. Current is passed either by bridging traces with a conductive object (typically some form of carbon-impregnated silicone pad) or by flexing one membrane to press part of it against another, allowing current to transfer between traces.

<gallery widths=250 heights=187>
File:AppleDesign Keyboard (NMB,B) -- membrane assembly 4.jpg | Three-layer membrane system with rubber domes
File:OTC OK-100M -- membrane layers.jpg                      | Spring over membrane (also three-layer)
File:Apple Keyboard II Mitsumi layers.jpg                    | Single-layer membrane system (conductive rubber)
</gallery>

See: [[:Category:Membrane switches|List of membrane switches]]

===Reed===
A reed switch consists of two metal contacts, hermetically sealed and usually with some inert gas inside a glass envelope. The slider pushes a magnet down in front of the reed capsule. In a normally open switch, the magnetic field causes the reed contacts to become attracted to each other and to make contact; the contacts are themselves not magnetised. The reed switch mechanism was invented in 1936 by W B Ellwood at Bell Telephone Laboratories.

Reed switches support both normally open and normally closed operation. Hysteresis is also a standard feature; the degree of hysteresis present depends on the specification. Latching reed switches require alternating magnetic polarity to cycle the switch.<ref>RRE — [https://www.reed-sensor.com/general-reed-switch-faq/ Reed Switch and Reed Sensor FAQ!]</ref>

The reed capsule can be orientated in a number of ways. Fujitsu and Omron B2R series switches have the reed capsule orientated vertically, which demands a tall switch to contain it. To save space, Clare/Pendar low-profile switches have the reed capsule arranged diagonally in the vertical plane, while FR Electronics, IZOT and Unitra Dolam opted to place it horizontally across the bottom of the switch. Fujitsu also used the horizontal arrangement.

Although reed switches are metal leaf contact switches, here they are considered separate from all other forms of metal contact switch, even those with sealed contact assemblies such as [[Futaba complicated linear]], because the contacts are operated magnetically instead of using physical force.

<gallery widths=250 heights=187>
File:ASM MEXICO reed -- opened.jpg           | [[Clare-Pendar Series S880]], opened
File:ASM MEXICO reed -- internals, rear.jpg  | Internals, rear view; here, the magnet attaches directly to the return spring
File:ASM MEXICO reed -- reed capsule.jpg     | Reed capsule connected to the terminals via leads
</gallery>

[[Fujitsu Magnetic Reed]] and [[Omron B2R series]] are fairly different in appearance, but the reed modules in each are very similar:

<gallery widths=250 heights=187>
File:FMR-VR ReedCloseup.jpg                  | Fujitsu Magnetic Reed reed module
File:Omron B2R-M -- reed module removed.jpg  | Omron B2R series reed module
</gallery>

See:

*[[:Category:Reed switches|List of reed switches]]
*YouTube — [https://www.youtube.com/watch?v=ClvP1HqqxTI Actuating distances of Reed Switches having different Release AT (Drop-out)] (depicts different amounts of hysteresis)
*YouTube — [https://www.youtube.com/watch?v=5d0eRaMLzZs How does a bi-stable / latch reed sensor work?]