Keyboard switches—“keyswitches” for short—are pushbutton electrical switches used in keyboards to register keystrokes.
The following characteristics are common to keyboard switches, and should help you differentiate keyboard switches from other kinds of pushbutton switch. Some of them require physical access to the switch.
Size and shape
Keyboard switches come in a surprising variety of sizes and shapes. One of the tallest types is Clare high-profile reed, at 28 mm tall (excluding the plunger). Micro Switch SW Series is a fraction of a millimetre shorter, and Micro Switch RW Series is just over 27 mm tall. Old Cherry switches—as tall as they look—are significantly shorter, at just under 19 mm. Most types are within or close to the range of 10–15 mm tall, again excluding the plunger; in particular, switch types made from the early 80s onwards generally fit this pattern to comply with German standardisation.
The keys on almost all keyboards are spaced at either ¾″ (19.05 mm) or its metric approximation of 19 mm. Keyboard switches will not exceed this spacing, although in many cases they will permit a tighter grid. For example, Cherry M8 can be put into a 13×14 mm grid, while Cherry MX can be put into a 16×16 mm grid. In some cases, such as Sasse Series 25, Cherry MY, RAFI full-travel, and RFT TSS 19 and TSH 19, the switches are just short of 19 mm square.
Keyboard switches are almost entirely SPST-NO. As such, most of them have only two terminals. Omron preferred three terminals for both B3G and B3G-S series, with the two outer terminals paired to function as a jumper, and the many Taiwanese types derived from B3G-S followed this design (the KPT and KPT-like types). Other types such as Alps SKFR/SKFS and Cherry ML are fitted with an integrated jumper, giving them four terminals. It is common for the jumper terminals to be a different size, shape or orientation to the switch terminals.
At least three series (Mitsumi miniature mechanical, Alps (S)KFL series and NEC “metal top”) had metal lids that were soldered to the PCB also, giving them as much as six solder points per switch. Just as with switches with internal jumpers, the legs from the lid are a different shape to the main switch terminals.
Switches with six or more identical terminals—a common configuration of other kinds of pushbutton—are not likely to be keyboard switches.
Some example terminal styles:
The travel of a switch is its vertical motion. A so-called “full travel” keyboard switch generally has 2.5 mm travel or greater; that is, each key can be pushed down by at least 2.5 mm. Laptop keyboards can offer as little as 1 mm travel (as given in the specifications for the Sony VAIO VPCZ21M9E 13″ notebook) but discrete switches will all offer at least 2.5 mm of travel.
2.5 mm is offered by several switch series, including RAFI RS 74 M and RS 74 C as well as Futaba ML. The RAFI low-travel switches were not really intended for keyboards, but were on occasion used as such. Futaba ML however was a widely-used mechanical switch found in machines such as the Acorn Electron and Memotech MTX 512.
More generally, switch travel will be at least 3 mm, and is generally 3.5 to 4 mm. Travel is a trade-off between sufficient movement for your brain to successfully register, and extra time required to push down on a key.
The visible part of a key—the keycap or keytop—is not part of a keyboard switch. Keycaps are separate components that attach to the keyswitch. They are usually a press fit onto the switch (that is, they simply push on tightly, also called a friction fit or interference fit) but in some cases they clip onto the switch instead.
In almost all cases, keyboard switches do not need to be pressed down all the way. The switch will register a keystroke when it is only partially pressed. The distance that the key must be pushed for it to register is called “pretravel”, and the distance that it can be pressed further after this point is called “overtravel”. Pretravel is generally around 50% of full travel, but this varies from switch to switch depending on the design.
Non-keyboard switches, such as Marquardt Series 6425—which are confusingly marketed as “key switches”—often have perceptually no pretravel. That is, they feel like the switch cannot be pushed any distance at all before it “snaps” and actuates. This would be punishing to type on, as it would require the keys to be struck very hard; actual keyboard switches start out at a lower force.
The force (as covered below under weighting) required for pretravel is often in the region of 50–90 grams of force, or with US switches, often one of 2 oz (around 57 gf, as with Hi-Tek High Profile), 2.5 oz (around 71 gf, as with Cherry M6) or 3 oz (85 gf, as with MEI T-5).
(I realise that a brass cylinder just under 1 cm in diameter and weighing 70 g would be 11 cm tall, but that would look mighty silly in the drawing! ;-) Even a solid gold weight would come out 5 cm tall at that diameter.)
As suggested above, a keyboard switch has to be comfortable to type on. If the keys on a keyboard require too much effort to press, typing will prove fatiguing, and the typist will be slowed down. If the keys are too light, it will be too easy to press the wrong key, and as some people rest their hands on the keys, they will type unwanted characters (which is already a problem with some lower-weight switches).
The weighting of a switch is the amount of force—downward pressure from your fingers—that is needed to operate the switch. This can be visualised by placing a metal weight onto the key.
The amount of force needed to register a keystroke varies considerably, with 60–70 centinewtons (cN) being typical for vintage switches. This is approximately equal to 60–70 grams force (gf), with 1 gram force is around 0.98 cN (they can be treated as equivalent for keyboard switches, as the difference is well within the tolerance of switches). 70 gf can be achieved by placing a 70 gram weight onto the key; if the pretravel (as noted above) is 70 gf, then the switch will typically sit halfway pressed, but it should register a keystroke at this point. Commercially-available switches can vary from as low as 30 gf/30 cN up to 75 gf/70 cN or more.
Switch manufacturers confusingly tend to give only one figure for the weighting of the switch. This will either be the force required to register a keystroke (as in, to press the switch half-way down), or the force required to fully depress the key. Both of these forces are referred to as “operating force”, and it is not always clear which is meant when a single “operating force” is given: do the mean pretravel, or full travel force? Typically a force of 80 cN or less will be the actuation force (force required to push the key far enough that it registers), and a force of 90 cN or more will be the force required to fully depress a key.
The following chart gives a rough approximation of the range of switching weightings. The hump on the left covers tactile switches that exhibit a sharp rise in force that then drops off after actuation. The shaded area is the general region in which the switch will register keystrokes, which varies both by switch design and by total travel.