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Switch design characteristics

The following are only brief notes. At present I do not have the technical data to assess these in more detail. These are simply notes to give a brief introduction to alternatives.

For individual switch functions such as alternate action, see switch operating functions.

Contents

Sensing

The vast majority of keyboards can be divided into two categories: conductive and contactless. The majority of keyboards in existence rely on some means of physically switching electrical current to detect keypresses; this is true of most rubber dome and membrane keyboards, as well as mechanical keyboards. Contactless keyboards are still electronic, but no physical switching occurs.

Most keyboards function as an arrangement of pushbutton switches. The switch contacts may be solid metal, or they can be screen printed onto thin plastic sheets. Such keyboards all share a particular characteristic: they cannot switch current cleanly. Physical switch contacts (be they sprung metal or plastic membranes) will rebound after they make contact, and it takes a small fraction of a second for the contacts to settle into their closed position. This behaviour is described as “contact bounce”, and it typically lasts no more than 20 milliseconds, with reed switches coming in around 2 milliseconds with their smaller contacts and subtle and gentle contact operation.

Without special processing, contact bounce will cause a single keystroke to be detected as a series of brief pulses before the switch remains in closed position until it is intentionally released. “De-bouncing” is the process of dealing with contact bounce such that the keyboard, keyboard or control panel only reports when a switch is actuated after the contact bounce has completed. Contact bounce has not generally posed a problem in decades in correctly-designed products, but as switches age or fail, the degree of bounce can exceed design tolerances and cause a single keystroke to register multiple times in rapid succession. This failure mode is referred to as “chatter”, although in Japanese that term (spelt as “チャッタ”) refers to bounce.

By 1971, Cherry had introduced a keyboard design with electronic circuitry capable of handling de-bouncing. However, in the 1960s and even into the 1970s, contact bounce was considered a significant challenge when using logic circuitry (such as transistor-transistor logic, or TTL). This led keyboard manufacturers to find ways to detect keystrokes without the need for physical switch contacts. Micro Switch adopted reed switches for keyboards in 1966 (with KB), but shortly afterwards in 1968 they introduced solid-state contactless switches based on Hall effect (SW series). Licon followed with ferrite core inductive switching by 1970, and RAFI introduced their magnetoresistive solid-state switches around the same time. Mechanical Enterprises opted to stay with conductive switching to keep costs down, but replaced the solid metal contacts with a flexible mercury-filled tube that would be pinched closed when the switch was released; their Mercutronic keyboard switches appear to have been on the market in 1969.

Micro Switch PB series included pushbutton switch units with integrated contact buffers: flip-flop–based circuits paired with NO+NC switches to render the contact bounce undetectable. Such an approach will only work with switches that have both normally-open and normally-closed contacts.

Encoding

Encoding switches self-encode their identity, such as the ASCII code representing the character typed. Such switches are wired into a bus with one line per bit of the output code. This idea never took hold, as advances in keyboard technology rendered the idea obsolete shortly after it was conceived.

Illumination

Switches may be illuminated for one of several reasons. This is often to provide status indication, such as for the caps lock key. Keys can also be backlit by the switches.

The source of illumination was originally an incandescent lamp, which by the 1980s were largely replaced by LEDs. A lamp placed in the centre of the switch is good for lighting the entire keycap, while a lamp placed in the corner can be used to light a window in the keycap. Generally a switch series would only offer corner or centre lighting, but Cherry offered both options between their US and German factories, with Cherry USA providing corner-lit M4 switches and Cherry Germany providing centre-lit M71 switches.

With a static lamp, the illumination brightness can vary by key travel. To avoid this, some switches place the lamp directly into the plunger, and the lamp moves with the key. Omron’s B2H and B2R switches offered this functionality, as did Alps KCC and Clare-Pendar S950, as well as Fujitsu’s taller reed switches. Such designs are more complex and often more fragile, as the lamp or LED requires sprung contacts.

Some switches offered two lamp positions: this is true of both Cherry M4 (one or two lamp positions depending on model) and Hi-Tek Series 725.

Tactility

Buckling rubber sleeves

“Buckling rubber sleeves” are discrete rubber domes with a hole in the top, which sit around the keystem and add tactility to the switch. Buckling rubber sleeves are most common in contactless switch systems where it is physically impossible to have inherent tactility in the design. Inverted sleeves are also widely recognised from their usage in Mitsumi’s KKQ and KPQ low-profile membrane types.

US patent 3767022 from Singer Co depicts a buckling rubber sleeve design as early as 1970. This was used with their optoelectronic switch system, which as a contactless system has no means of offering tactility of its own.

Terminals

Design

Current keyboard switches (Cherry MX, Omron B3K, Matias) all follow the same design principle that there is no distinct movable contact piece. The movable contact’s terminal is formed from the same piece of sprung metal as the contact itself. This is why it is very common to receive a pack of switches with one or more bent terminals. (It is not clear how this is addressed in automated assembly, where machinery would be less impressed with incorrectly-shaped parts.)

A number of mechanical types are notable for having found a way to have both terminals be stiff and not prone to being bent; examples include:

Sealant

See sealed terminals on the Deskthority wiki.