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Switch types

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The following definitions are explained in terms of major manufacturer literature and advertisements.

Mechanical

While it remains true that “mechanical” is not precisely defined, the historical trend within the industry seems to be that “mechanical” denotes metal contact switches (also known as “hard contact”) that are internally physically operated. That is, switches where the contacts are directly pressed or released by the plunger or by (as with Sasse series 25 and Alps SKC* types) an interposed part. Use of the term “mechanical” is not new, going back at least as far as 1979.

Many switch types are defined as mechanical by the manufacturer, including the following:

Monterey International also used “mechanical tactile click keyswitches” in their 1992 catalogue under the K110 keyboard and KP110 keypad listings.

The definitions above however are not found in context against other offerings from the same manufacturer and era. The following are examples where “mechanical” is contrasted directly by the manufacturer against other contact types:

In ITW’s case, they describe mechanical contacts as follows in the patent:

The mechanical type of keyswitch has the advantage of being relatively low in cost, and for many applications this factor makes it desirable to employ such a mechanical keyswitch. However, mechanical keyswitches have a number of disadvantages that make them undesirable for use in applications where high reliability is required and the added cost of a analog switch is, therefore, considered to be warranted. These disadvantages include contact bounce, the possibility of arcing, lower life times due to pitting and corrosion and possible deformation of the contact members.

Thus far, mechanical has been contrasted against solid state, membrane and conductive rubber. The above description also strongly implies that the switch contacts are metal.

For my purposes, “mechanical” will be taken to mean directly-operated metal contact switches. This also includes ball contact switches, on the basis that Tokai classified MM9 series as mechanical.

Corner cases

An awkward case is Dananetics DC-50 series. Meryl Miller of Datanetics affirms that these are “diaphragm switches”, putting them into a classification of their own. He notes that the Mylar membrane facing the actuator has the responsibility of separating the metal switch contacts, which are glued to the membranes. Although these switches are metal contact internally, they also use a three-layer membrane. Although it might seem reasonable to class them as mechanical due to being metal contact (the switch contacts are the same pieces of metal as the terminals), from manufacturer standpoint they were never classified as mechanical.

Alps SKCL/SKCM and related types use metal foil contacts. Alps specifically refer to their foil contact system as “mechanical” in their catalogues.

In Modern Data, April 1970, Mechanical Enterprises Mercutron mercury tube switches were described as using a “mechanical switching approach based on the movement of mercury in a sealed flexible tube.” 1970 was around the time that Licon Series 550 ferrite core switches came out, and less than two years after the introduction of Micro Switch SW Series Hall effect switches. As such, having a moving tube filled with mercury that is pinched by an actuator to separate the mecury when the switch is opened, was classed by MEI as “mechanical”, in comparison to solid-state designs. Just as with DC-50, these switches are hard to classify, as they used an extremely unusual design.

Reed

General Instrument advertised Series S950 as a “mechanical keyswitch” type in the S950 catalogue entry, while Clare(-Pender) Series S820 and S880 are described as a “reed keyswitch” type. Sadly the company names here are a guess. Both pages have a chapter title of “Reed and Hard Contact Switches”, but the company names and document titles are lost. (S950 is GI-branded, which is the only reason that this series is ascribed to parent company General Instrument instead of CP Clare or Clare-Pendar.)

Reed switches are still metal contact, but the switch contacts are closed magnetically, allowing the contacts to be sealed against moisture and debris ingress. As such, so far as I am concerned at least, these are not mechanical. The term “hard contact” is used here with a suggestion that this refers to exposed contact switches; Cherry also used the term “hard contact” in some publication. However, US patent US4370533 for Fujitsu FES-360 switches (filed in December 1980) notes the following:

Switches are divided into two types, that is, switches having a mechanical contact element, such as reed switches, and switches having a non-contact switch element such as hall IC. The present invention is directed to a keyboard comprising the former type, i.e., switches having a mechanical contact structure.

Fujitsu would later separate the types out in the May 1985 edition of their magazine, with Fig. 5 “Development of keyboard switches” (page 429) giving the following descriptions of their switch product lines:

As reed switches are less common, and documentation on them even less common, terminology regarding sealed versus non-sealed metal contacts is still scarce.

See How reed switches work (magnetically operated switches) for a clear explanation of reed switch operation, including normally-closed contact types.

Solid state

The normal definition of “solid state” is the use of semiconductor circuitry. Thermionic valves (vacuum tubes) are excluded from this: even though electronic computers using valves replaced electromechanical computers, computers did not become solid state until the introduction of transistors. Solid state storage uses semiconductor memory to hold the data. A broader use of the term “solid state” covers anything without moving parts, which for keyboards excludes everything except capacitive touch screens, as even resistive touch screens have a flexible plastic cover as part of the sensor mechanism. Even so, a number of keyboard manufacturers marketed their full travel switch technology as “solid state”, from as far back as the 1960s.

For keyboards, “solid state” refers to switch types where there are no switch contacts of any kind, be that metal (as in mechanical switches), conductive rubber (as in some Mechanical Enterprises T-15 variants) or silver–carbon ink (as in most membrane keyboards). Keystroke sensing is achieved using capacitance or by electronically detecting the presence of a magnet. Such keyboards may still require one or more additional parts as part of the sensor.

Hall effect

“Solid state” is an awkward choice of term in the context of full-travel keyboards. All such keyboards require moving parts, with at minimum a plunger and some kind of spring. However, the oldest switch type so far discovered that was marketed as “solid state”—Micro Switch SW Series—did indeed have actual semiconductor sensors. Introduced in 1968, the Micro Switch SSK—the original brand name of SW Series—was advertised to be the “first of its kind”, using an integrated circuit within each switch to detect a pair of magnets within the plunger via Hall effect.

Hall effect provides a means to to detect the proximity of a magnet entirely electronically. As the magnet is moved near the Hall sensor chip, current flowing through a special conductor area within the chip is diverted by the magnetic field from the permanent magnet, creating a minuscule potential difference across the conductor. Following amplification, this potential difference can be read in an analogue manner to indicate proximity, or the Hall sensor can set its own trigger and release thresholds and provide digital switch output.

RAFI’s Hall effect switches and keyboards are described as “contactless” and “solid state” depending on the document: “contactless” in the Electromechanical Components catalogues, and “solid state” in the Standard Keyboards catalogues.

Ferrite core

Ferrite core is a type of solid state switch introduced at least as far back as 1970, going by an advertisment for Licon Series 550 keyboards and switches. Ferrite core switches use a very simple 1:1 transformer to detect keystrokes; pulses of electricity are fed into the primary side and are read from the secondary side. Unlike “proper” transformers, the wires are not wound around the core; instead, they are simply passed through or around the core. Licon Series 550 was described in the advertisement as “All solid state”.

At least two general forms of ferrite core switch exist. Licon-designed ferrite core switches have a stationary core, and one or two movable magnets in the plunger. In their rest position, the magnet or magnets sit in proximity of the ferrite core and stop pulses of electricity passing from the strobe line to the sense line. The magnet or magnets are moved out of the way when the plunger is depressed, allowing electricty to pass into the sense line.

ADI and IMS ferrite core switches put a narrow ferrite core inside the plunger. Here, the strobe and sense lines are simply PCB tracks that pass around circular holes in the PCB. To couple the lines together, the ferrite core moves downwards with the plunger and into the space between the PCB tracks.

Foam pad

Foam pad—or foam and foil—switches are where things start to get awkward. This is a form of capacitive keyboard, where the plunger position is read by a variable capacitor. A foam pad is fitted to the bottom of the plunger, affixed onto which is a plastic-coated foil disc. The distance between foil disc and the PCB affects the capacitance registered at that key position. Keystrokes are detected when the foil disc is resting on top of the PCB; the solder mask on the PCB and the plastic coating on the disc prevent a short circuit. The compressible foam pad allows the plunger to continue being pressed past the point that the foil disc reaches the PCB, to provide overtravel. As such, the complete assembly will not operate without the use of an extra moving part (the foam pad) beyond the plunger itself. Worse, the foam pads are a weakness in the design: as foam can stiffen with age and lose its flexibility, some older keyboards fail due to the foam pad failing to re-expand after a key is struck. This is a problem that does not occur with semiconductor and ferrite sensors.

In the 1979 Cherry Electrical Products keyboards catalogue, Cherry use “solid state” to describe their foam pad keyboards. This terminology is not unique to Cherry. Key Tronic also described their keyboards as having “Solid-state capacitive switches”, as in these KB 5151 advertisements. Computer Products United likewise advertised their unbranded BTC 5339, 5160 and 5151 keyboards as having “solid-state capacitance low-profile key switches”. (No brand is cited, but inspection of the keyboards indicates that they are BTC and not Key Tronic due to the LED and key placement, and 5339 is a BTC model number. This is the larger, wedge-shaped variant of the 5339, rather than the slimline version.)

Leaf spring

Cortron used the description “Solid-state, capacitance unit” for their CP-4550 keyboards. Although no definitive details have been found, there is good evidence to indicate that these are metal leaf capacitive switches extremely similar to those of Digitran. Here, instead of a foil disc, there is a metal leaf spring that is pushed down onto the PCB. Overtravel is achieved instead by a special prong on the leaf spring. These somewhat fragile pieces of stamped-out metal stretch the “solid state” description even further.

Membrane

Membrane keyboards use one or more sheets of thin plastic, called “membranes”, as part of the switching mechanism. In most cases, there are three sheets: two are flexible printed circuits, and one is a spacer placed in between the other sheets. This is the system used in virtually all keyboards made today. The use of “membrane” is found in multiple patents for keyboards using these switches as well as the literature from many manufacturers, and is not in dispute when it comes to full three-layer membrane assemblies. The manufacturer descriptions of two-sheet and single-sheet membrane arrangements are not widely known, but Mitsumi’s later single-membrane types (KPQ and KPR types) were documented in their catalogue as membrane also.