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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

“Solid state” is an awkward choice of term in the context of full-travel keyboards. With keys being moving parts, keyboards cannot possibly be classified as solid state. In all examples, there are at least three parts within a moving assembly: slider, return spring, and capacitive or magnetic element.

In the 1979 Cherry Electrical Products keyboards catalogue, Cherry use “solid state” to describe their foam and foil keyboards. Referring to foam and foil as “solid state” seems something of a stretch, because overtravel is achieved by a compressible foam pad, while with Hall effect, the return spring is the only component that alters its form during a keystroke. I have yet to encounter an explanation for why foam and foil keyboards require the foil disc to be very close to the PCB to register, but it seems that there is (or at least was) no good way to register the foil disc at sufficient distance that overtravel could be achieved without a foam pad, and that means that the sensing system requires a non-solid part. Foam and foil will still function without the foam pad, but providing overtravel is another matter entirely.

The real problem with foam pad switches is that the foam degrades with time, and can be crushed flat after a key is pressed, leaving the key inoperative. This does not offer a fraction of the longevity of Hall effect and ferrite core designs, whose sensors are truly solid state.

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.)

In most contactless designs, the parts relating to the sensing arrangement are solid and rigid. Topre’s electrostatic capacitive switches are another example of a contactless sensing system with non-rigid components, but Cherry had already allowed for solid state switches to have flexible switching components as early as 1979.

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.

In general, one could argue that “solid state” in keyboard context encompasses all of the following:

I would like to define “solid state” as any electromagnetically-sensed type whose switching components are entirely rigid, but Cherry and Key Tronic have made this more awkward than necessary.

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 true transformers, the wires are not wound around the core; instead, they are simply passed through or around the core.

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.

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.