Jump to page content

Switch types

Contents

By sense method

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.

Many switch types are defined as mechanical by the manufacturer, including Alps CL/CM, Omron B3G-S, SMK J-M0404 series and Tokai MM9. However, these definitions are not found in context against other offerings from the same manufacturer and era. The following are examples where membrane and mechanical are contrasted directly by the manufacturer:

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.

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.

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

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.

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.