Switch operating functions
- Momentary action
- Alternate action
- Secretarial shift
For decades, the term “momentary” has referred to a switch which is actuated when and only when the plunger is pressed or held. A normally-closed switch will remain open until the plunger is released, and a normally-open switch will remain closed until the plunger is released. “Momentary” describes the operation of a pushbutton switch, and is contrasted against a number of other switch types that remain in whatever position they are left, such as toggle switches, rocker switches and slide switches, as well as alternate action pushbutton switches. All keys on a modern keyboard use momentary switches; historically a small number of keys may have been double action or alternate action depending on the design.
The term “momentary” was also used to describe switches that actuate only for a brief instance. As the switch reaches the actuation point, it would actuate and then immediately disengage. Typically the switch would not actuate again as the plunger is released, but this could also be arranged if required. These designs tended to be designed around the assumption that the operator would press the plunger as far as possible; it appears that generally they could be held in the actuated position by pressing the plunger only part of the way.
One use for such switches is doorbells, in order to prevent continuous ringing. A switch designed for this purpose is described in US patent 2198659 “Momentary action electric switch”, filed in 1939. Both designs depicted in this patent actuate briefly as the switch is both pressed and released, making it unsuitable for electrical signalling. The description of how it prevents the switch from being held in the actuated position is unconvincing, with there being no mechanism to urge the plunger away from that position in either direction.
US patent 2586056 “Momentary contact switch of the push-button type” filed in 1949 describes a more advanced design where the switch contacts are only cycled when the switch is pressed, and not when it is released. Both normally-open and normally-closed terminals are offered. It does appear that the switch can be held in the actuated position, but the design does provide hysteresis against attempted actuation without fully releasing the plunger. The objective of this switch was to reduce the risk of excessive power drain in systems with pushbutton-controlled solenoids, by preventing operators from keeping solenoids active for too long.
Such momentary switches seem to have become of more interest in the 1960s. US patent 3204070 “Momentary switch using resilient leaf spring actuator” filed in 1963 for example depicts a design similar to the 1949 design. The slightly earlier US patent 3153130 “Momentary contact switch” from 1961 uses a totally different design. US patent 3392252 “Momentary switch having normally engaged contacts actuated by a conductive member” filed in 1966 depicts normally-closed design where the circuit is momentarily interrupted.
Honeywell filed US patent 3375340 “Momentary actuating arrangement” for a keyboard switch placed into Micro Switch PB series, formed from a momentary mechanism built around a microswitch. This design, too, can be held in the actuated position by pressing the plunger until the microswitch engages. A harder press is required to clear the additional mechanism that releases the plunger. The reason for this switch design within keyboards is not indicated in the patent.
Micro Switch introduced solid state keyboards in 1968, and with it two-of-N coding. This arrangement cannot deal with two concurrently depressed keys, so each Hall sensor is designed to only draw current for a brief moment, making sure that under rollover conditions each switch will have quiesced before the next switch becomes active, with no two non-modifier keys ever being active at once during normal typing. Such switches were described not as “momentary” but rather as “pulse” output. The pulse duration for Micro Switch SW and SN Hall sensors was 10–100 µs, and unlike mechanical switches, the pulse duration was controlled entirely electronically and thus there was no longer any way to hold the switch in its actuated position. A few years later, Cherry introduced a pulse variety of M5 or M6 switch, described in US patent 3739127 “Keyboard pulse switch”. Their terminology matched up with Micro Switch’s, rather than using the traditional “momentary” description.
Alternate action switches change between their operated and unoperated states each time they are pressed. Unlike more typical switches that are flipped back and forth (such as rocker switches and toggle switches), an identical action applied to an alternate action switch (pressing it downwards with your finger) alternates the switch state. As a result they are sometimes referred to as “push-push” as two presses are required to complete the cycle. Where the plunger remains held in the down position while the contacts are closed, the terms “latching” and “latching action” are also used. “Locking” is another term, as the switch is “locked” in the down position on alternate presses. “Lock” is also used to refer to how these switches are often used for lock keys (in particular shift lock, alpha lock and caps lock) in computers and typewriters. Alternate action was widely implemented and was found in switch series from Micro Switch, Cherry, Alps, SMK, RAFI, Mitsumi and many others. Apple used alternate action switches with the Apple IIe onwards, with AppleDesign Keyboard from 1994 finally replacing Caps Lock with a regular momentary switch.
Most designs of alternate action switch use a “heart cam” design, a roughly heart-shaped cam track parallel with with plunger’s motion. (The term “heart cam” was used by various manufacturers including Alps and Omron and is covered by US patent classification Class 200/524 “With heart-shape cam”, defined as “Subject matter including a cam track having a generally cardioid shape which is utilized to hold the position of the solid conductive element in engagement or disengagement.”) A follower point moves around this track, and when it reaches the top of the track, it holds the plunger in its locked position. This gives a visual indication of the state of the switch. The cams on the heart-shaped track appear to exist to ensure that the follower traverses the track in the correct direction.
The existence of the cams means that the follower must be permitted to move perpendicularly to the track. Where the follower point is a rigid piece such as a turned metal point or a metal or plastic pin, this is achieved by placing a spring behind the follower. With Alps KFL this is a flat spring, while Micro Switch SD, RAFI RS 74 and RS 76 and SMK JM-0400 use a coil spring. The alternative choice is for the follower itself to be flexible. With Datanetics DC-50 and DC-60 series and all known Mitsumi types a piece of sprung metal wire was used; other options are flat sprung metal in Micro Switch SW and a flexible plastic arm in Alps (S)KCL. Stackpole KS-200 keyboards used a different technique, where the follower wire is rigid and pivots instead; the return spring provides the pressure to keep the wire within the heart cam (per Deskthority topic Stackpole latching keys).
Cherry however preferred rotary mechanisms. They offered a non-latching arrangement for their E series microswitches, as seen in Cherry Precision Switches catalogue C-663 (dated 1965). Here, their E33-00G and E34-00G “push-push” assemblies use a rotary intermediate part within an external assembly. The diagram shows that after actuating the switch, the plunger returns to its home position.
Cherry would later use a similar design with M6 switches. US patent 3770923 filed in January 1972 depicts an alternate action system with a rotary latching system. This design would be used again in MX series switches.
Two others manufacturers opted for a rotary system in their keyboard switches: Licon and Omron. Licon’s design is known only from US patent 3691333 “Alternate action mechanism”, filed in March 1971 for their Series 550 keyboards. Omron’s design (as shown in US patent 4495391) is very similar to that of Cherry’s, but the latch wheel has a rotational symmetry of 180° instead of 120°. The Omron rotary arrangement was used in B3G and B3G-S series.
Alternate action switches have significantly lower rated lifetimes than momentary switches: alternate action models typically have in the region of 1% or less of the rated lifetime of the equivalent momentary models.
Secretarial, or secretary shift is a shift lock mechanism in electronic keyboards that mimics the way that shift lock works on a manual typewriter. In typewriters, the shift keys physically shift the type basket, and pressing shift lock also locks the type basket into position. Releasing the type basket involves pressing shift.
Lock keys in electronic keyboards typically handle lock keys using either an indicator lamp or by way of an alternate action key. To reduce training costs when moving staff from manual typewriters to electric typewriters or terminals, some products were fitted with a mechanism that mimicked the mechanical shift lock of manual typewriters. Here, the shift lock keyswitch would be mechanically locked when pressed, and then released by either shift key.
Secretarial shift was offered by a number of electronic keyboard manufacturers. The following are listed in approximate date order.
No details on the Micro Switch SW Series implementation have been discovered; its existence is known from Product Brochure SW (373). The date of introduction is not known, but as SW Series was introduced around 1968, it is possible that Micro Switch had this design ready at this point. Secretarial shift (as secretary shift) was also offered with SD Series, introduced in 1975 or 1976, and likewise no details on the mechanism are known.
Clare-Pendar’s design is covered by US patent 3626120, filed in September 1970. A sliding plate held by a coil spring is pulled into a notch in the keystem of the shift lock key. A vertical bar is placed above each shift key, allowing the keycap itself to push on the bar and slide the plate aside, releasing the shift lock key.
Cherry supported secretarial shift with M6 series, and described their mechanism in US patent 3678255, filed June 1971 (although the switch shown in the patent looks more like their 201/201 reed switch type). Where Clare-Pendar used a coil spring, Cherry used a flat spring that rests against a keyswitch. The sliding plate of Clare-Pendar’s design is replaced with a rotating plate attached onto the shift key. This is one of the very few designs that has been observed; it can be seen in an Anderson Jacobson AJ 510 keyboard.
Cherry’s patent allows for both shift keys to release shift lock, by way of a connecting rod between a pair of dedicated shift switches. In practice it seems that manufacturers preferred to have only left shift release shift lock. This is the case with the Anderson Jacobson keyboard, as it is also with the ADDS CONSUL 980 keyboard (per the service manual).
US patent 4071719—filed in July 1976—covers mechanical shift lock for one of the older series of Licon/Cortron ferrite core switches. ITW filed a later patent, in February 1980—US patent 4295012—covering the successor ferrite core switch series.
Marquardt’s “butterfly” switch series supported secretarial shift, and this was used in Olympia typewriters. No patent or documentation has been found for their mechanism.
Double-action switches—also referred to as two-stage, bi-level or double-depression switches—are a derivative of double-pole switches. Where double-pole switches make both circuits simultaneously, double-action switches connect the circuits in sequence, with a significantly harder push required to connect the second circuit to prevent overshoot from connecting the first circuit, especially if the first actuation is tactile. Many people will be familiar with this behaviour from digital camera shutter buttons, which you press half-way to lock the focus and exposure, then press fully to take the shot, with two distinct tactile events.
The first circuit is connected at a lower operating force (in keyboards, 88 cN for Alps SKCLKB, 85 cN for Cherry M9), but the second circuit requires a much heavier press (in keyboards, 490 cN for Alps SKCLKB, 400 cN for Cherry M9). In keyboards, the second circuit was used to signal autorepeat, and was used in the Apple III and many electric typewriters. There is also a Cherry M8 variant with two switching stages, but without the heavy spring used to prevent inadvertent actuation of the second stage, as noted earlier (found in the Cherry version of the Siemens SIMATIC S5 PG675 keyboard). Switch series with support for double action include Cherry M9, Mitsumi full-size mechanical, Marquardt Butterfly and Alps SKCL.
The advantage of using a double-action switch for auto-repeat is that the operator has direct control of when auto-repeat begins (with no need to wait through the initial delay). The disadvantage is that auto-repeat cannot be applied to every key, as it would mean having complex and expensive switches on every key and require double the size of the switch matrix. Double-action switches are normally found in electronic typewriters under backspace, enter and space.
Numerous brands produced double-action keyboard switches, usually for typewriters. These include Cherry (with Serie M9), Marquardt (with “butterfly” switches), Alps, Micro Switch and Mitsumi. Apple’s bill of materials for the Apple III indicates that they were using Datanetics DC-50 double action, something that seemingly did not exist during Meryl Miller’s time there, and that has so far never been seen.
Micro Switch 1SW200 contains the double action switches from Micro Switch SW Series. These Hall effect switches still only have one Hall IC, but it has a larger die area with two separate Hall elements. A special arrangement of pins on stiff springs provides the clear “bottom out” sensation at the end of stage 1, and a heavy force curve for stage 2.
Alps provided double action in both KCC series (or a series directly related to KCC) as well as in KCL/KCM series. In the former case, the switch only has a single contact assembly, with some kind of metal dome embedded in the base of the switch for stage 2. In the latter case, there are two contact assemblies (one on each side) with a Datanetics DC-50–like arched actuator spring to drive one of them, in a design similar to Alps SCK.
Alps also provided double action in KFF series, as found in the Apple III and typewriters.