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Alternate action

Alternate action switches change between open and closed on each alternate cycle.

In most cases, they use a “heart cam” design, a roughly heart-shaped cam track parallel with with plunger’s motion. A follower pin 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 pin traverses the track in the correct direction. The existence of the cams means that the follower pin must be permitted to move perpendicularly to the track; this is often achieved by placing a spring behind the pin (a leaf spring in the case of Alps KFL, and a coil spring in SMK J-M0404 and RAFI switches), but in some cases (as with Datanetics DC-50 and DC-60 series, and Alps SKCL) a flexible plastic arm or steel wire is used instead.

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.

The only other manufacturer known to use a rotary system in a keyboard switch is Omron; their 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°.