|This article requires illustration in the form of diagrams — needs diagrams of how these things work, especially the cam track if anyone could provide a photo large enough to study|
Alternate action refers to switches that alternate between open (disconnected) and closed (connected) each time they are pressed.
Alternate action switches cycle between open and closed each time they are pressed. Typically, the slider latches in a lower position when the switch is closed, and returns to its fully raised position when the switch is pressed again. With an extra mechanism holding down the slider, the contacts are maintained in the closed position until the key is pressed again, and the slider is released.
The mechanism that provides this functionality normally generates unintentional tactile feedback, but the rotational mechanism used in Cherry MX Lock is linear.
Alternate action switches are found under several names:
- Alternate action
- Used by Cherry, Datanetics, MEI; this is the most common term
- Used by RAFI
- Used by Pendar
- Used by Himake
APEM use "alternate action" for switches that do not visually show status (the actuator always returns to the home position), and "latching action" for those which do. Clare-Pendar used "alternate" for non-latching switches, and "alternate (remain in)" for latching switches.
The most prominent application for alternate action switches was lock keys, but they were also found providing additional (often left unexplained without appropriate documentation) functionality in vintage keyboards.
This method is described in US patent 3770923, filed in 1972 by Cherry. A wheel with three pairs of teeth is affixed to the slider by a post, and this wheel is free to rotate. A lug within the switch rotates this wheel every time the slider is depressed, and on alternate cycles, the wheel engages with a second lug within the switch. The space between each pair of teeth is right-angled, and after engaging with the upper lug, holds the slider down. When the slider is pressed again, the wheel is rotated further, and as it encounters the upper lug again, it is given another push. This rotary approach imparts no noticeable force, and thus leaves the switch completely linear.
The notch bar approach used by Omron B3G-S Lock is a development of Cherry's method; US patent 4495391 was filed for it in 1982. Instead of a wheel with three pairs of teeth, a bar is used instead. Where the total rotation per lock-unlock cycle is was 120° with Cherry's notch wheel, it is 180° with Omron's notch bar.
The vast majority of alternate action switches use a cam track: a roughly heart-shaped linear cam track. A follower arm traverses this track, and the dip in the centre top captures the follower and thereby the slider. Ramps within this track ensure that the follower always proceeds the correct way around the track. To keep the follower pressed against the track and ensure that it engages the ramps, some source of pressure is required. In Alps SKFL Lock, the follower pin sits in front of a leaf spring, while RAFI full-travel key switch and SMK lock use a helical pressure spring. Alps SKCL Lock uses a flexible plastic arm as the follower, and resistance to flexing provides the pressure.
The cam track can be moulded into the slider, with the follower located in the switch (as used by Alps, SMK and Mitsumi), or the cam track can be placed in the switch, with a follower arm connected to the slider (as used by RAFI and Futaba).
Currently, Cherry and Omron are the only brands known to have opted for any method besides a cam track.
Alps SKFL Lock slider with cam track
Base of Futaba lock with stationary cam track insert
The follower pin in SMK lock has its own slide-in module
Alps SKCL Lock partially disassembled
RAFI RS 74 M SPDT latching slide block partially removed