Switch design characteristics
The following are only brief notes. At present I do not have the technical data to assess these in more detail. These are simply notes to give a brief introduction to alternatives.
Switches may be illuminated for one of several reasons. This is often to provide status indication, such as for the caps lock key. Keys can also be backlit by the switches.
The source of illumination was originally an incandescent lamp, which by the 1980s were largely replaced by LEDs. A lamp placed in the centre of the switch is good for lighting the entire keycap, while a lamp placed in the corner can be used to light a window in the keycap. Generally a switch series would only offer corner or centre lighting, but Cherry offered both options between their US and German factories, with Cherry USA providing corner-lit M4 switches and Cherry Germany providing centre-lit M71 switches.
With a static lamp, the illumination brightness can vary by key travel. To avoid this, some switches place the lamp directly into the plunger, and the lamp moves with the key. Omron’s B2H and B2R switches offered this functionality, as did Alps KCC and Clare-Pendar S950, as well as Fujitsu’s taller reed switches. Such designs are more complex and often more fragile, as the lamp or LED requires sprung contacts.
Some switches offered two lamp positions: this is true of both Cherry M4 (one or two lamp positions depending on model) and Hi-Tek Series 725.
Buckling rubber sleeves
“Buckling rubber sleeves” are discrete rubber domes with a hole in the top, which sit around the keystem and add tactility to the switch. Buckling rubber sleeves are most common in contactless switch systems where it is physically impossible to have inherent tactility in the design. Inverted sleeves are also widely recognised from their usage in Mitsumi’s KKQ and KPQ low-profile membrane types.
US patent 3767022 from Singer Co depicts a buckling rubber sleeve design as early as 1970. This was used with their optoelectronic switch system, which as a contactless system has no means of offering tactility of its own.
Current keyboard switches (Cherry MX, Omron B3K, Matias) all follow the same design principle that there is no distinct movable contact piece. The movable contact’s terminal is formed from the same piece of sprung metal as the contact itself. This is why it is very common to receive a pack of switches with one or more bent terminals. (It is not clear how this is addressed in automated assembly, where machinery would be less impressed with incorrectly-shaped parts.)
A number of mechanical types are notable for having found a way to have both terminals be stiff and not prone to being bent; examples include:
- Alps SKCC, SKCK/SKCM and SKCP series: the “switchplate” contact assembly uses a foil sheet as the movable contact; one terminal bears the stationary contact, and the other is pressed against the foil sheet.
- Futaba MD, MR and ML series: the terminals screw into the switch, and push through holes in separate pieces of metal that form the two contacts.
- MEI T-5 series: the movable contact is a piece of spring wire wrapped around one of the terminals.
- Omron B3G-S and Alps-style: these use a “switchplate”-like contact assembly where the movable contact appears to be a phosphor bronze leaf spring that is attached to one of the terminals
See sealed terminals on the Deskthority wiki.