RAFI RS 74 M
|This article requires additional photographic illustration|
|Family||RAFI full-travel key switch|
|Switch type||Linear, alternate action, changeover|
|Sense method||Metal leaf|
|Actuation force||80 cN|
|Total travel||2.5 mm|
|Keycap mount||RAFI mount|
|Switch mount||PCB mount|
RS 74 M switches share the same overall design as the 4 mm RAFI RS 76 M switch, but have a reduced travel of 2.5 mm. They are targeted at a unit of 0.75", or 19.05 mm, and the switch shells are themselves this exact width.
The switch uses a square loop of sprung metal that is kept away from a metal contact by the slider. When the slider is depressed, the metal loop lowers until it reaches the static contact. The switch has two solder points, and the slider recess appears to form the single PCB fixing pin. Actuation is at 1.5±0.5 mm; the lug on the slider which raises the loop travels further than the loop drops.
The switches are available in several forms: linear, linear (illuminated), latching, and latching (illuminated). So-called "illuminated" switches simply have room for an LED in the top-right corner; they are not intended to provide backlit keycaps.
RS 74 M was patented together with RS 74 C in German patent DE2504091A, filed in 1975.
According to RAFI, RS 74 M was discontinued in July 2005.
RS 74 M switches appear to be intended for keypads, not desktop keyboards (for which, you would use the RS 76 M or RS 76 C); they have an actuation force of 80 cN within a pretravel distance of 1.5 mm, which makes them quite stiff. Based on a 1984 keyboard, they can also suffer from binding when struck at the corner.
QWERTY keycaps are double-shot; other legended keycaps are engraved and painted; RAFI's catalogue suggests that the engraving is not performed with a laser, as the infill process is not stated as being applied to lasered keycaps, only the "labour intensive" low volume engraving process. Relegendable keycaps are also available.
RS 74 M was covered in the 2001 Electromechanical Components catalogue, allowing some part numbers to be retrieved. Some switches have been discovered however were already end-of-life when that catalogue was published.
Tentatively, the series appears to be divided as follows:
- 3.12960.nnnn — alternate action
- 3.12961.nnnn — momentary
- 3.17960.nnnn — illuminated
|3.12961.001||IP40||Momentary, with increased ESD strength|
|IP40||Momentary (red cover)|
|3.17960.101||IP40||Alternate action, illuminated|
|IP40||Alternate action changeover|
|3.17960.351||IP54||Alternate action, illuminated|
Standard momentary switches can be found with and without a clear plastic cover. The clear cover is depicted on type 3.12961.001, which has sealed terminals and is described as having "increased ESD-strength". The cover is non-conductive and appears simply to insulate the contacts. A variant with a transparent red cover has also been discovered; nothing is known about this version.
The exact number of configurations is not clear. The basic unsealed momentary type supports two stationary contacts; a DPST/SPDT version has yet to be seen. Curiously, the SPST alternate action and SPDT alternate action variants have different part numbers moulded into the the bases (".374" and ".512" respectively) but these bases appear to be identical, supporting two stationary contacts. The momentary illuminated switch uses a different base where the top-right accepts an LED; this base also accepts a latching mechanism and will be shared with alternate action illuminated.
All known sliders are white; the cream slider of the alternate action version depicted above is due to yellowing of the plastic.
The IP54 splash-proof variant was introduced in 1980.[Citation needed] Instead of the clear top, it sports a diaphragm between the switch and its cap. An insert punches through the foil and connects the keycap with the slider.
Illumination and alternate action are available in the IP54 sub-series.
- Gercom portable unit (illuminated switches minus the LEDs)
- Neve Necam 96 audio workstation keyboard
- Siemens field programming unit 670 C (has the splash-proof variant)[Citation needed]
- Thorn EMI Liberator keyboard
RS 74 M switches were discontinued from production in 2005 but old stock was still available in 2013; in April 2015, RAFI confirmed that spare switches are no longer available.
- Engineering Materials and Design 19 (1975) p. 77: The new Rafi RS74 keyboard system from Cole Electronics consists only of key switches and a printed circuit board — a matrix frame is not needed — giving low profile with an overall height of 15 mm.
- Conversation with RAFI, July 2013
- Patent DE 2914954 . For a photo of the sealed switch, see "Rafi-System RS74, spritzwassergeschützt IP 54" in Konstruktion im Maschinen-, Apparate- und Gerätebau (1981), p. 169.
- Reddit — Help identify switch, old terminal keyboard *PROJECT*
- The Register — Liberator: the untold story of the first British laptop part 1 (page 4) Dated 2012-11-12. Retrieved 2015-08-01.