RAFI RS 74 M

This article requires additional photographic illustration

RAFI RS 74 M

Manufacturer	RAFI
Family	RAFI full-travel key switch
Introduced	1975
Discontinued	2005
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
Patents	DE2504091 (1975) GB1533154 (1976) DE2914954 (1979)

RAFI RS 74 M is a low-travel switch in the RAFI Full-Travel Keyswitch family. It is the mechanical counterpart to the Hall effect RS 74 C.

Description

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 sprung loop and the static contact both feature gold crosspoint contacts, as with most Cherry switches.

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.

Key feel

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.

Keycaps

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.

Variants

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

	Part	IP rating	Type
		IP40	Momentary
		IP40	Momentary, illuminated
	3.12961.001	IP40	Momentary, with increased ESD strength
		IP40	Momentary (red cover)
	3.12960.101	IP40	Alternate action
	3.17960.101	IP40	Alternate action, illuminated
		IP40	Alternate action changeover
	3.12961.211	IP54	Momentary
	3.12960.311	IP54	Alternate action
	3.17960.351	IP54	Alternate action, illuminated

IP40

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.

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  SPST alternate action base marked ".374"

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  SPDT alternate action base marked ".512"

IP54

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.

Keyboards

• 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

Availability

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.

Gallery

• 

  RAFI RS 74 switches

• 

  RAFI RS 74 switches

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  PCB traces

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  Opened switch

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  Opened switch viewed from the rear

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  Slider, spring, upper shell

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  Upper shell is hard to remove without damage

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  Gold crosspoint prism on the static contact

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  Gold crosspoint prism on the sprung contact

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  Slider fully depressed

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  Linear illuminated variant; the switch design is open and has no transparent top

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  Slider removed

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  Spring removed from the slider

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  RAFI keycaps targeted at the RS 74

Documentation

• RS 74 and 76 excerpt from Electromechanical Components 2001