MEI T-5 series

MEI T-5 series
Manufacturer	MEI
Switch type	Linear
Sense method	Metal contact
Rated lifetime	20M
Bounce time	4 ms
Actuation force	3.0±0.5 oz (85 cN) (SPST); 5.0±0.5 oz (142 cN) (DPST, HS, latching)
Pretravel	0.080±0.030″ (~2.0 mm)
Total travel	0.170±0.010″ (~4.3 mm)
Keycap mount	Proprietary
Switch mount	PCB mount

MEI T-5 series (provisional name MEI WEAB) is a series of comparatively tall profile linear and latching action switches introduced by MEI and currently manufactured by Datalux. Some variants were also sold by RS under RS part numbers.

Design

The switch contacts take the form of two turned metal terminals, around one of which a piece of sprung wire is pressed, with both ends facing away from the terminal. One end of this wire rests against the centre post, and the other rests against the second terminal. A loose plastic arm is dropped into the base of the switch; it appears that slider rotates this inwards away from the side of the switch, thus pushing the contacts apart. A small nub prevents the contact wire from riding over the top of this arm. The second end of the wire stops the whole wire from rotating around the terminal. The documentation indicates that the switch is specifically designed to allow the contacts to close under their own power, to reduce contact bounce and produce more consistent bounce times. Contact bounce is given as 4 ms maximum. The use of heavy terminals, together with the mounting studs, relieve the need to use a mounting plate to secure the switches, and indeed these switches are found PCB mounted despite their size.

Travel is cited as 0.170±0.010″, around 4.3 mm. Pre-travel is cited as 0.080±0.030″, around 2 mm. Switch travel in the variants sold by RS is long at 5 mm, and the stem height for the keycap is 9.4 mm on the straight stem variants. The straight stem keycap mount is cruciform with a cross ca. 4.8 mm wide and arms ca. 1.2–1.3 mm thick. An illuminated version (T-5L) also exists. The shell is around 17.9 mm tall in total, and 15.8 mm square.

The switch is designed to provide two sets of contacts, allowing for SPST normally-open (NO), DPST NO and independent SPST NO + SPST NC; this last form can be wired up to produce a changeover switch. Latching action is catered for, as is illumination using a lamp that fits into a post in the centre of the switch, within the wide diameter return spring.

The top of the switch is removed by pressing four corner clips in with a sharp implement. Although the top and the slider appear to be square, they are not; the top and slider have 180° rotational symmetry, not 90°. The holes in the sides of the slider correspond with the contact separator arms (with typically one present, for SPST configuration). The slider arrangement is documented as having "precision tracking surfaces" between the slider and shell, which "prevent binding even when double-width keytops are installed".

Availability

As of 2016, Datalux report that this series is still in production. They appear to have lost all documentation for it, though, and will not acknowledge any awareness of the details or history of the product line of any kind.^[1] RS no longer offer the switches for sale.

Variants

Details on T-5 series are still scarce. T-5 series is divided up into sub-series. Sub-series T-5C has a very tall upright cruciform keystem. Sub-series T-5L is illuminated and has a flat square slider that surrounds the centre lamp. The central post that supports the lamp is also present in T-5C, but it is not fitted out. The partial document listed below also mentions T-5S and T-5H types, but the pages describing these forms are not included.

The part number schema is as follows:

T-5S-A-CC(-HS)

T-5: T-5 series
S: Sub-series; C = tall straight stem; L = illuminated; H and S also cited but missing from documentation
A: Action: M = momentary; A = alternate action (latching)
CC: Contact arrangement: NO = SPST NO, OO = DPST NO, OC = SPST NO + SPST NC
HS: Heavy spring (single pole momentary action only)

Form C (SPDT changeover) is not available, but this can be achieved using the "OC" version by joining two of the terminals together.

Discovered variants

Type	Part	Configuration	Slider	Keystem	Gold plating	Markings
T-5C	T-5C-M-NO	Momentary SPST NO	Black	Tall upright	Dual	MEI, bottom (RS)
		Momentary SPST NO	Grey	Tall upright	Single	MEI, bottom; WEAB, top
T-5C	T-5C-M-NO-HS	Momentary SPST NO, heavy spring	Black	Tall upright	Dual	MEI, bottom (RS)
		Momentary SPST NO	Black	Short angled		MEI, bottom; WEAB, top
		Alternate action SPST NO	Black	Short angled		MEI?
T-5L	T-5L-M-NO?	Momentary SPST NO, illuminated	Grey	Flat	Single	WEAB, bottom

Flavours

MEI T-5 switches can broadly be divided into two flavours: dual gold plated and single gold plated. The switches sold by RS have both terminals gold plated, but the switches were also sold with only one terminal gold plated, specifically the one that doubles as the stationary contact. The dual gold plated version has been found with black sliders, while the single gold plated version has been found with grey sliders, both the supposed T-5L-M-NO shown above and the Compukit UK101 switches.^[2]

Switches with single gold plated terminals have flattened tips to the terminals, while the RS switches with dual gold plated terminals have tapered tips.

RS 337-217 with dual gold-plated terminals and MEI branding on the base
Supposed T-5L-M-NO with single gold-plated terminal, and WEAB branding on the base

The two MEI WEAB switch types depicted below (momentary and alternate action) are unclear. One photo suggests that the momentary action switches have a lower-carat gold plating while the alternate action switch is of the true single gold plated form (one terminal deep gold colour and the other terminal a silver colour). The photos are simply not clear or precise enough to make any determinations about these switches and how they fit. The alternate action switch is not "WEAB" branded, though.

WEAB

Jacob Alexander's original name for these was "MEI WEAB". This seemed logical as the MEI switches used in the Volker Craig VC404 terminal keyboard were branded "MEI" on the base, and "WEAB" on the top of the slider retention frame. Subsequently, this situation has become increasingly unclear. The Compukit UK101 switches resemble T-5C, but have "WEAB" moulded into the top of the slider adjacent to the keystem; again, the base is branded "MEI". However, the visually corresponding T-5L switch has no slider branding, and the base is instead branded "WEAB", with no mention of "MEI".

No meaning for the term "WEAB" has ever been found. It does not remotely resemble the part numbers, and it is not known to have been any parent company or subsidiary of either Mechanical Enterprises or its successor, Datalux. It does not appear to be a series or model name, as "T-5" is the only term that Datalux have offered to describe the series,^[3] which was given unprompted prior to confirming this name by way of the RS catalogue excerpt.

As such, neither the term "WEAB" nor the slider colour can be conclusively related to any subset of T-5 switches (by design or by age) at this stage, as too few examples of these switches are known. The rarity of the switch hinders the rate of progress of understanding it.

WEAB angled stem

The switches found in the Volker Craig VC404 are of a form that does not correspond to anything in T-5 catalogue extract. Unlike T-5C, the keystems are a conventional height and are angled to provide a stepped layout. The momentary action switch uses a spring with nine or ten turns, so this is likely to be a medium weight switch. The alternate action switch has a spring with the same number of turns but greater length, making the switch heavier; this may explain why the HS suffix does not apply to alternate action switches.

Note that the switches do not have the redundant centre post for lamp terminals, unlike the T-5C switches sold through RS.