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MEI T-15 series

Contents

Overview

Mechanical Enterprises T-15 series remains poorly understood. In both the patent (US patent 4600819 “Switch assembly” filed in March 1985) and the Electronic Engineers Master (EEM) 1985–86 advertisement, the switch is depicted bare without a cover. For some reason, MEI offered their switches both with and without a cover. This is why, when the cover is removed, the plunger does not come out: the switch is designed to function without a cover. In their EEM 1988–89 and 1989–90 advertisements, T-15 was said to be produced and tested on fully-automated machinery in the US.

At the time of writing of the EEM 1985–86 advertisement, the optional “hinge cover” was not yet available, being expected in the coming August (1985). The “hinge cover” contains the mounting points for the stabiliser wire, but also allows for “sub-panel mounting”.

T-15 series (and T-15S in particular) bears some resemblance to DN series, including the ultra-low profile, the bare assembly design, and the sealed contacts. Where DN series presses metal balls against concave contact surfaces, T-15S series presses a metal spring against the outside surfaces of the contacts. The simplified version of T-15 uses bare conductive rubber to bridge the stationary contacts.

Keystem

The 1985 advertisement states:

“The T-15 keytop mount is the most frequently used cruciform style (equivalent to Cherry MX and others). Therefore several keytop manufacturers in the U.S. and abroad are able to supply, without a tooling charge sculptured and conventional keytop sets for the T-15.”

T-15S

T-15S is depicted as uncovered, with the spring-in-tube design. It is not stated whether “T-15S” refers to the entire series, or just the spring-in-tube variant, or just the uncovered spring-in-tube design, or some other combination of characteristics. T-15S is described as “sealed contact”, which the common variety with the dual orange rubber pieces is definitely not, so that is likely to be a different subseries.

Surprisingly, it was possible to order clicky and alternate action versions of the switch. This is why the switch only has a single piece of rubber on the left, as the space to the right is left open for the placement of a click or alternate action mechanism.

The 1985 advertisement lists the following part numbers:

Part Description
T-15SB Basic momentary
T-15SC Momentary with click
T-15SA Alternate action

The photos below are not the most detailed (in part due to how small the switch is) but they should give an idea about the design. This is the covered version of T-15S, which may have a different subseries name. These are sample parts that Meryl Miller collected a long time ago; two have been destroyed in order to depict their internal structure, and one remains intact.

View full-size image Top view of covered variant
View full-size image Cherry MX mount test using Tai-Hao keycap
View full-size image Cover removed from the switch
View full-size image Single elastic piece on one side in the sealed version
View full-size image Clip-in contact rails
View full-size image Switch contacts; the rubber tube has been cut open to show the spring inside

Specifications

Momentary Alternate action
Contact rating, DC resistive 1 W max
Voltage 24 V max
Initial contact resistance 100 mΩ max
Plunger travel 3.8 mm
Pretravel 1.9±0.6 mm
Operating force to make 65±20 gf
Temperature range, operating −20°C to 65°C
Temperature range, storage −40°C to 85°C
Contact bounce 5 ms max
UL flammability rating 94-HB
Life, actuations 20×10⁶ 10×10⁴

Keyboards

In their 1988 and 1989 Electronic Engineers Master Catalog advertisements, MEI offered stock 16, 20 and 64-position keypads and keyboards based around T-15 switches. These were matrix-only assemblies.

The only complete keyboards that MEI are known to have produced are the Microtype range of compact units, later known as Datalux SpaceSaver.

Documentation

All literature scanned by Bitsavers unless otherwise noted.