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MEI T-15 series


Mechanical Enterprises T-15 series remains poorly understood. In both the patent (priority date 8th March 1985) and the 1985 advertisement in EEM 85 86 (Electronic Engineers Master Catalog 28th edition), 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 slider does not come out: the switch is designed to function without a cover.

At the time of writing of the EEM 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”. When I took possession of Meryl Miller’s T-15 samples, I let Jacob Alexander take the rest of the T-15 switches, and he got the only one that accepted a stabiliser wire, which he has never knowingly documented (I did not realise until too late that this part was different, otherwise I would have requested that Meryl send that one to me). In the only photo I have seen of it (from Meryl), there is no suggestion that it supports plate mounting, and the other switches have covers that support neither plate mounting nor stabiliser wires. There is however a step at either end of the cover that could be used, Cherry M8–style, to allow the cover to hold the switches in place, without actually retaining them fully.

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 T15 uses bare conductive rubber to bridge the stationary contacts.


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 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 pretty terrible (in part due to how small the switch is) but they should just about give an idea about the design. This is the covered version of T-15S, which may have a different series name; these are sample parts that Meryl Miller collected a long time ago. One was destroyed in the disassembly attempts, 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


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