MEI T-15 series
|Introduced||1988 or earlier|
|Total travel||3 mm|
|Keycap mount||Cherry MX mount|
|Switch mount||PCB mount|
T-15 series (formerly known as the MEI switch) is a low-profile switch from Mechanical Enterprises (MEI). T-15 series switches were patented by Robert Twyford, the founder of MEI. Although most examples are branded "MEI", unbranded switches have also been encountered; these may have come after the reincorporation of MEI as Datalux.
The switch contains two conductive rails running around the inside; loops of this rail protrude from the bottom of switch, two per side, forming the the terminals. The slider is supported by one or two curved pieces of elastic, resembling segments of a very thick rubber dome. As this elastic block bows outwards under pressure, it connects two metal rails together.
The two rails are identical, and there is a matching gap on either side of the switch. There may be either one or two pieces of rubber under the slider. Both designs shown in the patent use one piece of rubber, with only one gap bridged during actuation. while all switches found in keyboards have two pieces of rubber, giving the switch redundancy through connecting both gaps between the rails.
There are two methods by which the circuit can be closed. The primary design in the patent is of a rubber sleeve containing a coil of wire, which bridges the gap between the ends of the rails. The elastic block deforms this sleeve, allowing the coil to touch both rails, forming a conductive bridge. This variant is depicted as Version A, below. The patent also describes a simpler approach, which is a conductive bar embedded into the elastic block. This design has yet to be encountered; the switches found in keyboards instead appear to have the elastic blocks directly coated with a conductive, carbon-based material.
The switch shell is 7 mm tall; with the slider, the height comes to 9.6 mm. Actuation is around 70 gf, and the terminal force is around 105 gf. Version A comprises 8 parts, while the production Version B has 7.
The switch has been criticised for its feel. The original design switch below does have a stiff, distinctly rubber feel that suggests that it would offer a poor typing experience; it is however hard to say how the feel compares to when the switch was made.
The date of discontinuation is not known.
The rails are connected by a miniature coil spring within a rubber tube. The photos show sample switches obtained in or before 1991; narrowing down the date further isn't possible.
The following photo, of a sample switch from 1991 or earlier, appears to show a variant of the switch where the stabiliser wire mounts directly onto the switch shell.
The rails are connected by an exposed conductive surface.
MEI switches in a Datalux SpaceSaver