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Single spring over membrane

There are several very similar designs of single-spring-over-membrane switch. Designs such as Oriental Tech spring-over-membrane and Alps spring-over-membrane use two helical springs: one spring applies pressure to the membrane assembly, and the other is the return spring.

Two springs are always required, but the functionality of the second spring can be achieved by other means.

True single spring

Two single-spring designs have only a single helical spring. The best-known approach is Oki’s gourd spring and tactile gourd spring, which use a non-inverting stepped spring.

Omron vase spring

Omron vase spring is very different to the gourd spring. The single spring consists of two sections that taper towards each other, with the lower section compressing first and providing the pretravel, and the much taller upper section providing the overtravel. The single recorded instance to date is from 1985; the keyboard (with a B5Y instead of B5G part number) appears to be dual membrane over PCB instead of triple membrane, but this is not clear).

Plastic prong

More common designs use a standard helical spring, supported by a stiff but flexible plastic prong which holds the spring above the membrane; this prong appears to be formed from the slider guide plate. As the key is pressed down, the helical spring is compressed. As pressure mounts in the helical spring, the plastic prong is deflected slowly towards the membrane assembly until it is pressing hard enough on the membranes to close the contact pads and actuate the key. The return spring can be compressed further, providing overtravel.

In many cases, such keyboard assemblies are unbranded. The following branded versions are known:

Matsushita spring-over-membrane

Identifiable by the Matsushita “M” logo on the sticker affixed to the metal backplate. The known examples are all integrated mount.

Keyboards:

Within the keyboard community, these keyboards are widely attributed to Amstrad. While it does appear that Amstrad had their own production facilities (and that they could have produced keyboards under licence), we also know that Amstrad keyboards were sourced from at least two manufacturers, namely Mitsumi and Matsushita. Jacob Alexander also possesses another Amstrad prong-over-membrane keyboard that bears Matsushita’s diamond logo on the membranes. A related CPC-464 keyboard exists that uses concentric springs; the OEM is not shown.