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
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).
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:
Identifiable by the Matsushita “M” logo on the sticker affixed to the metal backplate. The known examples are all integrated mount.
- Unknown Panasonic keyboard — click the orange “MORE” buttons for the pictures
- Acorn A3010 (and possibly also the A3020) — a label can be seen that matches that of the unknown Panasonic keyboard, and the site owner has confirmed that it bears a Matsushita logo
This is only so far known to be used in the SMK-made variant of the Apple Keyboard II. From the only disassembly shots known to exist, it is not clear whether SMK’s design is in fact the same, as a plastic prong is seemingly not present. This keyboard may use a different means to provide the full amount of pretravel.