Spring over membrane
Spring-over-membrane keyboards use a spring to apply pressure to the membrane assembly. The most common arrangement involves two springs—one for actuation and one for the return force—but designs with only one coil spring also exist. The use of only springs stands in contrast with rubber dome–based designs.
Numerous manufacturers produced keyboards with two springs per key. These include:
- Oak Full Travel Membrane
- Alps spring over membrane, with self-contained plunger modules from which the actuation spring projects from a hole in the base
- Alps moulded spring over membrane, a cost-reduced variant of the above that does not use discrete modules
- Mitsumi spring over membrane
- Oriental Tech spring-over-membrane, with one spring below the plunger and one below the keycap
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.
Matsushita (now Panasonic) are the only manufacturer confirmed to have made keyboards of this design. The exact source of Amstrad keyboards remains a mystery.
Single-spring designs have only a single helical spring. This one spring provides both membrane pressure and return force. Such a spring needs to be a special shape in order for this to work. Two such designs are known at present.
Oki “gourd 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).