Keystroke sensing
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A contact mechanism is the electrical mechanism in the switch used to register a key press by the keyboard's controller logic. Completely different switching mechanisms may use the same contact mechanisms – for example, rubber domes, Cherry MY switches and certain buckling spring implementations use a membrane contact mechanism despite the fact that these switches have otherwise completely different characteristics. Consequently terms such as "membrane keyboard" or "capacitive keyboard" are ambiguous, and should be avoided unless discussing the contact mechanism itself.
Membrane
Membrane contact mechanisms use thin plastic sheets with exposed electrical traces on them. These sheets, made from a material such as mylar, are extremely thin: those in a Dell KB522 for example are around 0.7 mm thick each, around the same thickness as a typical sheet of paper.
There are two styles of membrane keyboard: pressure actuator and conductive actuator. Pressure-actuated designs use three membrane sheets. The bottom sheet contains circuit traces on the top, and the top sheet has the traces on the bottom. A separator sheet containing holes is placed in between; the holes allow specific points on the top membrane to be pressed down onto the bottom membrane, upon which circuit paths are connected. Various types of actuator exist; the most common three are rubber domes, helical springs, and the plastic hammers in buckling spring switches.
Very little pressure is required to press the membrane sheets together, and it is not possible to feel the plastic flex downwards by around 0.7 mm; with the rubber domes removed, the membrane layers simply resemble a "futuristic"-looking solid-state capacitive panel.
The other type of membrane keyboard uses conductive actuators. A conductive element in the actuator makes contact with the membrane sheet and conducts current between adjacent traces. The two most common actuator types are carbon-coated rubber domes and the carbon-impregnated rubber foot in the Mitsumi hybrid switch. Conductive actuator membrane keyboards are significantly less common; most membrane keyboards are pressure actuated.
The membrane layers must be kept flat; in older keyboards, they were backed by a sheet of steel, but in newer keyboards, the rear case will support the membranes directly.
Membranes are cheap, flexible and compact, but have poor long term reliability, and are easy to damage. They are most associated with rubber dome keyboards (including scissor switch keyboards), but they are also used with some high end switch designs such as buckling springs.
Capacitive
A capacitive contact mechanism makes use of a property of capacitors - that their capacitance is related to the distance between the terminals (conductive contacts) that make up the capacitor. A switch which uses a capacitive contact mechanism will move a conductive pad towards or away from a contact connected to a signal generator. The capacitance between these two is measured (often using a third contact attached to a sensor) and the switch actuates or resets depending on changes in capacitance. Capacitive contacts have a long expected lifetime, but are also quite expensive and have thus fallen out of popularity for general purpose usage. They are also sensitive to contamination which can alter the behavior of the capacitive contacts, but usually these problems can be resolved by cleaning the contacts. Capacitive sensing lives on as the basis of most touch screen and trackpad technology used today.
As of 2013, Topre is possibly the only remaining producer of capacitive keyboards for mainstream usage. Old IBM designs such as the IBM Model F and beam spring keyboards used capacitive sensing. Some foam and foil designs from Key Tronic and BTC in the early 1980s made use of them too. Capacitive contact mechanisms do not suffer from issues of ghosting or blocking due to the fact that they are not dependent on the flow of current through the contacts.
Metal leaf
A metal leaf contact mechanism consists of two thin metal plates that are pressed together by the switching mechanism. They last longer than membrane contacts, but are still more wear prone than non-ohmic (i.e. dependent on current flow) contact mechanisms. They are commonly associated with individual microswitch designs such as the Cherry MX and most of Alps Electric's switch designs.
Somewhat related are designs that use two metal filaments that are kept separate at rest, and then pushed together by the depression of the switch. The NMB Hi-Tek ("Space Invader") switch uses such a mechanism, as did a popular type of linear switch (manufacturer unknown) that was used in many 1970s terminal keyboards such as that of the DEC VT100.
Magnetic reed
A magnetic reed contact mechanism consists of two metal contacts, hermetically sealed and usually with some inert gas inside a glass envelope. These contacts are brought into contact with each other, when a magnetic field pulls them in a given perpendicular direction. This is usually done by bringing a magnet close to the place where the contacts can connect.
The magnetic reed switch mechanism was invented in 1936 by W B Ellwood at Bell Telephone Laboratories. It was commonly used in computer interface devices (keyboards) commonly until the 1980s.
Hall effect
Hall effect keyboards use a solid state sensor to detect the movement of a magnet. A typical implementation of a hall effect switch consists of a linear switch which has a magnet attached to the end of key slider, whose movement when the key is pressed actuates the switch. They are extraordinarily reliable, with an expected lifetime measured in billions of key presses. However, they are too expensive for general use, and are usually reserved for military and aerospace applications.
Opto-electric
Opto-electric keyboards use a light source (usually infrared), and a light detector (e.g. photo-resistor) to detect the presence of a key press. In known keyboards this is done by breaking the line between the light source and detector, and registering the change. These switches are highly reliable due to their non-contact nature, but often quite susceptible to dust if design improperly.
It should also be noted that patents for keyboards exist where varying powers or different wavelengths of light are used to detect different keystrokes. However, none of these switching mechanisms have been seen in actual keyboards.
