Keyboard technology

Keyboard construction, in four layers, of a typical notebook computer keyboard


There are many types of keyboard, usually differentiated by the switch technology employed in their operation. Since there are so many switches needed (usually about 80-110) and because they have to be highly reliable, this usually defines the keyboard. The choice of switch technology affects key response (the positive feedback that a key has been pressed) and travel (the distance needed to push the key to enter a character reliably). Newer models use hybrids of various technologies to achieve greater cost savings.

Types:

Dome-switch keyboard


How a dome-switch keyboard works: Finger depresses the dome to complete the circuit



Dome-switches mesh with keys (keyboard is upside down in this image)

Dome-switch keyboards are kind of a hybrid of membrane and mechanical keyboards. They bring two circuit board traces together under a rubber "dome" or bubble. The inside of the top of the bubble is coated in graphite. When a key is pressed, it collapses the dome, which connects the two circuit traces and completes the connection to enter the character. The pattern on the PC board is often gold-plated.

This is a common switch technology used in mass market keyboards today. It is considered very quiet, but purists tend to find it "mushy" because the collapsing dome does not provide as much positive response as a hard closing switch. These are also a good choice for office or consumer environments because they are generally fairly quiet. This switch technology also happens to be most commonly used in handheld controllers, such as those used with home video game consoles.

Dome-switch keyboards are also called direct-switch keyboards.

Scissor-switch keyboard

On a scissor-switch keyboard, the keys are attached to the keyboard via two plastic pieces that interlock in a "scissor"-like fashion, and snap to the keyboard and the key. Scissor-switch technology does not use a rubber-plunger assembly [citation needed] like most other keyboards. Scissor-switch keyboards are normally responsive and crisp. These keyboards are generally quiet. The keys require little force to press. This keyboard technology is mainly used in laptops.

Capacitive keyboard

In this type of keyboard, pressing the key changes the capacitance of a pattern printed on a PC board. Usually this permits a pulse or pulse train to be sensed. Unlike "dome switch" keyboards, the pattern will be covered by a thin, insulating film. Capacitive keyboards are inexpensive, and resist wear, water, foreign objects and dirt. They are common in PC keyboards.

Mechanical-switch keyboard

Mechanical-switch keyboards use real switches, one under each key. Depending on the construction of the switch, these keyboards have varying responses and travel times. Notable keyboards utilizing this technology are the Apple Extended II, and its modern imitator, the Matias Tactile Pro. These two keyboards use ALPS switches.

Cherry Corporation of Germany also makes mechanical switches used in special purpose and high end keyboards. In India, the TVS Gold mechanical keyboard is very popular despite costing about five times a membrane keyboard.

Buckling-spring keyboard

Many typists prefer buckling-spring keyboards. [1][2] It is a common misconception that the IBM Model M and its derivates are mechanical-switch keyboards. In fact, the Model M uses membrane-sheet switches, much like those found in a dome-switch keyboard. The buckling spring mechanism (U.S. Patent 4,118,611) atop the switch is responsible for the tactile and aural response of the keyboard. This mechanism controls a small hammer that strikes the membrane switch.[3].

In 1993, two years after spawning Lexmark, IBM transferred its keyboard operations to the daughter company. New Model M keyboards continued to be manufactured for IBM by Lexmark until 1996, when Unicomp purchased the keyboard technology.

Today, Unicomp manufactures new buckling-spring keyboards and repairs old IBM and Lexmark keyboards. Unfortunately, the later Lexmark-manufactured Model M keyboards are of inferior quality to the original IBM-manufactured Model M. The plastic used is of lower grade (density) and other features such as a detachable heavy duty keyboard cord were replaced by cheap substitutes.

Hall-effect keyboard

Hall effect keyboards use magnets and Hall effect sensors instead of an actual switch. When a key is depressed, it moves a magnet, which is detected by the solid-state sensor. These keyboards are extremely reliable, and are able to accept millions of keystrokes before failing. They are used for ultra-high reliability applications, in locations like nuclear power plants or aircraft cockpits. They are also sometimes used in industrial environments. These keyboards can be easily made totally waterproof. They also resist large amounts of dust and contaminants. Because a magnet and sensor is required for each key, as well as custom control electronics, they are very expensive.

Laser keyboard

A laser projection device approximately the size of a computer mouse projects the outline of keyboard keys onto a flat surface, such as a table or desk. When the laser is interrupted in the position of a key, a keystroke is registered. This type of keyboard is portable enough to be easily used with PDAs and cell phones, and many models have retractable cords and wireless capabilities. However, sudden or accidental disruption of the laser will register unwanted keystrokes.

Also, if the laser malfunctions, the whole unit becomes useless, unlike conventional keyboards which can be used even if a variety of parts (such as the keycaps) are removed. This type of keyboard can be cumbersome to use since it is susceptible to errors, even in the course of normal typing, and its complete lack of tactile feedback makes it even less user-friendly than the cheapest membrane keyboards.

Membrane keyboard

Main article: Membrane keyboard Membrane keyboards are usually flat. They are most often found on appliances like microwave ovens or photocopiers. A common design consists of three layers. The top layer (and the one the user touches) has the labels printed on its front and conductive stripes printed on the back. Under this it has a spacer layer, which holds the front and back layer apart so that they do not normally make electrical contact. The back layer has conductive stripes printed perpendicularly to those of the front layer.

When placed together, the stripes form a grid. When the user pushes down at a particular position, his finger pushes the front layer down through the spacer layer to close a circuit at one of the intersections of the grid. This indicates to the computer or keyboard control processor that a particular button has been pressed. Membrane keyboards do not generally have much of a "feel", so many machines which use them issue a beep or flash a light when the key is pressed. They are often used in harsh environments where water or leak proofing is desirable. Although used in the early days of the personal computer (on the ZX80, ZX81 and Atari 400), they have been supplanted by the more tactile dome and mechanical switch keyboards. However, membrane keyboards with interchangeable key layouts, such as the IntelliKeys and Discover: board are still commonly used by people with physical, visual, or cognitive disabilities as well as people who require assistive technology to access a computer.