Hardware Reference
In-Depth Information
the foam pads and the circuit board, thus restoring the keyboard to a “like-new” condi-
tion. Unfortunately, over time, the corrosion problem will occur again. I recommend us-
ing some Stabilant 22a from D.W. Electrochemicals (
www.stabilant.com
)
to improve the
switch contact action and prevent future corrosion. Because of such problems, the foam
element design is not used much anymore and has been superseded in popularity by the
rubber dome design.
KeyTronicEMS, the most well-known user of this technology, now uses a center-bearing
membrane switch technology inits keyboards, soyouare likely toencounter foam-switch
keyboards only on old systems.
Rubber Dome Switches
Rubberdomeswitchesaremechanicalswitchessimilartothefoamelementtype,butthey
areimprovedinmanyways.Insteadofaspring,theseswitchesusearubberdomethathas
a carbon button contact on the underside. As you press a key, the key plunger presses on
the rubber dome, causing it to resist and then collapse all at once, much like the top of an
oil can. As the rubber dome collapses, the user feels the tactile feedback, and the carbon
button makes contact between the circuit board traces below. When the key is released,
the rubber dome re-forms and pushes the key back up.
The rubber eliminates the need for a spring and provides a reasonable amount of tactile
feedback without special clips or other parts. Rubber dome switches use a carbon button
because it resists corrosion and has a self-cleaning action on the metal contacts below.
Therubberdomesthemselvesareformedintoasheetthatcompletelyprotectsthecontacts
below from dirt, dust, and even minor spills. This type of switch design is the simplest,
and it uses the fewest parts. This made the rubber dome keyswitch reliable for several
years. However, its relatively poor tactile feedback has led most keyboard manufacturers
to switch to the membrane switch design covered in the next section.
Membrane Switches
The membrane keyswitch is a variation on the rubber dome type, using a flat, flexible cir-
cuit board to receive input and transmit it to the keyboard microcontroller. Industrial ver-
sions of membrane boards use a single sheet for keys that sits on the rubber dome sheet
for protection against harsh environments. This arrangement severely limits key travel.
For this reason, flat-surface membrane keyboards are not considered usable for normal
touch typing. However, they are ideal for use in extremely harsh environments. Because
thesheetscanbebondedtogetherandsealedfromtheelements,membranekeyboardscan
be used when no other type could survive. Many industrial applications use membrane
keyboards for terminals that do not require extensive data entry but are used instead to
operate equipment, such as cash registers and point-of-sale terminals in restaurants.
