Hardware Reference
In-Depth Information
called
buckling spring
keyboards because of the coiled spring and rocker used in each
keyswitch to provide tactile and audible feedback.
Figure 15.6
A capacitive buckling spring keyswitch.
A capacitive switch does not work by making contact between conductors. Instead, two
plates usually made ofplastic are connected inaswitch matrix designed todetect changes
inthecapacitanceofthecircuit.Themembraneversionoftheswitchusesupperandlower
electrical contact sheets separated by an insulating sheet with small holes.
When a key is pressed, the plunger moves the top plate in relation to the fixed bottom
plate,oritpressesahammeroverthemembranes,allowingthetopandbottomcontactsto
touch. The buckling spring mechanism provides for distinct over-center tactile feedback
with a resounding “click.” As the capacitance between the two plates changes or the
two membranes make contact, the comparator circuitry in the keyboard detects this as a
keypress.
The tactile feedback is unsurpassed because the buckling spring design provides a relat-
ively loud click and a strong over-center feel. The only drawback to the design is the cost.
Bucklingspringkeyboardsareamongthemostexpensivedesigns,whethertheyusecapa-
citive ormembrane switches. Thequalityofthefeelandtheirdurability makethemworth
the price, however.
Although some of IBM's older keyboards featured capacitive keyswitches, most current
IBM/Lenovo keyboards use either membrane, rubber dome, or other lower-cost key-
switches. In 1991, IBM spun off its keyboard/printer division as Lexmark, which then
spun off the keyboard division as Unicomp in 1996. Today, Unicomp still manufactures
and sells “IBM” keyboards with buckling spring membrane switch (“clickety” as some
would say) technology. You can purchase new Unicomp (IBM) keyboards direct by
