How Projected Capacitance First Evolved_____
While looking for a holiday, in the early 1980s, Ron Binstead noticed that,
although Travel Agents used to advertise their holidays on cards in the
window, they could get the same, and further information through their computer, inside the
Ron thought it would be great if the customer could get
some of this information by operating a computer keypad directly through the
To this end, in 1983, Ron and his brother Malcolm, decided to
make a keypad that could sense a person's finger through the thick glass of a
They decided to use the change in
frequency, caused by the finger approaching the input of a Resistor/Capacitor
(RC) oscillator, to detect the finger through the glass.
Although a very small change in frequency was
detected, it was found that temperature, humidity, and other environmental
factors, such as passing busses also caused a similar change in frequency,
making the product rather unreliable.
It was realised, however, that
various improvements could be made to overcome these
factors would affect all the keys in a similar manner and so, by taking this
"common mode" effect into account , many of these environmental factors could be
2) By assuming that the
keypad was not being touched at "Power-up", a capacitance reading could be made, for all inputs.
values could be stored as the "no-touch" reference values for each of the keys.
It was also assumed that no-one would ever operate every one of the keys at the same time, and so, the keys that changed the least could
be treated as "non-touched" keys, and act as
references for the other keys.
These assumptions led to remarkable improvements in the sensitivity and
stability of the keypad.
Over long periods of time, however, the product
was still proving somewhat unreliable, due to the fact that the keys did not all
change by exactly the same amount as environmental conditions changed.
effects within the keypad, such as small changes in capacitance caused by the
build up of dirt on the glass, or localised environmental changes caused, for
example, by the Sun's shadow slowly moving across the screen, affected some
keys and not others.
Fortunately, however, these effects are
fairly slow, when compared to the speed of change caused by touch. They could, therefore, be eliminated by slowly, but constantly, tracking any
changes and adjusting each keys original "no-touch" value to approach the current value.
This solved the problem and enabled the keypad to operate through the
shop window successfully, for days or months, without any false triggering.
This created the first "Dynamic Capacitance" Window Keypad .
Because of the application for which it was developed
, Ron Binstead called this technology "THRUGLASS Technology"
This technology later became known as "PROJECTED CAPACITANCE or PROJECTIVE CAPACITIVE".
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