What is Projected Capacitance Technology ?

Projected Capacitance Technology is a method of detecting an object at a distance ( proximity sensing ) ,  either though air, or through other slightly conductive, or non-conductive materials.
The object to be detected, may be conductive, such as a metal, or non-conductive, such as a person.
It is often used to detect a finger operating a touchscreen or keypad, through thick glass.

The technology is very similar to capacitive technology but with three major differences:

1) More than one capacitance measurement is involved.

2) Only changes in capacitace are used.   Absolute values may effectively be ignored.

3) A relatively high degree of data manipulation may be used to compare, and discriminate between,  these different capacitance measurements.

This enables a technology, that is readily influenced by a multitude of environmental variables, to be used to accurately detect much smaller changes in one selected variable, such as the close
 proximity of a finger.

How do these three factors enable Projected Capacitance to work so well ?

Many factors, such as ambient temperature and humidity, affect all capacitance sensing elements in a keypad/touchscreen array.

A person standing close to the keypad will affect its capacitance, as will a passing bus. These effects, however, will be global.

A finger, on the other hand, affects one key more than it affects others.

The touch action of a finger also has a certain profile - touch happens within a few seconds, and will usually be gone after a few more seconds.
     Environmental effects usually happen more slowly and affect more than one key.

Parts of a keypad/touchscreen may change with time by natural ageing processes, or through external factors, such as damage, or dirt accumulation.
     These changes will greatly alter the capacitance environment, either globally, or locally.
     Projected Capacitance, continually tracks these changes.

By taking all these variables into account, it is possible for a finger to be detected with great sensitivity and accuracy, over very long periods of time , with no false triggering

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