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2017-September-19

Capacitive touch sensor
(theory)


Input capacitance

The image above explain input capacitance and shows, what items are significant. Overal input capacitance is sum of touch pad capacitance, connection wire capacitance and optional input capacitor (10pF in the figure).

Static values are automatically compensated and capacitance variations are converted to frequency changes. Further detection is based on special algorithm that use time division and multiple frequency. There is also built in automatic compensation, that eliminate temperature and humidity changes. To work properly in RF noise, sensors have advanced digital filtering stage. This ensures reliable function.

Finger acts as conductor put over touch pad and GND plane, input capacitance significantly arises

When finger is out of touch pad, input capacitance is lower than if finger is near touch pad. This is monitored by touch sensor and as soon as capacitance change with time match pattern that define „touch”, sensor output is activated.

Sensitivity is adjustable parameter. Simple devices have special control pin, that can be used to set input sensitivity to low, middle or high level. I2C controlled devices has special registers, where can be set sensitivity as well as other parameters.

Some sensors can even detect touch intensity. For example TSM12 can detect intensity in three steps. Application then may respond by small (f.e. volume) change to soft touch and by high volume change to hard touch.



Relative permittivity of some substances
Substanceεr
Vacuum1
Air1,000 59
Expanded polystyrene1,03
Wood2,04 - 7,3
Amber2,6 - 2,8
Plexiglass3,4
Glass3,8 - 19
FR-44,5
Ice4,8
Porcelain5,5 - 6,5
NaCl (monocrystalline)5,9
Marble7 - 8
Silicon11,7
Germanium16 - 16,4
Water81


For more application circuits and component explanation see detail documentation available on our web.


, 2017