Circuits and fields
Capacitance is the ability of two separate plates to hold electrical charges because of the difference in potential and change in electric field between the plates - Circuits and fields introduction. Capacitance is directly proportional to the dielectric constant and the area of overlap between the plates, and inversely proportional to the distance between the plates. A capacitive sensor is a device that perceives a change in the capacitance whenever something comes in contact with the sensor. A capacitive sensor usually has a positive plate and a negative plate. The difference in the potential of these two plates produces a constant electric field between them. But there is this so-called fringe capacitance due to non-idealities of any device. The fringe capacitance is formed because of the fringe electric field flowing from the positive to the negative plate that is not part of the electric field across the gap between the two plates. A capacitive sensor works by detecting the change in the fringe capacitance. A decrease in the fringe capacitance could be caused by a human hand placed near the sensor. Whenever a hand comes in contact with the sensor, it affects the fringe capacitance by interfering with the path of the fringe electric field, thus causing a change in the fringe capacitance in the sensor.
How a capacitive sensor perceives a change in the fringe electric field.1
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A specific application of a capacitive sensor that uses this concept is a proximity sensor. A good example would be a touchpad on gadgets like mp3 players and cellular phones. Whenever a human hand comes close to the touchpad of an electronic gadget, it changes the detected fringe capacitance of the sensor, thus it indicates that touchpad has been touched.
Analog Devices, Inc., Capacitance Sensors for Human Interfaces to Electronic Equipment, 2006, retrieved 12 April 2008 <http://www.analog.com/library/analogdialogue/archives/40-10/cap_sensors.html>