Cypress Semiconductor CapSense CY8CKIT-022, Руководство пользователя, Страница: 17 / 33

CY8CKIT-022 CapSense® Liquid Level Sensing Shield Kit Guide, Doc. #: 002-09825 Rev. ** 17
Kit Hardware
2.3 Theory of Operation
For a detailed explanation on the theory of operation of this kit, refer to AN202478 - PSoC 4 -
Capacitive Liquid Level Sensing .
This section provides a high-level description of the principle of capacitive sensing and describes
where the CY8CKIT-022 kit and PSoC devices fit in a capacitive liquid level sensing system.
2.3.1 Principle of Capacitive Liquid Level Sensing
Capacitive liquid level sensors are conductive pads or traces laid on non-conductive material such
as PCB, plastic, or glass. The intrinsic capacitance of the PCB trace, pads, and other connections to
the sensor results in a sensor parasitic capacitance (C
P
). When a capacitive sensor is excited by a
voltage source, an electric field is created around the sensor. A small number of electric field lines
couple with the nearby ground, while most of the electric field lines are projected into the nearby
space of the liquid container, as shown in
Figure 2-7 .
When a target object such as water approaches the sensor, the electric field couples with it and adds
small amount of liquid capacitance (C
L
) to the existing C
P
, as shown in Figure 2-8 . The capacitive
liquid level sensing technique involves measuring this change in the capacitance of a sensor when
water is near the sensor.
Figure 2-7. Capacitance and Electric Field of a Capacitive Liquid Level Sensor
Figure 2-8. Added Capacitance (C
L
) When Liquid Approaches a Capacitive Sensor
PCB
Ground Ground
C
P
C
P
Container Wall
Liquid
Sensor
PCB
Ground Ground
C
P
C
P
Container Wall
Liquid
Sensor
C
L