Cypress Semiconductor CapSense CY8CKIT-024, Руководство пользователя, Страница: 13 / 29

Kit Hardware
CY8CKIT-024 CapSense
®
Proximity Shield Kit Guide, Doc. No. 001-92568 Rev. *B 13
2.2 Theory of Operation
This section provides a high-level description of the principle of proximity sensing and describes where the CY8CKIT-024 kit
and PSoC devices fit in a capacitive proximity-sensing system.
2.2.1 Principle of Capacitive Proximity Sensing
Capacitive proximity sensors are conductive (usually copper/indium tin oxide) pads or traces laid on a nonconductive
material like PCB or glass. The intrinsic capacitance of the PCB trace or pads or other connections to a capacitive sensor
results in a sensor parasitic capacitance (C
P
). When a capacitive proximity 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, as
Figure 2-4 shows.
When a target object such as a finger, hand, or conductive object approaches the sensor, the electric field couples with it
and adds a small amount of finger capacitance (C
F
) to the existing C
P
, as Figure 2-5 shows. The capacitive proximity-
sensing technique involves measuring this change in the capacitance of a proximity sensor when a target object
approaches the sensor.
Figure 2-4: Capacitance and Electric Field of a Capacitive
Proximity Sensor
Figure 2-5: Added Capacitance (C
F
) When a Hand
Approaches a Proximity Sensor
PCB
Ground Ground
C
P
C
P
Overlay
Proximity
Sensor
PCB
Ground Ground
C
P
C
P
C
F
Overlay
Proximity
Sensor