Liquid-Tolerant Design Considerations
CY8C20xx7/S CapSense
®
Design Guide
Doc. No. 001-78329 Rev. *E
61
Figure 6-6. Schematic View of Polarity Select
6.1.2 Guard Sensor
Figure 6-7. PCB with Shield and Guard Sensor
A guard sensor is a copper trace, as shown in
, that surrounds all the sensors on the PCB, which is used to
detect the presence of a continuous water stream or a large liquid spill. When a water stream or a large liquid spill is
present on the sensing surface, a large-capacitance C
ST
is added to the system, as shown in
. This
capacitance may be several times larger than C
LD
. Because of this, the effect of the shield electrode is completely
masked and the raw counts measured by the sensor will be the same as or even higher than a finger touch. In this
situation, a guard sensor will help; when it detects a water stream, it will block the other sensors from triggering.
PCB
Shield Electrode
Guard Sensor
Sensor Pads
