Capacitance Presence Sensor
This type of sensor is often used on assembly lines and in controlling industrial processes.
The capacitor is formed by 2 plates embedded in an insulating dielectric material.
A signal applied to plate A is coupled to plate B, where it causes a current to flow in an external circuit.
When no object is present , the coupling between A and B is minimal, so only a small current flows in the output circuit connected to B.
The voltage across the load resistor B is small, but not zero, under the no-object condition. It is
called the " baseline response".
When an object is in the field of the capacitor formed by A and B, the capacitance between the plates changes, which causes a change in the amount of coupled signal.
Mechanism
Large conductive object