SINGLE capacitor metallic and EARTHED figure.As the object approaches so the 'plate SEPARATION' of the capacitor changes, becoming significant and detectable when the object is close to the probe.Magnetic field proximity sensors are RELATIVELY simple and can be made using a permanent magnet. The magnet can be made a part of object being detected or can be part of the sensor device It can only be used for the detection of metal object and is best with ferrous metal.