A mechanical resonance system is used in VP version fork sensors. The sensor's electrical circuitry excites and maintains the mechanical element in resonance. A piezoelectric crystal is subjected to an electrical stimulus. This electrical excitation produces physical deformation of the crystal, resulting in vibration of the probe element at its natural resonance frequency. When there is no material around the fork, the vibration is reduced and recognized by the electronic circuitry. As a result, the relay output and local LED indication change.