Abstract: A circular layer of piezoelectric material is supported on a larger substrate disc of a thin somewhat flexible material. Flexing of the disc generates an electrical signal. The substrate disc is weighted at its periphery, and the central portion of the disc is supported on a member for transferring motion or vibration that it is desired to detect. In the preferred embodiment the center support is provided by a pair of bosses projecting into the otherwise hollow interior of a two-part casing that encloses the disc-weight assembly. The weights impart a peripheral inertia that makes the composite unit sensitive to minute vibrations.