One aspect of the present patent application is an energy harvesting device comprising a composite structure including a base spring and a piezoelectric structure. The base spring has a base spring surface having elevated portions separated by a recessed portion. The piezoelectric structure substantially crosses the recessed portion. In one aspect the piezoelectric structure includes a piezoelectric element that is bonded to the elevated portions. In another aspect, the base spring has a base spring stiffness. The piezoelectric element has a piezoelectric element stiffness. The base spring stiffness is less than the piezoelectric element stiffness. In another aspect, the composite structure has a natural frequency of vibration, and this natural frequency of vibration of the composite structure is automatically adjustable. In another aspect, the piezoelectric elements are stacked. In another aspect, the piezoelectric structure is located in the recessed portion.