A device for harvesting an external source of energy includes an electricity generating device, a flexure, and a restoring spring. The electricity generating device generates electricity based on movement of the flexure. The flexure has a first stable position. When disregarding presence of the restoring spring the flexure also has a second stable position different from the first stable position. The restoring spring is positioned so when the flexure snaps from the first stable position toward the second stable position the restoring spring provides a restoring spring force to restore the flexure toward the first stable position. Substantially greater force is needed to snap the flexure from the first stable position toward the second stable position to overcome the restoring spring force than is required to snap the flexure from the second stable position toward the first stable position.