I am trying to do a model of piezoelectric energy harvester. basically consists of a single-supply pre-biasing circuit and a buck converter circuit supplied by a battery. I need a constant voltage aroung 8V to do pre-biasing, thus the input voltage of buck converter is 8V. I use a schmitt trigger and a pulse to control the switch. (Vcc > 9V, turn on switch; Vcc< 7V, turn off switch).