This paper presents a fully self-powered interface circuit with a novel peak detector for piezoelectric energy harvesters (PEH). This circuit can be utilized to scavenge energy from low power environmental vibrations in 10s of mu W range. Synchronous switching technique is used to extract maximum available power where switching instants are detected independently from excitation changes of the PEH. The proposed peak detector senses voltages higher than power supply for a wide frequency range of input vibration. The simulations with an output voltage range of 1 to 3.3 V show power conversion efficiency between 79% and 88% for an input power of 13.2 mu W.