the vibration energy harvester is intended to convert ambient environmental energy into electrical energy. it has great potential to be an alternative to the conventional battery in low-power electric devices. a traditional vibrating energy harvester consists of a linear oscillator that operates in a narrow frequency band. nonlinear energy harvesters provide a promising solution to widen the operating bandwidth. based on the system stability states, the nonlinear vibration energy harvesters can be classified as mono-stable and multi-stable. in this study, a tunable multi-stable piezoelectric energy harvester is proposed. the apparatus can be manually tuned to achieve three states, namely tri-stable, bi-stable, and mono-stable. it has a compact structure compared to the existing designs. [...]