Remote low power devices that will constitute the internet of things can be powered by scavenging energy from environmental mechanical vibrations. Triboelectric generators (TEG) are devices that can be used to convert vibration motion into electrical energy. In this paper we explore how freely vibrating TEGs excited indirectly by external forces may be designed. The design of TEGs depends on several variables and application specifications. First the system is modeled and the related parametric variables are studied. A design algorithm is proposed to achieve resonance condition. Based on the algorithm single-impact TEG and double-impact TEG devices were fabricated and their vibration behavior measured. The double-impact TEG showed four times improvement in voltage output and two times improvement in operation frequency bandwidth. Finally, the proposed system model was simulated using MATLAB for the two TEG designs. It was shown that the simulation supports the experimental results.