A miniature sensor node is an attractive solution for internet-of-things (IoT) applications as it connects the physical world to computing systems with minimal disturbance. Replacing discharged batteries of the deployed sensing systems is difficult due to cost issue. Thus, to extend the system life time, energy harvesting techniques have been developed as one of the most important features for the miniature sensing systems to recharge the batteries. As system size decreases down to a millimeter scale, battery capacity and available power become extremely limited, necessitating efficient energy harvesters. In this presentation, we discuss low-power circuit design techniques such as a self-oscillating switched-capacitor harvester, a maximum-power-point tracking circuit using a transferred charge calculator, and a reconfigurable photovoltaic-cell network. These techniques help to develop energy-autonomous miniature IoT devices even in low-energy environment.