An improved ultra-low-power wireless sensor-station supplied by a photovoltaic harvester

In this work we describe a system optimized to use the few microwatts generated by a 4 mm2 photovoltaic energy harvester to acquire, process and wirelessly transmit information about the environment temperature and light irradiance. The energy needed for the described power-expensive operations is obtained by charging a buffer capacitor while the system is in a low-power-consumption state. A voltage level detector senses the charging status of the capacitor and wakes-up the sensing station every time enough energy is stored. Thus, the environmental parameters are monitorized and transmitted with an asynchronous and intermittent strategy. The optimization work related to this paper has persued the target of minimize the unerasable continuous power consumption of the voltage detector during the charging of the capacitor. We reached the goal of reducing it from the 50 μW of the previous version of the system to the current 5 μW, with a significant usability improvement. Simulations and experimental evidences demonstrate that our station can work totally battery-free when just illuminated with 100 W/m2 of light irradiance, transmitting data with a 802.15.4 compliant protocol and potentially occupies a volume of 1 cm3.