Design of a low-power autonomous energy-harvesting system for direct plant health monitoring

Agritech and precision agriculture are emerging as key approaches to enhance the sustainability and efficiency of modern farming systems. Among the various techniques proposed to achieve this goal, the direct monitoring of plants through electrical measurements represents a promising and innovative solution. Building on previous work conducted within the same research group, this thesis focuses on the design and development of an autonomous, battery-free, and low-power system capable of assessing plant health status through stem impedance measurements. The main objective of this work is twofold: to achieve complete energetic independence by exploiting solar energy harvesting, and to design and validate an alternative low-cost and low-power measurement architecture that improves upon the traditional relaxation oscillator circuit previously employed. To this end, an initial prototype was realized using an STM32WL55JC1 Nucleo board combined with an energy-harvesting shield provided by STMicroelectronics. The system was used to develop a dedicated firmware managing the charging and discharging phases of storage capacitors, the measurement and data transmission processes via the LoRa communication protocol, and the overall power management strategy ensuring autonomous operation under variable lighting conditions. Two measurement circuits were implemented and compared: a validated reference circuit based on a LMC555 timer in oscillator configuration, and the proposed Capacitive Divider Measurement System, designed to further minimize energy consumption, and to minimize the electrical stress experienced by the plant during measurements. The prototype was experimentally validated through a long-term measurement campaign conducted on a tobacco plant placed inside a controlled-environment grow tent, where environmental parameters and irrigation events were continuously monitored and correlated with impedance variations. Finally, a custom PCB was designed, fabricated, and assembled to integrate all the developed functionalities, as solar energy harvesting, dual impedance measurement systems, and LoRa communication, into a single board conceived as a dedicated shield for the LoRaTO platform previously developed by the research group. The resulting system successfully demonstrated reliable and energy-efficient operation, featuring an almost maintenance-free design with a theoretically infinite lifetime, confirming its potential as a compact and sustainable solution for precision agriculture applications.