Abstract
Internet of Things (IoT) sensors are becoming increasingly popular in agriculture for a wide range of applications, such as fine-tuning irrigation schedules and enabling more efficient and sustainable crop management. This work presents the design and validation of a low-power IoT system for monitoring the growth rate of apple fruits in real time. The system uses a non-invasive flexible and screen-printed strain gauge to detect dimensional changes in apples on the plant over time. A Wheatstone bridge circuit, coupled with a multiplexed instrumentation amplifier and an analog-to-digital converter (ADC), ensures accurate resistance measurements on multiple sensors at one time. Wireless data transmission takes advantage of the LoRaWAN architecture using an STM32-based development kit. With a smart low-power design that minimizes power consumption at about 15 mJ per measurement cycle every 15 minutes, the system supports long-term deployment and allows future integration of energy harvesting. Experimental validation shows an average error of less than 1% throughout the resistance measurement range covering almost two decades of variation, confirming the feasibility of the proposed approach for the management of precision orchards.