LoRa Based Moisture Sensing System

Abstract: Water is an important parameter for crop growth, and the information about the moisture content in soil at different depths is very useful for farmers to determine the best time to water the soil and to irrigate farmland so as to maximize their yield. This thesis project aims to develop a prototype of a multi-depth moisture sensor probe that is part of a large sensing system used in agriculture. The sensor probe has three sets and is required to last for 6-12 months of usage and to be reproduced at a low cost. The sensor probe consists of three sensor boards, on each of which has two different capacitive based sensors and one analog temperature sensor. The three boards are placed approximately 20 cm from each other in the probe. During this project, the two capacitive based sensors were developed, one with arc-shaped plates operating at a frequency less than 1 MHz, and one with electrodes in the form of annular rings operating at a high frequency, approximately 100 MHz. The moisture content in the soil is calculated based on the measurement of the frequency, which depends on the dielectric constant of the soil. For the implementation of the sensor probe, three printed circuit boards (PCBs) for the sensor boards were designed using Altium Designer and then ordered; an STM32 Nucleo board with low power microcontroller was used and the software was implemented in STM32CubeIDE. The lifetime of the sensor probe was calculated for different duty-cycles. With a duty-cycle of 15 minutes, where the sensor probe is active for 1 minute and in sleep mode for 14 minutes, the lifetime of the sensor probe would only be 16 days. With a duty-cycle of 120 minutes instead, with the sensor probe being active for 1 minute, the lifetime is increased to 130 days (less than4,5 months). Due to challenges with the high frequency capacitive sensor, the multi-depth sensor probe does not fully work, and thus cannot be tested with a large testbed. Further work needs to be conducted on the high frequency capacitive sensor and the communication with the gateway.