Battery-less NFC Sensors for the Internet of Things

2Case Study 1: Soil Moisture Sensor

2.1. Motivation

Soil moisture measurement is a key indicator for certain applications such as irrigation water management. Irrigation regulation is essential in order to satisfy the water requirement of crops without wasting water and to properly treat the soil, controlling plant nutrients in order to save energy. Most sophisticated systems are automated using electronic climate-based controllers, whereas the simplest repeat a set of scheduled tasks at a set time. Low-cost monitoring systems are demanded for personal use at home or to monitor a particular crop, such as in a greenhouse or plant store. In these cases, irrigation is often manual or semiautomatic and the conditions are usually specific for each crop. Therefore, for appropriate plant care, soil moisture control is an important aspect. There are several soil moisture measurement techniques (Reynolds 1970) but most of them are not suitable for amateur or low-cost applications. Most typical portable low-cost meters use a sensor connected to the analog-to-digital converter (ADC) of a microcontroller, which translates a physical value to the corresponding magnitude. The result is often shown in a display or acquired by an external data logger. In all cases, these methods use a battery to supply energy to the electronic components.

Several wireless soil moisture sensors are found in the literature (Gutiérrez et al. 2014; Yunseop and William 2015). However, those sensors require an expensive ...