# An IoT Reader for Wireless Passive Electromagnetic Sensors
## 1 Introduction  
In the last few years, many electromagnetic, microwave or radiofrequency (RF) sensors have been reported in the literature (some good examples are [1; 2; 3; 4; 5; 6]). One important advantage of electromagnetic or RF sensors is that they can be easily integrated with antennas, resulting in wireless devices (e.g., [5; 6]). One of the current trends in electromagnetic sensors is the use of radiofrequency identification (RFID) approaches. A typical RFID system consists of two elements: the reader and the tags [7]. Each tag contains a unique identification code. The reader interrogates the tags located within the read range, and the tags reply to the reader with their identification codes. In order to follow this operation protocol, most tags contain two elements: an antenna and a RFID chip. During the last decade wireless sensors based on RFID chips have been proposed [8; 9; 10; 11; 12; 13]. However, the inclusion of RFID chips increases the price of tags and sensors, limiting its success. For this reason, chipless passive technologies (RFID tags and sensors) have been proposed [14; 15; 16; 17; 18]. In spite of its drawbacks, for instance, the shorter range and the more limited number of bits or information than in chip-based RFID sensors, chipless sensors are a simpler and cheaper alternative for passive sensing and the best solution for disposable sensors. Most of the previous works focus only on the sensor part, while ignoring the reader. This philosophy leads to the use of general purpose, complex and highly expensive laboratory equipment which can be only operated by trained personnel to read the sensor response. For this reason, the development of specific reading systems for electromagnetic sensors is crucial for the success of this technology. Electromagnetic chipless sensors can be designed as fully passive wireless devices. These characteristics make this kind of sensors a good candidate for the Internet of Things (IoT). The IoT is the connection of physical items to communication networks (e.g., the Internet) that enable these items to collect and exchange data [19]. This global infrastructure represents the next step towards the digitization of our society and economy [19; 20; 21; 22; 23]. The development of sensor systems with networking and cloud capabilities is crucial for the final establishment of the IoT [20; 21; 22; 23]. Some recent proposals on sensors, interfaces and platforms for the IoT can be found in the literature [24; 25; 26; 27; 28; 29]. These works show potential applications of the IoT on different areas, such as industrial processes [24; 25], home monitoring [26] or health and telemedicine [27; 28; 29]. However, up to the authors' knowledge, there are no IoT platforms for reading the response of passive wireless electromagnetic sensors. In this work, a novel IoT reader (IoT-R) for passive wireless electromagnetic sensors is proposed. This reader scheme can be used to interrogate passive electromagnetic sensors within a short-range wireless link. For example, this is useful for the remote monitoring of harmful substances. The proposed IoT-R is based on an innovative layer scheme which is firstly presented in this article. The implementation of this proposal results on a portable, easy-to-operate and low-cost solution. Moreover, the addition of IoT functionalities to the reader allows remote sensing from computers or handheld devices. Other applications such as automatic cloud storage or cloud computing (e.g., cloud postprocessing of the data sent by the reader) are possible with the proposed technology. The remote operation of the IoT-R is fully automated and efficient, reducing the human errors in acquisition, processing and storage of sensing data. Finally, a full implementation of an IoT-R for wireless sensors is presented in this article. The objective is to show a fully functional demonstrator of the proposed technology.