An LC Passive Wireless Multifunctional Sensor Using a Relay Switch

This paper presents an LC passive wireless sensor for monitoring both humidity and pressure by using a relay switch. The LC sensor system consists of an inductor, two capacitors, and a relay switch. The two capacitors are successively connected to the inductor controlled by the relay switch, forming two different LC resonant circuits. The relay switch is actuated by induced voltage, which is coupled from the readout coil. The theoretical model is proposed and analyzed, and operation equations are derived. To demonstrate the proposed scheme, a print circuit board inductor, two capacitive sensors to monitoring humidity and pressure, and a relay switch are constructed. Ambient humidity and pressure are tested by utilizing the sensor system. The capacitance derived from the detected resonant frequencies indicates that the sensitivity of the humidity sensor is 277.1-fF/% RH, and the sensitivity of the pressure sensor is 38.3 fF/kPa.