Simultaneous Remote Sensing of Temperature and Humidity by LC-Type Passive Wireless Sensors

This paper presents an integrated wireless passive sensor for remotely monitoring both temperature and relative humidity. The sensor consists of a coil and a capacitor to form an inductor-capacitor (LC) resonant circuit, which oscillates electrically at its resonant frequency. The inductor was a singlelayer planar spiral copper inductor and the capacitor was fabricated by the silicon-on-glass process, which utilizes graphene oxide films as the sensing material. The change of the capacitance due to environmental humidity variation shifts the resonant frequency, while environmental temperature affects the resistance and capacitance of the LC circuit and changes the resonant frequency and quality factor. By monitoring the real portion magnitude maximum of the impedance and the resonant frequency for the sensor, it is possible to get the capacitance and resistance from which the temperature and humidity can be extracted. The results presented here show that the sensitivity of the passive wireless sensor is about -17.80 kHz/%RH and 7.32 Q/%RH at 25 °C from 55%RH to 95%RH, and it is about -7.69 kHz/°C and 6.27 Ω/°C at 65%RH from 10 °C to 40 °C.