Passive Wireless Sensing of Temperature Using RF Technology Available to Purchase

Sensor networks are emerging as an attractive technology for deployment in monitoring applications due to their flexibility, small size, ease of installation, low cost and low power characteristics. Since the wireless nodes use batteries as energy sources, their operating lives are limited by the life of the batteries. Replacing batteries periodically in a sensor network can be a burden as the nodes may be dispersed over a wide area. Moreover the battery increases the size of the sensor nodes and offsets one of the primary advantages of such networks. This study proposes a design for wireless sensing of temperature based on passive RF-technology. The experimental setup includes: 1) signal generator 2) sensor tag 3) receiver. The carrier signal is produced by function generator. The sensor tag consists of a voltage divider circuit combined with loop antennas. The sensor tag filters the carrier signal and transmits back a modulated signal of the same frequency. The variable resistor acting as a thermistor is connected as a voltage divider. The change in amplitude of the output signal is relative to the temperature change. Experimental results show that the passive RF design can be effective way for wireless temperature monitoring. The results can be generalized to any sensor that converts the measured signal into a change of resistance.