Abstract:
The large-scale deployment of sensor nodes in difficult-to-reach locations makes
powering of sensor nodes via batteries impractical. Besides, battery-powered WSNs require
the periodic replacement of batteries. Wireless, battery-less sensor nodes represent a less
maintenance-intensive, more environmentally friendly and compact alternative to battery powered
sensor nodes. Moreover, such nodes are powered through wireless energy harvesting. In this research,
we propose a novel battery-less wireless sensor node which is powered by a dedicated 4 W EIRP
920 MHz radio frequency (RF) energy device. The system is designed to provide complete off-grid
Internet of Things (IoT) applications. To this end we have designed a power base station which
derives its power from solar PV panels to radiate the RF energy used to power the sensor node. We use
a PIC32MX220F32 microcontroller to implement a CC-CV battery charging algorithm to control the
step-down DC-DC converter which charges lithium-ion batteries that power the RF transmitter and
amplifier, respectively. A 12 element Yagi antenna was designed and optimized using the FEKO
electromagnetic software. We design a step-up converter to step the voltage output from a single stage
fully cross-coupled RF-DC converter circuit up to 3.3 V. Finally, we use the power requirements of the
sensor node to size the storage capacity of the capacitor of the energy harvesting circuit. The results
obtained from the experiments performed showed that enough RF energy was harvested over a
distance of 15 m to allow the sensor node complete one sense-transmit operation for a duration of
156 min. The Yagi antenna achieved a gain of 12.62 dBi and a return loss of −14.11 dB at 920 MHz,
while the battery was correctly charged according to the CC-CV algorithm through the control of the
DC-DC converter.
