Physical Clock Calibration of Wireless Sensor Node Based on Indoor LED Light

Accurate and stable clock is one of the important guarantees for normal operation of wireless sensor networks (WSNs). As the quartz crystal ages and the ambient temperature changes, the clock of each sensor node will inevitably drift. Clock performance deterioration of wireless sensor node can lead to its out-of-step in the time synchronization of the whole network. Regular physical clock calibration of each sensor node is an important prerequisite for achieving network-wide time synchronization. This article proposes a novel physical clock calibration method, which leverages the fact that the light-emitting diode (LED) light intensity changes with a stable frequency which equals twice the frequency of ac voltage in the low-voltage grid. By tuning to the light emitted from indoor LED lamp, the method can intelligently extract a clock reference signal and achieve network clock calibration of all these sensor nodes in a specific light coverage area by referring to such a common clock reference. An actual circuit has been developed to validate the proposed method. Within a 7 m lighting range generated by an ordinary commercial 36 W, 3000 lumen LED lamp, the experimental circuit can accurately and stably receive a 100 Hz clock reference signal emitted by the LED lamp. Using this reference signal to calibrate the clock of the sensor node can greatly improve its long-term stability, reaching approximately
$3\times 10^{-{9}}$
/week.