Optical Wake-up from Power-off State for Autonomous Optical Sensor Nodes

Abstract

Wireless sensor nodes spend most of their time in standby mode and wake up periodically to send the measurement data. Conventionally, the wake-up function is realized using a radio-frequency oscillator with an amplifier to recognize an activating radio signal. Therefore, even during standby operation, the sensor node utilizes a certain amount of energy, which can be critical for an energy-harvesting source. In this study, we propose a novel approach of an optical wake-up for autonomous sensor nodes, which employs a solar cell as a wake-up signal detector. The bright light flash coming from another node or a smartphone exposes a solar cell, which activates the sensor node. Unlike photodiodes or RF-antennas, solar cells do not require any additional energy to detect the light signal. Therefore, the proposed electric circuit allows the sensor node to wake-up from a complete power-off state. The solar cell of the novel wake-up receiver has a sensitive area of 8 mm x 10 mm. The wake-up signal can be recognized from a maximal distance of 25 cm at ambient illumination from 0 to 1600 lx with a transmitter optical power of 20 mW. At power-off state the power consumptions are the lowest among all existing off-the-shelf wake-up receivers: 248 pW at 0 lx and 627 nW at 1600 lx.