  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.
