Battery-less luminance sensor biomimicking human sensory nervous system

Abstract

With the evolution of materials science and microfabrication processes, energy harvesters have become sophisticated, achieving power outputs in the range of several milliwatts, and have become a promising alternative to conventional batteries. Although their output power is insufficient to continuously operate a wireless sensor module, energy harvesters can operate small integrated circuits, including timers, watches, and ring oscillators. In this study, we emulated the human sensory nervous system to develop a battery-less sensor with a built-in analog-to-digital converter. The human sensory nervous system comprises a sophisticated sensing mechanism that digitalizes external stimuli by pulse-density modulation. To mimic this behavior, we integrated a ring oscillator with photovoltaics, allowing it to function as a luminance sensor with an event-driven operation. The oscillation frequency of the ring oscillator changes with respect to the operating voltage; hence, the output voltage of the photovoltaic modulates the frequency by more than two orders of magnitude. The sensor exhibits oscillation frequencies of 10 kHz and 7.7 MHz corresponding to luminance levels of 25–25 000 lx. Its response times are 40 μs and 15 ms when the light source is turned on and off, respectively. Battery-less sensors expand the opportunities for the application of energy harvesters in biomedical, wearable, and environmental sensing.