THIS SELF-POWERED BADGE is all about energy-harvesting light and efficiently charging a supercap to power an ePaper display once in a while. The tiny display shows a loop of several calculated patterns inspired by #plottertwitter. This badge requires no battery and no charging cable and is energy-neutral.
HARDWARE - A MSP430 with integrated FRAM and external crystal was selected, providing low power consumption during active state and deep sleep mode. The EPD booster circuit can be turned-off with a hardware switch to achieve lowest possible deep sleep current. One external booster component was changed for improved efficiency during display HV generation. The PMIC for harvesting and providing the power supply is from E-Peas/Fujitsu and supports MPPT.
SOFTWARE - The firmware supports a shorter image update, achieving around 60% energy savings compared to the default update mode. GPIOs are set to lowest possible leakage. MCU sleeps almost all time, just wakes up to trigger an update if sufficient energy is available.
RESULTS - Deep sleep current is around 1.5µA and the needed energy for an typical image update is around 4mJ.
DISCUSSION - At daylight, minutely update rates are possible depending on light intensity, at full sun light even down to seconds. Indoor light based on Halogen showed fast update rates of several minutes, even down to dim 100 Lux. Harvesting efficiency of LED bulbs is much lower and would require more PV cells or cells with improved efficiency. At this point the badge does not really charge under singular LED illumination unless its very bright.