Solar powered ePaper Clock 24/7

Low power 1.1" ePaper Node - Powered by Light and a Supercap and thus battery-free

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A simple clock demo for a 1.1" Always-On E Ink display and ATmega328pb, updating every five minutes. During the day its powered by light through PV cells and storing the additional energy in a supercap. During the night this harvested energy is used to allow continuous operation until next day to achieve a 24/7 coverage.

Hardware and software are optimized for low power operation during deep sleep phase and operational phases as well. As a result there are no coin cells needed, the device is running battery-free while keeping the form factor as lightweight as possible.

All testing currently done on window desks, looking to south-east, north-west and east working OK around winter solstice and with several rainy days in series; so should work easily in spring/summer/autumn too. Now looking for further improving towards indoor usage with artificial LED lights.

Feel free to use this demo as staring point for your own projects via FTDI adapter and Arduino IDE.

Hardware specification

  • MCU: ATmega328pb (16MHz, 32KB FLASH, 2KB SRAM)
  • External RTC: MCP7940M
  • LoRa Chip: RFM95W, Antenna EU_863_870 or u.FL connector for external antenna
  • EPD: 1.1" Plastic Logic, 148x70pixel
  • Flash: Winbond W25X40Cl, 4Mbit
  • Low power design: Deep Sleep 2.4uA
  • PV cells: IXYS SLMD121H04L
  • Energy-harvesting PMIC: E-Peas AEM10941 w/ MPPT
  • Storage device: EDLC supercap 1.5F


A description of the #PaperiNode components and the schematic can be found at here at Hackster..

  • 1 × PaperiNode Solar powered E-Paper Node for LoRaWAN
  • 1 × FTDI programmer Needed to updated new sketches via Arduino-IDE

  • First indoor LED light measurements

    Robert Poser01/30/2021 at 13:53 0 comments

    To get a better feeling for the limits of the PV-powered device under indoor LED illumination I modified one of our ikea floor lamps with a phillips hue LED bulb as can be seen below. The color of the light can be changed and also the intensity can be configured between 0...~3000Lux. The solar cells are oriented towards the LED bulb; the rest of it is covered with a beautiful hand towel... my wife was really happy with this setup too:-) To measure the light intensity I used a light meter MT-912 for less than 20 bucks, which works for me much better than some of the available mobile phone apps, especially with LED and dim light.

    BildWith this setup I was now looking for the minimal light intensity which is just recharging the supercap back to its last state for a given update rate (e.g. 5mins.). So the clock demo was running and the light intensity was changed until I found a stable condition (for around 12hours) at which the supercap voltage remained unchanged during each update (because it just reloaded before the update). Luckily the voltage is measured via a voltage divider and printed on the screen so no manual measurement needed here. This measurement was done for 5 mins update rate, three, one and 10mins update frequency, the related minimum LED light intensity can be seen below.

    Typical indoor intensities for living rooms may vary around 200Lux..400Lux, so what we see is not super performand for indoor usage  yet... the used mono crystaline IXYs PV cells work really well when exposed to full sun light but with this dim artifical lights we can just harvest a couple of Micro-amps.

    Nevertheless; i repeated the same measurement by using the earlier mentioned epaper update mode with low power settings(shorter update time, 2GLs). The arduino clock sketch uses this mode when the supercap voltage is below a certain voltage threshold (e.g. 4.2V) automatically. It can now be seen that about half of the light intensity is only needed for a given update rate.  I thinks thats some good data to share and a big step forward. 


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marazm wrote 01/07/2021 at 17:32 point

Can I use mruby on this device?

  Are you sure? yes | no

Robert Poser wrote 01/30/2021 at 13:12 point

I think the ATmega328pb MCU is to small for mruby.

  Are you sure? yes | no

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