Ultra-Low Power Public Signboard

We recently installed an ultra-low-power 84″ equivalent epaper sunlight-readable digital signboard for a city council in Silicon Valley, California.

When I say ultra-low-power it is not hyperbole, the display itself uses zero power to display, the driver board electronics use about 2.1W total and the media player is ticking along at approximately 1.4W. A comparably sized sunlight readable LCD display has a power requirement of 1450W to 1850W, ie well over 400 times the power requirement.

Here is the signboard:

Digital View digital epaper signboard

When the display changes the image the power consumption increases to a total of approximately 18W for 6 seconds, and as image updates are typically infrequent, the average stays incredibly low.

Here is a table with the data for all the local parts to the display:

Signboard – idleSignboard – writeLCD sunlight readable
(3000 nits)
Displayzero1450W to 1850W
Media player1.4W3W1.4W to 3W**
LTE modem4W6.25W4W to 6.25W
* Total, each driver uses approx 0.7W, ** Or more if a PC or other device is used as a media player

Key Components

Display: As indicated in the table above the e-paper display, made using three 42″ E Ink panels, does not use any power to show the image, it is literally zero power. The actual display only consumes power when the content is updated and we have included that in the figures for the driver board as shown above.

Controller/Driver: The E Ink driver board is our model EPM-100, there is one for each panel. They consume about 0.7W each when idle and together with the panel consume about 14.6W when writing full screen content. I mention full screen as it is possible to write to part of the screen. Update 1 Oct 2021: We have new code to further reduce the idle power consumption and will provide revised figures on the reduced power consumption shortly.

Media Player: For this project we used a Raspberry Pi 3B+, which is very compact, low power at approximately 1.4W when idle and 3W when processing, and quite capable for this signboard application. The image is downloaded as a single image, the Pi then splits the image into three parts and writes the image to the display one panel at a time. There are three panels and each takes about 2 seconds to update.

LTE modem: The 4G LTE modem used here has an idle power consumption of 4W with a peak during data transmission of 6.25W.

Digital View e-paper public signboard
Showing the displays during the day and at night lit by two LED lights
Digital View e-paper public signboard
Showing two images running from a playlist

Further Power Reduction

With suitable circuitry and applications it would be possible to reduce the power further, here are a few ideas:

  • The EPM-100 driver could sit behind a power switch so that it is only activated when needed for writing to the display. We made such a circuit board and it could be triggered by a command from the Pi.
  • Both the Pi and the driver board could sit behind a remotely activated power switch so both of them are only powered on when needed. This would need a suitable command passed from the modem.
  • The modem, the Pi and the driver could sit behind a timer controlled power switch. For example the system is only powered on at preset intervals.

Solar &/or Wind Power

With such a low power display system solar and/or wind power may be an option. Clearly there are design considerations as the positioning and size of the solar panel is important and this may conflict with the display installation. Also, solar is dependent on clear skies so cloudy environments need larger solar panels and larger batteries. Wind power clearly relies on wind and a suitably sized fan or turbine.

Dark Sky Signage

Thanks to being a reflective digital display technology these can be completely Dark Sky compliant, they emit zero light as a display technology. For example here is a photo of these displays, clearly very visible in sunlight but also visible purely from ambient light at night.

Update May 26, 2022

This is an update of the original blog posted on August 10, 2021: We have continued to update the firmware to provide greater monitoring as well as built-in failsafe methods such as a watchdog and reset capabilities.

Additionally, for the single screen signboard which didn’t have lighting, we recently installed a test LED light strip as shown in the photos below.

The photo below shows the installation of the LED strip. This also has an ambient light sensor with a delay so vehicle headlights do not turn the light off when passing.