Trade offs to get to low power: The Display

A project log for Newt: Always On, Low Power Digital Assistant

An always-on digital assistant that fits on a desk/nightstand/mirror/wall/fridge. Weather, alarm, timer, calendar, habit tracker, to-do list

Darian JohnsonDarian Johnson 03/20/2021 at 05:190 Comments

The largest power draws on an always on smart display are:

  1. the display
  2. the microcontroller 
  3. the internet connection method

This Log entry focuses on the display

Adafruit's breakout for the Sharp 2.7 in Memory Display

Given the desire for low power, TFT displays were out. The backlights would drain any battery in a matter of days.

OLED displays are better (from a power consumption perspective), but they still require 20 mA of power.... fine for a periodic display, but no good for what I wanted.

After looking around, there were really only two viable options

  1. a eINK display
  2. a Sharp Memory Display

E-Ink is all the rage these days. They can retain their screen image without any power (ultimate low power) and they aren't too expensive... but, there's one big drawback: the process of updating the screen - which can take 1-3 seconds and is somewhat jarring (here's a great video on the process). The sequence wouldn't allow a user to quickly toggle between screens, so eInk was a no-go.

That left the Sharp Memory Display as my only option. Sharp Displays are low power (the 2.7in version only requires 10-35 uA). They look as sharp as an OLED (without the current burden). They are relatively easy to use (SPI).

The drawback: they are  just plain ol' expensive... $20/each to purchase 250 (per Digikey).

Running BOM (@250 units)

ComponentFunctionDeep Sleep (uA)Costs (USD)
Sharp Memory Display (LS027B7DH01A)Display10-35$20.00