questions to power consumption

i am an absolute beginner (and even worse with an enterprise programmer background) and thankful for every hint and critic :-)

for my version of a word clock i am trying to use an e-paper display with an espruino pico

the display is this one: http://smart-prototyping.com/NOA-Labs/E-­ink-E-paper-new-version-display-module-3­.3V-2.04-inch-177x72.html

so far i have the components and i am on my way to migrate (luckily provided from manufacturer, some quite complicated command sequences) the arduino code to espruino

the final goal is to run from battery, for this i would be thankful if someone can answer my questions

my take on power consumption

• assuming 2s operating time every 5 minutes (its a word clock, update every 5 minutes is enough for me), 24s per hour
  
• Pico deep sleep needs 0,02mA/h
  
• Pico operating needs 32mA/h
  
• Display operating needs 8mA/h (but it is an e-paper, power is only needed to update the display, content stays after that)
  
  

Pico current draw in mA/h = 0,23 mA/h = (0,02mA/h:3600s*(3600s-24s)) + (32mA/h:3600s*24s)
Display current draw in mA/h = 0,053 mA/h = 8mA/h:3600s*24s
final power consumption ~ 0,29 mA/h

• assuming 1900mA/h from 2x AA eneloops (2.4v)
  
  • according to the specification the display works from 1.9v - 3.3v
    
    

operating time ~ 0,76 years = (1900mA/H:0,286533333 mA/h:24h:365d)

i know that is just a theoretical value, voltage drop will probably happen earlier

Questions

• Is my take on the power consumption correct?
  
• Are 2.4v enough to drive the pico?
  
• Is it possible to control the VBAT Pin to only activate at designated timer?
  
  • So far i think i need a transistor, but maybe it is possible without an extra component.
    
  • Are there alternatives to my approach? (can a GPIO pin drive the display? i am not sure about the current draw)
    
    

updated

• was 2s operating time per hour, but should be (60min/5min)*2s=24s
  

• Is my take on the power consumption correct?
  
  

I didn't rework the maths, but it sounds about right. In reality you'd hope the display update would take less than 2 seconds as well.

• Are 2.4v enough to drive the pico?
  
  

Just :) It'll run down to 2v, but the voltage regulator will drop maybe 300mV. You could connect the batteries straight to the 3.3v line though.

• Is it possible to control the VBAT Pin to only activate at designated timer?
  
  

No, but the IO pins will supply 20mA, so as long as the display didn't peak above 8mA then they should be good enough.

Also, the display itself may only draw 8mA when writing - it may be much less when idle.

Hope that helps!

The other thing to add is at the moment, the Pico (despite having an RTC) isn't great at timekeeping since it uses the internal RC oscillator - you'd need to solder a crystal on the end of it to make it accurate enough for a proper clock.

thx for the help

i did read about the clock problem, my idea is to use GPS or DCF77

Great! There's already a DCF77 module for Espruino here as well: http://www.espruino.com/DCF77

Having said that, I found my module to have very bad reception when in the house and it only got the time occasionally... I think in my case it was all the interference caused by WS2811 LEDs though - using epaper would be much better.

GPS sounds great though, and potentially (it's a bit of a hack) you could work out how far off Espruino's clock was relative to GPS and could then just multiply or divide it.

right now i still work on preliminaries like learning C and Arduino-style to migrate the provided code (display text or image) to espruino