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I am building a raspberrypi zero eink photoframe. My aim is to create a photo frame that I would be able to put on the wall without cables hanging around, battery powered. In order to achieve this I thought that an eink screen would be a good idea as it will not require too much power to refresh the screen and keeping the pi waking up and turn off using cronjobs.

My understanding after reading a bit around is that cronjobs that deal with turning on/off the raspberrypi would not work as it does not have a clock. So I was thinking of a solution that involves the addition of a RTC (real time clock) to the pi. Using an RTC and a cronjob, my overall plan would be:

  • The raspberry pi get turned on
  • A script fetches the new image that need to be displayed connecting to my photo library
  • The picture get displayed
  • Turn off the pi
  • Turn on after X minutes/hours

Two questions:

  • Would that be possible with a combo of RTC + battery?

  • What would be an estimate of the battery life of the frame?

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  • The pi is also not capable of turning itself on and off (at least, not the models I've checked). So you'll have to fix that, and tie the timer into that as well. Does the turn-on time have to be accurate, or just happen periodically?
    – user10489
    Commented Jun 21, 2021 at 11:38
  • Thanks, I was not aware of that. It should just happen periodically, no need to an exact time. I was thinking of this solution only to preserve the battery.
    – VDC
    Commented Jun 21, 2021 at 13:59
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    Turning it off and on periodically is a good idea, it will just take some implementation. A timer circuit to turn it on would not be hard, but it would be a bit easier if it didn't have to be accurate.
    – user10489
    Commented Jun 21, 2021 at 17:22
  • Have you read this question about Pi zero on battery? raspberrypi.stackexchange.com/questions/40181/… In my opinion I would try to use an Arduino with sd-card shield instead.
    – Robert
    Commented Jun 21, 2021 at 18:53
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    Something like the Sleepy pi might work.
    – user10489
    Commented Jun 21, 2021 at 19:22

1 Answer 1

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The raspberry pi has no power management features. It has no power saving mode, and it can't turn itself on or off, so cron really isn't much help here at all.

However, there are multiple external kits that can do parts of what you need for the external power. The Sleepy pi board is one example of this. It includes an RTC clock and an arduino optimized for power management. It can handle both clean shutdowns, partial battery management, and scheduled, triggered, or programmed wakeup.

There is not enough information here to determine how long a battery would last. That entirely depends on the size of the battery and how much you can optimize the system. For example, a typical raspbian setup may take 2-3 minutes to boot, but an optimized bare bones linux could boot in 5 seconds according to the Sleepy pi website. After booting, it will take some time to connect to the network and download images, etc., and then some unknown amount of power to update the e-paper screen. And then some unknown (but presumably under a second) time to schedule the next wakeup with the sleepy pi's arduino and shut down the pi.

The sleepy pi and its arduino uses an extremely small amount of power in standby mode, which is fairly insignificant in comparison to what the pi needs when on, so that could last quite a while.

Rather than ask how long the battery would last, after designing the whole system and benchmarking the time and power needed to update your image and calculating the necessary power budget and select your update frequency, you could then ask how large of a battery you would need to last as long as you would like...

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