Computing stuff tied to the physical world

Not long enough

In Hardware on May 9, 2012 at 00:01

My second solar setup did not fare well:

DSC 3086

Started during the day, with the supercap charged up in bright daylight behind a window, it was able to power the JeeNode for about 16 hours – and then in the middle of the night it gave up:

    L 02:36:44.743 usb-A600dVPp OK 17 30 4 0 0
    L 02:37:48.074 usb-A600dVPp OK 17 31 4 0 0
    L 02:38:51.402 usb-A600dVPp OK 17 32 4 0 0
    L 02:39:54.725 usb-A600dVPp OK 17 33 4 0 0

Times are in UTC and we’re in the CEST time zone, so this was two hours later – i.e. around 4:30 AM.

I left it there for another few days, but unfortunately once dead this setup never recovers. The main reason for this is probably that the RFM12B starts up in a very power hungry mode (well, relatively speaking anyway) with a 0.6 mA current consumption – because it starts with the crystal oscillator enabled.

Maybe the self-leakage of the supercap was still too high, and would be (much) lower after a few days in the mostly-charged state, so I’m not ruling out using supercaps just yet. But as it stands, not getting through even a single night is not good enough – let alone being used in darker spots or on very dark winter days.

More experimentation needed!

  1. jcw. Is it time to re-visit your idea of putting the power to the RFM12 under the control of the micro?

    • I’ve been thinking about that – but it would only address the power-up issue. The RFM12B only draws 0.3 µA in sleep mode, which is what the sketch uses whenever it’s not transmitting.

      This particular failure of not getting through the night puzzles me – for some reason the supercap isn’t keeping as much charge as my previous measurements seemed to indicate. Very odd…

  2. But if you have the ability to “really” put the radio to sleep AND monitor the supply voltage, then you should be able to keep going with very little power — possibly without the ability to actually transmit anything :-)

  3. Surprising result!?! Maybe the charge current from the pv cell is just a bit low.

    I have just ordered some of these: http://www.ebay.de/itm/1x-5-5V-50mA-0-25W-68x35mm-Solarmodul-Solarzelle-Monokristallin-vergossen-/260998974767?pt=Bauteile&hash=item3cc4c04d2f

    Quite big at 68x35mm², but fits to the size of the jeenode not so badly and has 50mA capabilty at 5.5V. And quite affordable at €3.25. Lets see…

  4. Dang! Too bad, this really sounds like a holy grail.

  5. the Atmegas are notoriously power hungry. have you thought about using the Energy Micro EFM32 ?

    • Fascinating chips – what sort of development environment is there for them? C / C++? Multi-platform?

  6. @Sayan, @jcw, on a more general note, migration from 8-bit AVR to 32-bit Cortex M3 (including EFM32 but also ST, LPC, etc.) is certainly worth some experiences.

    The closest IDE for EFM32 is likely leaplabs’ Maple. It was supposed to merge with Wiring (the father of Arduino’s IDE actually) and to be extended for other M3 CPUs, but development is stale.

    In the middle, the Texas MSP430 (16-bit) could be a contender.

  7. I just posted a video about an effective indoor photovoltaic energy harvesting solution implemented in the open source Firefly Cap design:

    http://www.kickstarter.com/projects/mossmann/firefly-cap/posts/224855

    I hope it is helpful for you!

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