Computing stuff tied to the physical world

Indoor solar energy

In Hardware on May 1, 2012 at 00:01

The amount of solar energy available indoor is very limited… a very small fraction of outside, I’m sure.

Still, this unit has been running in the house here for a few years now, and not near a window either:

DSC 3074

It’s a attractive goal: gadgets which you buy (or build) once and then use essentially forever!

We don’t use the alarm clock mode of this thing, so the beeper never goes off, but it does listen for the DCF77 clock standard transmitter in Germany once a day to stay in sync. It’s also slightly glow-in-the-dark, so this clock remains readable at night.

The fact that this clock works so well tells me that, with proper care, we should be able to run simple nodes inside the house with a solar cell of perhaps a few square centimeters, just like this clock.

And, whether battery- or supercap-powered, that’s precisely what I’d like to get going…

As with the AC-mains connected ultra-low power supply, I suspect that reliable startup will be the hardest part. Such an energy source will have very little spare energy and charge up very slowly, so when the JeeNode comes out of power-up (or brownout) reset, it’ll have to be careful to not cut off the hand that is feeding it, so to speak.

  1. one reason why this one works so well indoors is because it’s a thin film solar cell. It captures another part of the light spectrum than crystalline cells and it has a much better behaviour in low and diffuse light conditions. The load is also probably optimized to make it work at MPP (maximum power point)

  2. I note that a typical RTC chip uses 0.5 microamps to keep time. That isn’t very much! Not sure about the current used to drive the LCD display, I think it’s proportional to the size of the display area. http://datasheets.maxim-ic.com/en/ds/DS1307.pdf

    • Without a backlight, an LCD panel like that only needs current when you want to change the display. Once the crystals have been twisted by a voltage they will remain there until twisted back by another voltage.

      So it will only draw current for a fraction of a second when the second field is updated, and once a minute the minute field, etc etc. As you say, it’s probably proportional to the cell area, but on the grand scale of things it will be minute.

  3. Worth looking at these – http://www.enocean.com/en/enocean_modules/stm-250-oem/ albeit not the price range we are talking about :-)… but the capability is…

  4. Just love the Dolphin marketing blurb… ” An integrated energy store allows unrestricted functionality for several days in total darkness ” i.e. a lipo keeps it working a while.

    • Funny, I was just about to post something about the Enocean “energy store” :D Thanks for the clarification @matynj.

  5. Didn’t realize there was no static LCD drive current. That explains why the “seconds” digits are often smaller in area.

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