Computing stuff tied to the physical world

JeeNode goes solar

In Hardware on Sep 3, 2010 at 00:01

Now that ultra low-power options are coming into reach for JeeNodes, lots of new scenarios can be explored.

The most obvious one is probably a solar-powered JeeNode … so meet the latest new node #5 at Jee Labs:

Dsc 1875

It also uses a 0.47 F supercap, same as yesterday, but now hooked up to a small 4.5 V solar cell (which can only deliver a few mA in bright sunlight), and a Schottky diode between the solar cell and the capacitor.

Here’s the “power supply” in more detail:

Dsc 1876

As you can see, the solar cell is tiny. A few square cm’s only. In fact, it takes quite some time for it to charge the supercap to acceptable levels. I had to place the cell in moderately bright sunlight for about half an hour to get to a 4 Volt charge. It was inching along, taking several seconds per 0.01 V increase.

To avoid losing all that charge right away in the power-up cycle, I modified the ATmega’s fuses to start in 258 clock cycles after power down, and to start up within 4.1 msec after reset. That way it will start up as quickly as possible at all times. The 258 CK setting is particularly nice, because it means the ATmega can get out of total power down within about 16 ┬Ás, fast enough to respond to a byte RX/TX interrupt from the RFM12B!

Does it work? Check it out: after connecting the JeeNode with the “radioBlip.pde” sketch pre-loaded… away it went – sending one packet every 60 seconds as node 5:

    OK 5 1 0
    OK 5 2 0
    OK 5 3 0
    OK 5 4 0

While exposed to the current partly-sunny / partly-cloudy light levels, the voltage on the supercap is still increasing. This is good – it means there’s a surplus of solar energy, even with these transmissions going on. That extra energy will be crucial if this thing is to last through the night…

If everything works out, this little Arduino-compatible bugger could well be the first JeeNode to become completely autonomous and transmit wirelessly… forever!

Time will tell :)

  1. I also did some experiments with a “green jeenode” a while ago. See my blog post on this (I went down the easy road):

    • Ah, thanks for the pointer. Your setup ought to work quite well, I’d expect…

      The above post wasn’t a serious attempt to keep going on sunlight, the cell is far too feeble for indoor + winter use. I’m deliberately pushing the envelope to get a feel for what sort of energy levels are needed as rock-bottom baseline.

      As it turns out, my solar experiment doesn’t last through the night. The cap doesn’t charge beyond some 4.3V (makes sense: 4.5V cell – 0.2V diode). For two days in a row, it died before midnight. The other reason it won’t last is that apparently supercaps have a fairly high self-discharge rate, at least mine does.

      One idea would be to keep this thing off at night, and then restart transmissions only when there is new light coming in to fill up again But even then, my 0.47F probably isn’t enough.

  2. It’s still very interesting. I wonder how Stef and his bigger cap and panels is getting on with the new uber-low power tips?

  3. Fantastic Article – really exciting. (My personal fantasy is for a node that can sit for months or years watching an unoccupied property or location (ie no mains electricity), and when some condition is triggered (eg burglars, eg squirrels in the attic, eg flooding) it will send a SMS message via a GSM card.)

    Many thanks – please keep pushing the envelope Nick

  4. Ah thanks for the Schottky diode tip. I did hear you need to put a diode but I just used a normal one which probably has a bigger voltage drop.

    Yesterday I connected up my solar cell, supercap and LED. I only had a half hour of spare time but I did see that it takes quite some time to charge up the supercap and I also noticed it did not get the near solar cell’s rated V (probably due to the not-Schottky diode and the lacking of sunshine).

    More experimenting with this stuff is pushed back on my todo list. I’m busy now with my CNY70 gasmeter monitor. Trying to keep the power usage down there is also a nice challenge. Got to get this thing working fast. My wife asked me this morning to power-on the “floor heating” again. Talk about timing.

  5. I have used a “Mini Solar Charger” no 38-3632 from this company for about 6 month now. It works great in indoor light but it is expensive (about 20 Euro). I have also tried to use a solar cell from a garden light but it was to weak to work indoor if the room board is attached. I’m looking forward to more reports from your experiments.

  6. I LOVE IT! I’ve been thinking of creating a garage door checker but didn’t know where to put the status lights … how about a solar lamp in the garden (wireless jeenode) … while this project has been growing in my head I was lovingly surprised to see you throw out low-power and then a solar article … BINGO! Great work/hw/site!

  7. Wow.. there is no end to the wonders that you keep putting into the world. :) I like it a lot!

  8. Heh, glad y’all like it. Just wanted to note that this particular solar setup is still too feeble to be useful. Hopefully some slightly larger panels can produce enough energy at all times to charge the supercap up enough.

    Re-purposing one of those solar garden lights would also be an interesting experiment.

  9. Fascinating stuff. Here’s a question for you. Do you think this is a low enough power consumption that we could use a piezo button to charge a cap enough to do a single send operation? Probably not, but just a thought. It would be kinda cool: a wireless button that requires active power.

  10. Well if you need a batteryless sensors check out EnOcean e.g.:

  11. Hi Jean-Claude, do you plan to build and sell a “solar” plug as shown with on the picture with this nice square solar cell ? jl

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