Computing stuff tied to the physical world

Through the night on a supercap!

In Hardware on May 26, 2012 at 00:01

Yeay! – The JeeNode made it through the night on a 0.47F supercap, for the first time ever at JeeLabs:

Screen Shot 2012 05 15 at 10 59 19

Sorry for the awkward / missing scale, here’s some context:

  • vertical is voltage: 50 = 2V, 100 = 3V, 150 = 4V, 200 = 5V
  • blue is VCC before sending, green is VCC after sending
  • graph runs from 11:45 yesterday to 10:45 the next morning, i.e. 23 hours
  • that’s two VCC measurements and one packet transmission every minute

The supercap had been charged by the solar cell for 3 days, no load. When connecting the JeeNode (BOD set to 1.8V, on-board 100 µF i.s.o. regulator, already running), I placed it in a cardboard box to block out the light:

  • the first upward blip is at 12:45, during 5 minutes of exposure to sunlight
  • then back into the box until 18:30, depleting the supercap for a few hours
  • after that, the node was kept in the light to try and charge up enough for the night
  • at 20:00, the charge had gone up to 4.42V and 3.86V, respectively
  • at around 6:30 the next morning, the lowest point was reached: 3.44V and 2.88V
  • from then on, the cell started charging again from the morning light (no direct sunlight yet)
  • looks like about 10% of the packets never arrived (probably mostly due to collisions)

At noon, the cap voltage had risen to 4.9V (note that the RFM12B is now operating above its official 3.8V max).

So there you have it: one packet per minute powered by solar energy, harvested indoor near a window.

Update – FWIW, this setup lasted a second day, but then it died again… we’re not done yet!

  1. Why does the voltage drop and then recover in the subsequent minute?

    • Because I plugged it in again, briefly.

    • No, I mean something different; maybe I’m getting something wrong but how come the before transmission VCC is so much higher than the after but the next minutes before voltage is almost the same as the before voltage previously.

  2. Nice work. Wonder if the raw power of the raw power of the solar cell is enough and how much power you are losing because of internal leakage in the supercap.

    Do generatie these graphs with Jeemon? Could you share the TCL code for that so that I can generate comparable data?

    My DX solar charger appears to work much better with the charging led disconnected. Could be a viable cheap solution.

  3. @adam, an attempt to explain the supercap voltage ‘dip’ is in the final comments here

    • Ah – yes, supercaps are far from normal caps. One oddity is that some of the charge seems to be buried “deep inside” – you can short out a supercap briefly, and when the short is gone it’ll still recover some voltage. Here, the same happens: a brief dip as the transmit packet draws a lot of current, and then deep charge replenishes the charge near the surface, so to speak.

    • Thanks martynj and jcw.

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