Computing stuff tied to the physical world

Solar top-up, full sun

In Hardware on Jun 9, 2012 at 00:01

Yesterday’s setup described a circuit with the JeeNode running on an AA Power Board and a little solar cell to top up the charge when there was sufficient light.

Since today was a warm day here with lots of sunlight, I thought it’d be nice to establish an outdoor baseline:

DSC 3302

From left to right (see how useful it is to have a whole bunch of multimeters?):

  • the voltage of the solar cell is 2.7V right now
  • the current supplied by it is 0.76 mA
  • the current drawn from the AA cell is 20 µA
  • IOW: less than an hour of sunlight is enough for a day

This is running the radioBlip2 sketch (including the recent survival tweaks), with the same modified JeeNode (no regulator, 100 µF cap) as used in many recent experiments here at JeeLabs.

Note that these values add up reasonably well:

  • the 2.7V from the solar cell ends up being distributed as follows: 0.65V forward drop over the 1N4148 diode, 0.75V voltage drop over the 1 kΩ resistor, and 1.3V over the (almost fully-charged) Eneloop
  • battery draw is 20 µA, and I’ve independently measured about 4.8 µA idle current draw from the JeeNode (i.e. w/o the MCP1702 regulator), so losses and efficiencies are actually quite good

Here’s the same setup in the shade (still bright sunlight outside) – sorry for the bad readouts:

DSC 3303

The cell voltage now drops to 2.0V and the current it supplies is down to ≈ 0.14 mA.

For comparison, some indoor charge currents from the solar cell:

  • near the white LED strips at my workbench: 60 µA
  • behind the window, but not in the sun: 80 µA
  • behind the window, in (modest) sunlight: 400 µA

With a Room Board attached and a permanent indoor setup, these figures will change, but it all looks promising!

  1. Thanks for all these posts on solar ‘live forever’ setups.

    I spent a bit of time thinking about this stuff on my own, and eventually concluded that buying cheap chinese ‘cell phone chargers’ was the way to go.. $5 gets you a big solar cell, a ~750mah lithium battery, and usually a boost converter that puts out 5.5v

    I usually tap into the raw battery voltage and feed that to a 3.3v regulator.

    • Good idea! This is the setup I am working with. the whole assembly fits perfectly with a little hot glue on the 2″ x 4″ box for my remote sensor pack. I have not opened it up yet, was just using the usb connector into the usb jeenode. It works great when there is sunlight, not so well inside. The LED on the side indicates it is charging inside, but I do not believe it is. There may be just enough voltage for the LED, but not enough to charge the battery. My next step is to open it up and start measuring some things. I think tapping into the battery while I am in there would be much more efficient.

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