Friday, January 10, 2020

Solar Lantern

One can buy solar powered garden lights everywhere now.  They are useful for lighting up a garden path, outside patio, or your Satnogs ground station, but why buy one if you can build it yourself - at much higher cost of course.  If it would be a rational economic decision, then I'll never build anything!

How much current can one get from a small, supposedly 1 Watt, 12 cell solar panel:

X-Ray Tube Current

According to my ancient 'X-Ray Tube' meter, about 7 mA under my desk lamp!

If you want to build one yourself, or repair an existing one that failed, here is a little circuit with only about a dozen parts, that should get you going:

There are a few tricks to this circuit.  Solar cells are diodes, therefore one could simply wire a small solar cell panel across the battery to charge it, but solar cells are also cheaply made and full of flaws, meaning that they are not good quality diodes and will slowly discharge the battery when it is dark.  Therefore it is necessary to add a series blocking diode, to avoid discharging the battery at night.

Solar Lamp Prototype Board

The battery is made of three Nickel Cadmium or Nickel Metal Hydride cells.  These cells can be trickle charged and they are not expensive.  The three cells will slowly charge up to 3.75 V during the day.  Using a tiny 6 V, 1 W solar panel, we don't need a charge control circuit.

NiCd batteries can overheat and catch fire when short circuited, so it is best to put a 150 mA PolyFuse in series with it - a polyfuse is much cheaper than burning your house down.  The PNP transistor is turned on/off with a Cadmium Sulphide photocell.  The level of light/darkness where it triggers is set with a 10k trimmer (depends on the photocell - one I have needs a 22k trim resistor).  Put the photocell out of view of the LEDs, unless you want a solar powered blinky light.

The light source is made of two high brightness LEDs in series (Sparkfun has extremely bright LEDs that will hurt your eyes if you look into it).  The maximum current flow is controlled only by the internal resistance of the batteries, the inefficient transistor and the internal resistance of the LEDs.  This is another reason for the PolyFuse - to limit the maximum current.

Two high brightness red or green LEDs in series, require about 3 V to glow.  This sets the minimum discharge voltage of the battery.  If you discharge a NiCd cell below about 0.9 V, then it will get damaged.

If you want to use a blue or white LED which require 3V or more to glow, then use only one LED.

You can put LEDs in series, but not in parallel.  If you parallel them, the LED with the lowest forward voltage will light up, the other one will stay in a dark mood and sulk in a corner.

The whole circuit can be constructed inside the lid of a glass flask, with the solar panel on top, so now you have a good use for an empty Nescafe coffee powder bottle.  I learned over the years that one should mount a project PCB on the lid of a box using a few nylon/brass stand-offs.  Doing this, makes it much easier to work on the project than when it is way down in the bottom of the box!

You can buy all the parts from an online high tech toy shop such as Jameco or Sparkfun

If you want to learn more about real world wind and solar power systems, there is no nonsense Real American Cowboy information at Missouri Wind and Solar:

Have fun!


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