There are essentially two approaches I've used with LM2596 regulators so far. The first approach is 6v solar panels into an external regulator. This is the system adaptable to a standard 6v hub or bottle dynamo setup. The second approach is my current approach and uses Maximum Power Point (without the tracking) regulation and 12v solar panels (18-20v open circuit, 16v maximum output).


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A few of us have been making ultra-low cost MPP regulators based on LM2596-adj regulators and documenting it on fieldlines.com. Here is the thread:
http://www.fieldlines.com/index.php/topic,146685.0.html
The actual Power Point regulation is done by regulating the solar panel before it gets into the regulator. I've used an LM723 regulator, OperaHouse uses zenar diodes. Both do the same thing, turn the LM2596 on when the maximum power point is reached. The LM2596 switches at 150k. Large capacitors store the energy while the solar panel voltage (and power rises). This happens all very quickly. The LM723 regulator has the advantage that the power point is adjustable, it's an old but efficient and robust regulator.

The LM2596-adj boards you buy on ebay have a number of pitfalls. OperaHouse gives some good advice on the thread above on how to find the best of the bunch. These boards most likely have counterfeit LM2596 chips as in bulk (1k) the chips themselves are more expensive than the boards cost including shipping anywhere in the world. These chips also are soldered (some may be welded) to the board so heatsinking can be difficult.

EDIT: The chips on the LM2596 boards are almost guaranteed to be FAkE. The genuine LM2596 pins are staggered. The ones on ebay are inline.

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The photos show a plain LM2596 regulator with current limiting (current limiting ie 3 pot LM2596 regulators are less efficient than single pot regulators) as you buy them on ebay. They are installed in a gutted external charger as you buy them from dealextreme at low cost. There is low-ESR capacitance (the most expensive part of this regulator). I used a 6v solar panel and made my own connectors out of Deans style connectors (really like them). This regulator is not a Maximum Power Point one as there is not enough headroom on the solar panel. This system (ie not a MPP regulator) would work well adapted to work off a 6v dynamo with the full-wave bridge rectifier as suggested above.

You could make an external charger or a USB dynamo charger this way. Keep in mind however if you make a Nokia USB charger you will need a decent amount of current on the port when you connect otherwise it will be current limited to 95mA. You could do this by spinning the wheel in the air before you connect the n900 (and then hop on straight away and cycle). This or install a switch on your handlebars or something like that to cut in while you ride. There are also problems with voltage rising on the input capacitors with dynamos, it could be worked out by monitoring the voltage levels. It doesn't appear to be a problem with the MPP regulator and 12v solar from what I've found.