My apologies to any of the 1400 or so people who've read this Instructable in the day since it was published; I described an idea I was sure would work, and I was so excited to get it out there where people could try it, that I didn't wait until I could get the parts I needed to fully test it myself, and I posted based on a partial test and an assumption about the performance of small motors that turned out not to be accurate.
Now normally I would take a faulty Instructable down (and run away and hide), but in this one instance I'm going to leave this online after editing it to correct my mistakes, because I thought that there was still a good idea here that could be developed even if it had to be by someone with more electronics experience than me, and I wanted to put this germ of an idea out there so that maybe it could inspire someone to come up with a solution that works. I offered the 1Yr pro membership upgrade I received when this went front-page as a prize, and as you can see from the comments a lively discussion ensued. (I gave the prize to jtlowe for his suggestion of using a clockwork rotating platform, but although there's no more prizes to give, I would still welcome any more suggestions you all can offer).
The problem is sun tracking: pointing a solar panel directly at the sun so that it can harvest significantly more light - and the difficult aspect of the problem is that the cost of adding a sun tracker to a solar panel in order to gain X% extra output has to be less than X% extra cost, otherwise it's more cost effective to simply add more solar panels.
The solution that I had was to take two small PV cells from a couple of solar garden lights, and connect them not in series or even in parallel, but head to head, connecting the ground lead from one to the ground lead from the other, and determining which of the two panels was receiving more light by looking at which one was able to generate more voltage than the other. For instance if one generated 2V and the other generated 3V, then the voltage between the two positive outputs would be 1 volt, and that volt would be used to drive a DC motor to turn the platform in the direction of the cell that was reading the stronger light signal. (Although in practice the voltage was actually less because driving current through a PV cell against its natural direction (since it acts a little like a diode) causes a voltage drop in excess of just the voltage that would be cancelled out, though that's not relevant to the problem)
Although that is indeed what happens which you can confirm by looking at the voltage on a voltmeter, what I didn't realize was that these panels don't generate enough amperage to drive even the smallest motor, as far as I can tell. I'm talking about 3V motors that need so little power that they'll spin from a single half-dead AA battery!
So what I'm going to show you here is half a solution, and I'm asking the smart readers of Instructables to help come up with the other half.
Step 1: What you'll need
Grab a couple of those cheap garden path solar lights, the kind they sell at the Dollar Store for... well, a dollar. (If you miss the occasional Dollar Store deal on those, damn, you'll probably be out of pocket by two dollars from a more expensive store! :-) )
That's it - to test the circuit you won't need any more components. Just your trusty volt meter, and a soldering iron... (I finally treated myself to a programmable temperature controlled iron from Radio Shack and I have to say I'm really enjoying using it compared to what I had before. The volt meter is that cheap one that's on sale for a couple of dollars every weekend at Harbor Freight)
When I was trying to drive the motor and realised I didn't have enough power I added a second pair of PV cells in parallel, so some of the images below have 4 cells and some have 2. As long as we're just using these as light sensors, 2 will do fine - I didn't feel it was necessary to reshoot the photos...