Five Minute Solar Phone Charger

Need to charge your phone?  Have nothing but a clothespin and some P-N junctions?  Have no fear!  The (solar) power is in your hands!

So, what's the plan, stan?

Well, we're going to make a solar panel from scratch that can charge up a simple phone.

See, cheap phones like mine are real simple to charge--they just need 5V and they'll take care of the rest.  They'll draw as much power as they can (my little nokia will draw up to an amp while charging), but they're smart enough to limit their current draw to whatever the source can provide.  

So, that means we need to make a 5V solar panel.  Solar panels are made by wiring up a set of solar cells--big silicon P-N junctions.  A solar cell puts out a very low voltage--0.65V open-circuit, or around .55V under an ideal load.  To get the 5V that we need to charge a phone, we have to wire up a bunch of solar cells in series.  

But how many solar cells?  Well, a solar cell without anything connected to it will produce .65V in bright sunlight, but as I draw power from the cell, the voltage will drop.  This relationship is called an I-V curve.  The max current the cell can output is based on the area of the cell, the intensity of the sunlight and the temperature of the cell, but the max voltage is always the same, regardless of the size of the cell.  
Every solar cell has a peak power point, a current-voltage combination that gives me the most possible power for that cell.  For my panel, I want to run my cells at their peak power point.  I happen to know that the peak power point of my cells is .5V at 130mA.  So, if I want to put out 5V, I need to stack up at ten solar cells in series, giving me an output of 5V.  If I'm drawing less than 130mA, my output voltage will "float" up, and if I draw more, my voltage will get pulled down.  Now, it so happens that phones are pretty smart and robust, so I can actually give them a bit more than 5V, and they'll be fine.  I'm actually going to add an extra cell, giving me a total of 5.5V coming off my cell.  This makes my panel a little more robust--the higher voltage means that, in lower light, it'll still put out enough voltage to charge up my phone, and it'll be less sensitive to shading or pointing it away from the sun.  

So, I'm going to make a solar panel with eleven solar cells in series, I'm going to wire the output into a phone, and it's going to charge.  Cool, right?  It's pretty quick and easy to do, too.  Here's a video of the whole process:

This is a pretty easy and cheap project.  You need:

Solettes--cut up pieces of solar cells.  You can get them as part of a DIY solar kit that a friend and I put together, or pick some up on ebay by searching for "solar cell pieces", like these:  http://goo.gl/C25he.

a cheapo phone charger that fits your phone.  Mine was a nokia, and I got a knockoff charger for $2.

Copper tape--available at michael's, Sparkfun (http://www.sparkfun.com/products/10561), and plenty of other places online, for cheap

A clothespin


and wire strippers, scissors and a soldering iron.  Ready, set, go!

To build anything, you need some proper tunes.  Did you know that Scarlett Johansson put out an entire CD of Tom Waits covers called Anywhere I Lay My Head.

Kill me now Pete, I'm in heaven

Chop up that purty phone charger.  Cut the charger part of it, so you're just left with a nice long cable and a phone connector at the other end.  Strip the ends of the wires.  

Cut a couple of small pieces of copper tape, just a little longer than the pads of the clothespin.  Solder each wire from the charger to the end of a piece of tape.

It's like you made an adorable little heart resuscitation device for tiny little mice having cute heart attacks

Now you're going to stick the tape onto the clothespin pads, with the wires coming out of the back of the clothespin.  I found this to be challenging, but not too hard.  Just right, really.  Persevere!

Smush the clothespin together to stick the tape on well.  It's like you made a little electrical alligator!

Now for the fun part.  Stack up the solettes, and then fan them out like a deck of cards.  Stacking them up makes a series connection between each of the cells, and now we just need to attach to the top and bottom of the stack to pull the power out.  

Fan them out pretty wide--the better they're spaced out, the more each solette will show, and the more current you'll be able to pull off the panel.  

Now fan yourself and talk about mint juleps in that adohrable suthehn gentlmuhn accehn yuhv been a-wohkin on

Now, clip the clothespin onto your fan.  It takes mighty skill to do so without breaking the solettes or ruining your delicate fan arrangement.  Stop complaining.  Sack up and do it.

The positive (usually red) wire going onto the bottom of the stack of solettes (the side that's not blue), and the negative wire goes to the top.

Sweet, you're done!  Plug your phone into the charger and go wave the panel in the sun.  Shout excitedly to all your friends and then go hug a sad banker.  You've powered something FROM THE SUN!