Instructables has quite a few guides on how to make Solar USB Chargers, including the very well done guide on how to combine a Lady Ada Minty Boost circuit with a solar + lithium ion battery. Great, but a bit expensive to make and not a very simple project for the weekend DIY person.
Well luckily for us I know how to make one for under $20 that is better in nearly every way and also completely fits into an Altoids Tin. Covert style.
(If you want a more powerful USB Charger, a Heavy Duty one, I have an instructable for that as well.)
Step 1: What You Need
USB Charging Circuit
Solar Panel 4V or greater
AA Battery Holder
AA Rechargeable Batteries
Altoid Tin (or whatever)
Melt Glue Gun and Glue
Cost is less than $30. I can make one for under $10 when I buy parts in bulk. I have a kit available at my website BrownDogGadgets.com which has everything you need to make this project. Also, if you're lazy, I do sell made versions in a variety of tin styles.
Step 2: DC to USB Converter
The central brain of our project is a DC to USB converter circuit. This takes our AA power and changes it into the 5Vs we need for charging USB. There are several ways of doing this.
1) You can make one yourself using Lady Ada's Minty Boost kit. It's $20 and requires soldering. It also charges almost every gadget under the sun, including new Apple Products.
2) You can buy a premade circuit off ebay, or even off my website BrownDogGadgets.com. They cost around $10 and work with most everything. Kindles, iPhones, iPods, GPS, Android Phones. They're great.
3) You can rip one out of a cheap USB charger. eBay and or Amazon are great pleases to look. Some of them don't work with Apple stuff, so be careful or buy several.
Whatever you do, don't try and make a 6V or 9V circuit and then use a voltage limiter to take it back down to 5V. That's sloppy and ineficient. You can do it, and it works, but there are far better options.
Step 3: Choose Some Batteries
We need to use rechargeable batteries for this project. I prefer NiMh AAs over everything else because they're easy to find, cheap, and reliable. You probably even have a few at home. Since we're using two AAs in this project our charger will have 2000 - 3000 ma of current. You could even have two sets of AAs in parallel and boost that capacity to 4000 - 6000 ma.
An added bonus from rechargeable AAs is that you can take them out of the Solar Charger, charge them up or replace them, and be on your way.
If you're making this project into a very small container you can use a smaller NiMh battery pack. You can find these at places like American Science and Surplus as well as many places on the web.
Step 4: Choose Your Solar
If we use two rechargeable AAs that put out a total of 2.4Vs we're going to need a solar panel that is at least 3 - 4Vs just to meet basic levels of charging. The higher the voltage of our solar cell (or cells) the less light we need to charge up our batteries.
Now we're also trying to fit this into an Altoids Tin, so we're limited in space. I have found some great 4V solar cells that perfectly fit into Altoids Tins. They're the same ones I use with my Solar AA Atoids Charger.
Sure, a bigger and better solar cell would give us added power, but it wouldn't fit into our tin. (Something that has annoyed me with nearly every Altoids Solar Guide out there.)
You could also use a combination of several smaller cells to get your four volts. For example, 2V cells are very cheap and small on ebay. You could easily connect two of those in a series to get your 4Vs.
Just remember that when charging NiMh batteries we don't want to throw more than 10% of their capacity at them at any one time. For instance if your battery has a capacity of 2000 ma we can only use a solar cell that puts out 200 ma or less of current. This isn't usually a problem unless you're using a massive solar cell or a big combination of cells. None the less, keep this in mind.
Step 5: Wire Up The Solar
First, we're going to wire up our solar panel.
I like to connect my 1N914 diode directly to the solar panel. When soldering make sure the black bar on the 1N914 diode is point away from the solar cell. The black bar is the negative side and prevents power backflow. If we put it in wrong we're going to prevent power from flowing into our circuit!
Then attach a long wires to both the diode and negative tab on your solar cell.
Then tape it up for protection.
Step 6: Connect The Battery Pack
Just connect all the positive wires (red) with all the negative wires (black).
Easy as pie. Sweet sweet solar pie.
Step 7: Prepare the Charger
So the charging circuit I use also has a couple of LEDs on it. These in no way effect the USB charging and in fact are annoying. I remove them because they're useless and a waste of power. This is why I'm cutting them off in the pictures below.
You should NOT start cutting things away unless you know what you're doing. Seriously Ben, I know you're reading this. Stop cutting now. This is how you lost a finger.
Anyways, if you've bought a cheap charger to use you have to take it out of it's plastic, and disconnect any random wires or battery packs.
What you should be mindful of is where power connects to your board. Mine has a nice little + and - sign to guide me. You need figure this out on your own depending on your situation.
Lastly, and most importantly, you should watch out for any switches. For instance mine has a little switch on the side to go from "Light" to "Off" to "USB." I just keep mine set to "USB" as we don't lose any power unless something is plugged in.
Step 8: Solder The Charger
Now all you have to do is solder that bundle of positive, in my project red, wires to the positive point on the board. Then all the negative, in this project black, wires to the negative point on the board.
You're done with the circuit. You can test it now.
Usually I can charge a gadget just from sun. If it's not sunny the circuit should charge via batteries.
Step 9: Cut Your Tin
I use Altoids Tins because they're the prefect size for this project. Plus very cute. I also enjoy mints.
Just use some tin snips to cut a hole big enough for the USB port.
If you want to be fancy you can also use a Dremel and make a nice rectangle hole in the side of the tin. I'm lazy, so I just cut it away.
Step 10: Glue Everything In
To be on the safe side I put down some electrical tape below my bare circuit board. Just to play it safe.
Now all you need to do is glue in your battery pack (though I use foam tape for that) and glue down your USB circuit.
Notice I use ample amounts of hot (melt) glue. Yummy.
Done and done.
Step 11: Enjoy
Now you're done. Enjoy the fact that you're being very green and clean.
Also, before someone chimes in... yes I do know batteries are not entirely green. If you want to be super green you should use some super capacitors for this project. They last forever, are super green, but are also quite expensive.
What I like about this project is that it's simple and handy. It makes for a nice gift.
If you need any parts you can always get them from my website, BrownDogGadgets.com. All the money I make goes to doggy treats and more projects.
Thanks for reading!