While Ladyada's Mintyboost design is great, there is a cheaper and much easier way to build one- with Sparkfun!
If you'd like to save a few bucks and skip assembling a board, a quick trip to Sparkfun can net an already assembled equivalent of the original Mintyboost.
By the way, please use what you save on this project to buy something else from Ladyada- she's got some sweet kits- especially arduino related, and she keeps everything open source and transparent! Her kit was the grand-daddy of all things Mintyboost, so please remember to support her!
Mintyboost Original- http://www.ladyada.net/make/mintyboost/index.html
Adafruit (Her kit store-check it out!)- http://www.adafruit.com/index.php?main_page=index
Step 1: Components and Tools
The Ladyada Mintyboost design (v1-2) uses something called a boost converter to "boost" the 3 volts you can get off a pair of AA's, up to the 5v required to charge a usb device. Her kit is a boost converter of her own design, and comes with a pcb and components that you need to solder together. An attractive kit, and the v2 kit can source 400ma for faster charging. Her kit, shipped to my house in PA, runs about $23.25.
However, if you're cheap, lazy, or know all about boost converters already and don't care to solder a pcb, there is another solution! Sparkfun sells a pre-assembled 5v boost converter, with a AA holder already attached. Hooray!
So, with a pre-assembled boost converter, our parts list shrinks immensely, as you might imagine. Conveniently, we can buy all the electronics components at Sparkfun.
5v Boost Converter http://www.sparkfun.com/commerce/product_info.php?products_id=8249
USB Type A Female Connecter http://www.sparkfun.com/commerce/product_info.php?products_id=9011
Total Price with Shipping: $15.44
Alright! We saved over 7 bucks, or ~ %30! You'll also need an Altoids tin of some sort. I used a cool round one, mainly because I had it already, and I could wrap the charge cord around it. You can use whatever will hold the components.
1. Soldering iron- Basic one is fine, just for soldering pins.
3. Drill/ Hammer/ Knife/ Hole Punch- To start the hole for the USB connector
4. File- To shapes and smooth off the edges of that hole.
4. Glue gun/ Double Sided Foam Tape- For insulation and holding things down.
**Must Read Stuff**
Boost Converter Background http://en.wikipedia.org/wiki/Boost_converter
Ladyada Mintyboost design process (Very Interesting)- http://www.ladyada.net/make/mintyboost/process.html
Step 2: Mounting the Connector
First things first. We'll need to cut a slot in the Altoids tin for the USB connector. Straightforward process. You'll need to draw or scribe in the shape of the USB profile. Make sure that it's oriented so that when the connector is in place, its mounting tabs will press against the bottom of the tin.
Using a drill or knife, punch out a rough outline. Clean it up with a file or some sandpaper, until you can just fit the connector in. Then, using a soldering iron, solder the connector's mount tabs onto the Altoids tin.
Step 3: Finish Up
Next, we'll connect the boost converter to the USB connector. The USB standard puts a 5V line and a ground on either side of the connector, with two data lines in the center.
USB Pinout: http://pinouts.ru/Slots/USB_pinout.shtml
Solder on the red wire from the boost converter onto the 5V pin, and the black onto the ground pin.
Finally, using some hot glue or double sided tape, affix the boost converter inside the Altoids tin. One note of caution: there are two exposed pins on the underside of the boost converter, and if they short on the tin, your charger will not work! Make sure to put a piece of cardboard over them, or insulate them some other way.
Step 4: Charging
To charge, you just need to plug in your device. So long as the AA batteries can supply the juice necessary to source 5V out of the boost converter, the charger should work functionally the same as any other USB outlet.
There are some devices that slightly modified hardware to charge properly; these are pretty well documented on Ladyada's site, and her kit can handle this issue. Right now, I have tested this charger on my Sansa e260- all the e200 players should work, at the very least.
So, have fun building your cheaper mintyboost, and make sure to use your extra cash to buy more adafruit stuff!