This is a quick way to build a cell phone battery booster while inside a Radio Shack or similar store. You may want to have one of these ready if you want to livestream or record an event, and don't have time to order a proper battery booster because the event starts in an hour or so. Build time is about five minutes.
The schematic is extremely simple: this is just a 1N400x diode in series with the batteries.
Cheapie wall-wart phone chargers output 5V nominal, but the actual voltage varies wildly, going as far high as six volts and usually stabilizing between 5 and 5.5 volts while under load. Cell phone manufacturers know this and design some tolerance into the phones.
A silicon diode drops around 0.7 volts (it's actually a curve: look up any datasheet) from the roughly 6V from the batteries, which brings us nicely where we want to be -- the little bit of extra voltage is handy in that it allows partially depleted batteries to still work with this circuit.
If you want to make a lot of these, here are suggested Digikey part numbers -- buy fifty and the cost of these chargers will end up being roughly $2.50 per.
http://search.digikey.com/scripts/DkSearch/dksus.dll?keywords=A31726-ND USB connector (watch your polarity!) this should let people use their existing phone cable
http://search.digikey.com/scripts/DkSearch/dksus.dll?keywords=BH14AAW-ND Battery holder for AA batteries
http://search.digikey.com/us/en/products/BH24DW/BH24DW-ND/2330509 Battery holder for D batteries
http://search.digikey.com/us/en/products/BH4CW/BH4CW-ND/1169389 Battery holder for C batteries