thanks to everyone’s great feedback, i’ve now got a usb battery that’s much more efficient and capable of delivering more current to a portable device. like i promised, i put together a slick little case to finish this hack off right. read on for some tips on building your own.
as mentioned by several readers, you’ll want to use a 5 volt regulator ic instead of the resistor/zener-diode combo i discussed previously. you’ll be able to charge your device faster while it is running and it’s much more efficient, which will give your 9 volt a longer battery life.
the best part is that it’s easier and cheaper to do it this way. just grab an lm7805 from your local hacker store and wire it up. connect the positive battery terminal to the 7805 input pin, the positive usb pin to the 7805 output pin, and connect the negative battery terminal and the usb ground pin to the 7805 ground pin.
which is which? if you hold the 7805 with the text facing you and the pins downward, the pins from left to right are: input, ground, output. it looks like this:
for the female usb connector, look down into the end of the connector so that it is oriented like so:
pin 1 is the positive (5v) terminal (which goes to the output of the 7805) and pin 4 is ground (which goes to the 7805 ground pin).
easiest way to test
several people were wondering how to tell if you are going to fry your device. here’s the easiest way to test when you are all done wiring:
cut a standard usb cable in half.
plug the male end into your usb battery.
connect the positive and negative ends of your multimeter to the red and black usb wires respectively.
if it reads something very close to 5v then you are wired correctly.
making a case
i chose to make my case out of polystyrene plastic. you can find this stuff in sheet form at most hobby stores. it’s the same kind of plastic used in your standard plastic model kit and the sheet form lends itself nicely to making flat sided objects