My first Instructable described how to build a compact power source that could power a Dell Axim PDA off of 8 AA batteries for extended use on long trips. It used a simple 7805 regulator and a few capacitors to filter the power. It could also be used to power a PSP, since both the Axim and PSP have the same adapter port and use the same voltage. However, when I finally got to go on a long trip, my friend asked to borrow my charger for his PSP but accidentally hooked the connector up to the 8AA pack backwards. This pretty much fried the 7805 chip, rendering the whole thing completely and totally useless. Although I didn't get to watch movies on my PDA coming home, this led me to create a new design, one that would: A) Protect against backwards connections so that simple accidents such as this wouldn't damage it B) Provide power for USB charging devices (like the iPod, Sansa, etc) in addition to PDA/PSP C) Provide 2A of power instead of 1A, my PDA running on full speed/brightness with WiFi and Bluetooth on apparently used more than 1A (7805 = 5v at 1A) and it would stop charging To do this, I decided to add a diode at the beginning of the circuit (diodes only allow power to flow in one direction, preventing the backwards flow that destroyed my previous model). To accommodate 2A instead of just 1, I added a second 7805 chip in parallel with the first one. Since each chip provides 1A, 2 of them provide 2A. I also pulled some USB connectors off of a dead USB hub and added one to the circuit so that I could plug iPod/Sansa/other USB charging devices into it and charge them on the go.
http://www.instructables.com/id/Ultimate-Portable-Power-Source-Axim-PSP-and-USB/