I've gotten bored and decided to finally get around to building myself a USB power injector for my 770. Thanks to Thoughtfix, I've got a firm grasp on what needs to be done and all that's missing is a bunch of knowledge with electronics. I'm not a complete dunce but I need some questions answered.

For whatever power source I use (I've still got my eye on a 9v battery), the voltage needs to be dropped to in the neighbourhood of 5 volts.

Some people have said to use a voltage regulator, some people have said to use a 5.3v zener diode. Which would be better? If the +0.3v from the diode would be dangerous, I could always throw a resistor in series after it. I also understand the regulator would eat up 2v defacto whereas the diode would only eat up 0.5v.

What happens if the voltage drops too low? If I feed the USB port 3v will it cause any damage to either the 770 or my peripheral? (ie an MP3 player)

The mp3 player will most likely just turn off. Depends on the device. Your 770 should not be affected beyond a simple reboot in case some program can't cope with a device going away. That is, unless you feed power to the 770 on some pin that is connected to the board. But that doesn't work with the correct voltage either

As for the method, use whatever consumes the least amount of power if you have the choice. This is not a matter of battery life but of heat generated. If you have a wall plug in reach, just use an active (powered) USB hub.

Well, I want the portability. If I'm within reach of a wall plug, I may as well pull out a laptop If an undervoltage won't be very dangerous I'll just go with the most readily available parts; which in this case would probably be the voltage regulator.

Has anyone else built one? Any cool additions? (I know Simon had a blinking light when power got too low, both he and Thoughtfix added a switch (which I'll do as well given the simplicity)).

im operating a hacked usb hub with 4 nimhs. voltage output is actually around 5.4v, no troubles so far >.<

undervoltage could be dangerous if youre using some sort of data storage, then your data could get lost

First off, I would really avoid a 9V completely. They'll run down very quickly although they have the advantage of being small. Also since the voltage is so much greater you'll take a big hit in efficiency as well as idle current consumed by the regulator itself.

Next, a zener diode will give you a shunt regulator which is not what you want-- it's kind of like driving a car with the gas pedal floored and using your brakes to govern your speed. Lots of heat, and extremely inefficient since it acts as a short to bring to voltage down to the desired level. A linear regulator -your second option- is better but still not very efficient, and a "normal" version will drop about 2 volts across it, minimum.
You can buy "LDO" (Low Drop Out) regulators which will drop under a volt typically, but with the same mediocre efficiency. I don't have enough time (or patience ) to explain all this but Wikipedia has a dandy article on the ins and outs of linear voltage regulators: http://en.wikipedia.org/wiki/Linear_regulator

4 AAA (or AA) rechargeable batteries will give you roughly the right amount of voltage (5.3V), and if you use an ordinary diode in series that will go down to about 4.6V, which will be fine. No regulatory required and no current consumed when not plugged in. Should last a very long time.

Highly unlikely. Too little voltage will be ignored, but obviously I can't guaranty that or be responsible if it melts in your lap.

Larry

Larry: Thanks for the info, I understood. (High-school electronics coming back to me, heh!) I think aiming for the right amount of voltage in the first place is the best bet.

I went with the simplest solution given available parts and created thoughtfix's version of the power injector.

I used the severed female ends of two USB A extension cables which hang about 2 inches out of a black project box. The project box came with a PCB board which made for easy soldering/packing. The voltage regulator showed 5.01v out on my multimeter and all's well.

My logitech keyboard only worked in xterm and the enter key brought up the onscreen keyboard. It didn't work in any other apps. I think it's the keyboard, it's wireless so kind of flakey in the way it works. My 40GB mp3 player mounted fine, though it uses fat16 so I'm stuck with short filenames like "Simps014.93j" instead of "Simpsons - Season 15". The USB key works fine when mounted.

I've never seen this before but I'll guess it's normal; the sdx# devices don't exist until something is plugged in, and depending on what I plug in, it's different. My mp3 player was sda, my USB key was sdb1. Maybe this is just the order I plugged things in to test? I'll check again later.

Much thanks to everyone that's ever posted here on this topic, and especially to thoughtfix who had a blog clear enough to explain this to someone who slept through half of his highschool electronics classes and never made anything more than a stun gun out of a disposeable camera.

my keyboard works in xterm and opera. it doesnt work in maemopad.
hint: press the "enter" key for linebreak, "return" will hide/show the onscreen keyboard!

Figured out the CR/LF trick with a USB KB here at work... still doesn't work with any other apps (I didn't try forms in Opera because I don't have a connection here (that the security director won't kill me for using)).

I can't recall if I've already mentioned this here before, but the neatest portable power hack I've ever seen is this geek lady's :

http://www.instructables.com/id/EGBQJPLCB2EP287KTZ/

(check out her crazy home page too...)