It's already been done, in this very thread : look for lbattraw's post at the end of page 2...

Crossbow : thanks a lot indeed for the very detailed explanation & pics.
So in the end you only use the casing and regulating circuitry of the battery box, sacrificing the USB part ?
What do you think of lbattraw's approach of using 4 AAA rechargeables for lower voltage and total output under 5V ? Is this not sufficient security ?
You are right about the skill issue also. I have just bought a Keysonic nano kbd specifically for the 770, I have a portable USB hub ready to sacrifice, and USB cables galore. I can solder and have a Dremel tool for the delicate assembly mods, but I do lack the tools and experience for the electrical part (multimeter, continuity testing etc. ).
I just know + from - and how to use a LED for polarity and continuity. Think I stand a chance ? :-)

Thanks !!

As far as powering a keyboard, finding where to supply the power is easy -- it's the outside pins of the USB connector. Please be careful with the polarity. If you supply power in this manner, you are supplying both the keyboard and the cable, so it should work fine.

Make sure what ever you use for power is regulated. I have seen some suggest using 4 x 1.2v rechargeables. This is dangerous. First, there is always the possibility of accidently installing alkalines, and second, I have seen rechargeables come off of the charger a little "happy". Would 6+ volts harm the 770? I don't know, but it seems like poor design, and I wouldn't try it on my 770. Finding something with a regulator in it like I did, or even designing one is not hard.

The hub is clearly optional, but due to the (current) limited size of RS-MMC cards, I wanted to be able to use a Flash drive or card reader. Besides, I used components that were available to me at the time.

I would have loved to build mine into a USB keyboard with a hub in it like was mentioned is a previous post. I also have the circuit board of the 2 port hub from an Apple USB keyboard (met with a tea accident.) I thought of adding that and a battery pack to a mini USB keyboard. This would make a nice single package, but the physical surgery to the keyboard was more than I wanted to tackle. Besides I think I will mostly just carry the hub with me. These days you can "scare up" a USB keyboard almost anyplace.

Another note. I just typed quite a bit on my 770 using an Apple keyboard (not the one that got tea on it). I had no keybounce events, so I guess it is just that mini logitech that has the problem.

fpp;

DO NOT use an LED to check for USB power!

Typical (classic) LED's run on about 2 volts. 5 volts at sufficient current can cause them to explode, possibly sending bits of plastic out at high velocities. 5 volts from a battery powered device will probably just damage the LED, but it is possible that a PC, or even a hub with a large filter capacitor could make the LED fail violently.

An LED and the proper resistor is safe, but is not "proper tools".

I would not connect anything I built/hacked/whatever to my 770 without testing with a good and trusted DVM.

That is correct.
;

Me too! I wish Nokia could fix that!

::: great waves of envy to you!! :::

If Nokia -- or someone, some great big generous hacker-type person -- would ever come out with an app that switches the 770 to/from host mode on the 770 itself, I'd be first in line to beg you to make me one of these things for money! I have a 20GB pocket HD I'd just love to hook up to the 770... and even if that didn't work, I'd figure out something else.

Nice, that you found that PS2 KB. I saw it at J&R and mentioned it in my "blog" w/o knowing anyone else had seen it (I'm late catching up on threads!).

Whew :-)
Have you ever noticed that on good days, when you are blissfully unaware of such dangers, they also sometimes ignore you ?... I've done just this twice, in good faith, very recently, to verify output polarity on two "wall-warts" (5V and not too puny output - like 2.5A). Admittedly I only lit the LED a fraction of a second each time, but I held it between my fingers and probably too close my eyes too. Given the ubiquity of such things in computers & other electronic gizmos I sort of assumed 3-to-5V should be OK... Thanks for the warrning !

Crossbow, you have convinced me. I've been shopping at lunchtime and found myself an entry-level-but-not-too-shabby multimeter, and will learn to use it.

Also your remark on batteries and rechargeables finally reminded me that my iRiver mp3 player came with a battery pack (like yours, only not USB) that I had never used and forgotten. Amazingly I managed to locate it and will repurpose it for the 770. It doesn't seem like I can take it apart without breaking it so I can't see if it has voltage regulation built-in, but if I understood you correctly I can make this my first test with the DVM: if I load it with four fresh akalines and get less than 5V output, then it does, right ?

I understand protecting users from themselves by making the "root" thing a bit difficult, but I agree the "USB host" ought to be more accessible. Dunno if technically such a machine can "flash itself", though. My iRiver mp3 player does just that, so it's possible...

You should see the KeySonic nano keyboard (ACK 3400U) - it's SO cute. About as thick as the 770 (without cover), barely taller, and some 50% wider. Short of the foldable BT stuff it's probably the best match you can think of - right down to the matte black finish. And it's a *real* keyboad, albeit petite - not something you'd want to write "War and Peace" on, but definitely beats anything on a PDA, past and future :-)
(must remember to snap & post a pic...)

LEDs, batteries
 

I have to chip in here (no pun intended). I've been using and destroying electronic components ever since I can remember and I've never had a LED explode. Granted, I've never attached one to 120VAC to see what it would do, but under logic-level (5V/3V) voltages with virtually unlimited current the worst I've ever seen is a little smoke and discoloration of the epoxy resin they're encapsulated in. I've seen chips and power transistors "pop their top" where a small piece of resin will fly off, but it's not at a velocity that would hurt someone. Anyway, it's more a situation of cracking than shattering; they're made to take a fair amount of abuse.

Digital devices are generally made to tolerate a +/- 5% variation in their supply voltage (5V) and an absolute maximum typically is at 6 volts. This means we're fine from 4.75 to 5.25V, and small spikes above that won't necessarily destroy the device. Rechargeable cells produce a nominal 1.25V, a little over 1.3V right off the charger and 1.2V after some use or idle time. Thus, we're looking at about 5.2 off the charger and 4.8 after some time. This is just fine, but we run into serious problems with alkalines which are sometimes almost 1.6V per cell. Even at 1.5V nominal, we're already at 6V-- bad news.

I've never seen a removable AA battery pack that was regulated. Any regulation will happen on the board of the device. Please just use rechargeables, and keep the DVM handy to double check polarity, voltage, etc. You're guaranteed serious damage if you hook things up backwards :(

Live via free 770:

I think I might have seen this @ DataVision under a different name. But a pic is always welcome!