My MP3 players seem to each batteries, mostly because I leave them on when I
connect them to my PC to move files around. It seems that the run from their
battery even when connected to the USB port?
Some may, I can't claim to have taken a survey of them all.
Any hacks out there that will get these things to run from the USB power, or
even recharge a battery when connected?
Yes it's possible, but not just a matter of uploading a file
into them or anything like that. The main limitation (for
anyone with the other prerequisite ability) is how much free
space there is inside the MP3 player- some are quite full.
Supposing you have only a single cell or two power supply
(that is, one or two AAA or AA batteries for ~ 1.4V (Peak)
to 2.8V total power when fully charged) you have plenty of
voltage margin to construct a simple linear current limitor
recharge circuit. While there are theoretically great,
fancy ways to do it, in practice on an MP3 player with
limited space you might want it as simple (read- small) as
possible).
One of the most common and quite sufficient linears for this
would be an LM317, and most manufacturers' datasheets
include a simple current-limiter circuit you could use. The
key is then how to integrate this additional hardware either
into the player housing or externally.
That can vary per your needs also. I have a player with
removable SD card storage and I find it seldom that I need
to hook it up to a system at all, rather than popping out
the SD card and accessing it via a card reader. Therefore
my needs are more those of recharging the whole player
and/or powering it while playing, as an audio source hooked
up to a non-mobile amp. Towards these ends I took an old
cordless phone recharging base (not a whole-phone type, was
a remote charging cradle) and moved the contacts in it,
adding a couple of small stainless steel screws into the MP3
player housing so when the player is set in the phone
cradle, it recharges somewhat like the phone did... though
it does not use the original phone recharge circuit, I added
a home-etched circuit board with the trickle charge circuit
similar to what I've described above, using an LM317.
Some MP3 players aren't so easy to open, it can vary a lot.
If yours has a screw or two, it might be much easier to do
it without breaking off too many molded pastic tabs, because
"often" the sections with the screw will crack apart with
the screw removed, then you have a crack you can peer into
and insert a tool to help (slightly) flex the rest of the
player to get the remaining plastic tabs apart without
cracking or overly stressing them. A good tool to use would
be an exacto knife with a thin blade bent at the tipe or a
tiny pick set (similar to dental tools). If you end up
breaking the tabs your only recourse might be putting the
outer shell back together with super-glue. If you end up
doing that I suggest putting electrical tape around the
outside seam (making sure it adheres good to the edge so no
cement gets under it) to keep the super glue off of the
exterior surface of the player, lightly clamping it shut
after applying the cement then immediately wiping off
excess. Otherwise the exterior may be cosmetically defaced
by the cement.
As for the interior circuit of the player, you'd need a very
fine tipped soldering iron to solder a couple of very thin
jumper wires from the mini USB socket's 5V & ground pins, to
the recharge circuit board you make, then jumper wires from
that circuit board to the battery contacts in the player (or
find a place on the player's circuit board to attach these
post-regulated power and ground leads.
If you don't already understand what I'm talking about with
"linear regulators" and "current limiting", this project may
be beyond your abilities... as I haven't even gotten into
fabricating a tiny surface-mount add on PCB yet. If you
wanted that inside the MP3 player you would have to take
exacting measurements to determine the available space and
shape you need the circuit board to have, and find a way to
mount it.
Having described this much of the project I'll end the post,
until you determine is this is a route you want to take. An
easier solution is to search for an MP3 player that does
already incorporate a recharging-via-USB circuit in it...
but I don't remember any makes or models that do so, only
that I very vagely recall there is at least one (or more)
that do.
