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MP3 player mod. Would this work?


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I've got a crappy little mp3 player from Walmart that runs off one AAA battery. I use it primarily for listening to podcasts while i drive, since it shows up like a regular usb thumbdrive in Linux (i'd have to reboot into windows to put podcasts on the zune). The only problem I have with it is that the only way to power it is off the one battery. It doesn't have any sort of power adapter available and its a pain to keep recharging the AAA battery to keep it going.

I'm thinking of a way to us the power from my car's battery to charge the unit. I have a power adapter in the car that powers usb devices (it measures at 5.2v w/ my multimeter). I figure I could just splice up a usb cable, and connect the ends of the cable to the + and - ends of the player to power it. Of course I'm sure I'd need some sort of resistors or something so I dont fry the mp3 player.

Heres a drawing to clear it up a bit.

width=640 height=202http://img184.imageshack.us/img184/5706/mp3bg0.gif[/img]

I have a few questions

1) Would this even work or is this completely stupid?

2) I measured the voltage of a AAA battery with my multimeter to get the 1.25v. Would this number be accurate for the mp3 player or could it somehow draw more or less?

3) Assuming my numbers are correct, what kind of resistors would be needed to bring the 5.2v down to 1.25v?

As you can see I dont know too much about these types of projects, so any help you could give me would be very helpful.

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The player will work on any battery that will fit in it, which includes 1.2v rechargable NiMHs to 1.5v alkalines so you have a little margin to work with.

You'll be wanting something that looks a lot like this (though it's simple and not hugely efficient, it's extremely easy to build):

USB                  Resistor                      MP3


+5v -------------[            ]---------------| + |

    D-  -----                                      |    |

    D+  -----                                      |    |

-0v ----------------------------------------|_-_|

The resistor, if the mp3 player chews ~150mA and if my math is correct, is probably gonna need to be around the 27ohm mark with a rating of 1W.

From the USB's V+ pin to the resistor to the positive end of where the battery would sit in the mp3 player, then connect the ground to the negative end of where the battery would go... sorted, plug 'n' play.

The other option if you want to get a little less kludgy is a voltage divider, which is one of these:

width=121 height=132http://ourworld.compuserve.com/homepages/Bill_Bowden/R.gif[/img]

In your case, you have 5v going in (V) at 500mA (I'm guessing, since it's USB) and the Vout should be 1.5v, and with those numbers we can calculate the R1/R2 values to give you 1.5v between Vout and ground...

According to http://ourworld.compuserve.com/homepages/Bill_Bowden/r2.htm (which happens to be where I nicked the picture above from), R1 should be 7ohms and R2 hould be 3ohms. Now an advantage to this method, if my electronic knowledge isn't completely useless, is that you can use the remaining 3.5v to do something else with rather than just wasting it. You could use it to... I dunno, power a couple of white LEDs to use as an in-car flashlight...

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