[project] 12v in laptop (5v) HDD .mp3 player. USB access, AUX output

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First off, let me say that I know next to nothing about electronics. Sad, considering that I'm going to school for a degree in electromechanical technology. I know how to fix broken things, connect components, and make my own cables which has carried me for the most part through my life as a computer geek. Generally I end up burning a lot of components since I just last week bought my first multimeter and beforehand would simply not even do any math and never cared about voltage, resistance, or current; I'd just plug things in and hope for no smoke (hey, I was messing around with electronics since I was about 7 years old and since most of the stuff I intended to do worked I never bothered to ACTUALLY "learn" anything about it). I may be able to replace parts and seemingly know what I'm doing with my mad soldering skills and common sense, but as far as beginning with an idea and building something from a bare PCB I'm clueless.

So hopefully being here will be a learning experience for me and supplament my schooling nicely. I don't really know if this is the right place to be posting about my proposed project since this forum seems more about the audio than the electronics, but quite frankly this seems better than any other place (since most electronics forums seem to chastise me for not knowing the ins and outs of electronics.

The first thing I want to do is this: I have a 10GB laptop (2.5") hard drive (and about a dozen smaller ones, from 4gb to 1gb) and I want to make a DIY MP3 player. The trick is this; I'd like it to be able to plug into the AUX on my car's head unit and draw power from the cigarette lighter (which is notoriously "dirty" power as I understand) but I'd also like to be able to plug it into a USB port on a computer (not at the same time, of course) and add/remove/access files from it.

I already have one of those $10 enclosures that turns an internal drive into an external, and it works nicely. I'd intended to use it with a 12v FM transmitter I picked up from wal*mart that plays mp3's from a flash drive (play, skip, stop, freq select buttons. Automatically plays songs in the order they were loaded onto the drive. I'm very willing to break this down for parts). The problem is that the FM transmitter doesn't seem to cooperate with the hard drive -- plus I've a lot of radio interference from other devices in my car and as such I get a very fuzzy signal when I do use a flash drive.

Alternatively, adding a USB port to my head unit would be an even better solution so long as I could skip ahead in a playlist with the player controls. Since USB is powered, I wouldn't need to plug it into my ciggy lighter and instead could leave my CD and heater plugged in (I have so many gadgets in my car -- both DIY and purchased -- that my hatchback looks more like a semi trucker's cabin). The downside is that it would involve extensively modifying the head unit with software, hardware, and even firmware, and I'm not going to do that this time of year without a garage to work in and with a warranty that would be void as a result.

I'm not asking for schematics and a pat on the head. I'd like to UNDERSTAND what I'm doing and why I'm doing it since the "build this, here's your A" type of deal is what I'm going through in school. I can't explain the control logic of most of the circuits I build for my classes, but I can build them from schematics like clockwork. Sadly, since I only have free time from 11pm - 4am, I don't have time to learn WHY it works or what I'm doing by sitting down with other students or instructors.

Yes, I suppose I could go and read books for dozens of hours and hope that a light went on, but I learn better by doing, and creating something I "need" is wonderful motivation to learn.

The kicker is this: I'd like to do this project for under $50 if possible. I have a ton of mid-to-late 90's computer components (all of the laptops from which I pulled HDDs) as well as misc. wires and cables. My tool cooection is massive, and anything REALLY exotic that I don't have I can get VERY cheap locally. I'm not concerned with how this thing looks at all since I've got some places I can hide it. And no, a laptop is NOT an acceptable solution as it's too huge and unwieldy and the screen would likely eat up a unecessary amount of power... though maybe I _could_ use one of those solar car battery charge toppers... hrmmm... well?
 
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