🚀 go-pugleaf

h1dd3n_h4m$73r@salmahayeksknockers.edu wrote:
> I've got a battery powered peripheral (FM Transmitter) that I'd like to power 
> from USB.
> 
> I know that USB puts out 5v, and that a "Double A" batter puts out 1.2 v,
> and assume that two would put out the same.

Depends if the batteries are in series or parallel. if they are in 
parallel they still put out 1.2v but can produce more current. If they 
are in series then they will put out 2.4v.

> 
> Can anyone point me in the right direction to make my little project?  I
> have a soldering gun, and can use it, but need an idea of what kind of parts 
> to put in between.
>

http://www.starmount.co.uk/s_usbpin.htm

Well to start here is the USB pinout:

"
Pin  	Name  	Description  	Cable Colour
1 	Vcc 	+5V DC 	Red
2 	D- 	Data - 	White
3 	D+ 	Data + 	Green
4 	Gnd 	Ground 	Black

Low power, bus-powered function  	Draw 100 mA (max)
"

(your can't actually trust those colors listed, but they might be right)

Basically you want to use a few resistors so we go to good ol' ohms law..

5v
----- = 50 ohms
100mA

so you want to ideally keep this down to 50 ohms total. It just so 
happens 12 ohms aren't the hardest resistors in the world to find.

12 ohms X 100mA = 1.2v (or 1.25v in reality)
(3x12 ohms) X 100mA = 3.6v (or 3.75v in reality)

double check the math.... so here is your circuit (excuse the crude drawing)

5v + pin 1 ---+[ 12 ohm ]++[ 12 ohm ]++[ 12 ohm ]+---- 1.2v+
			                        |
                                                 |
                                                 |
GND - pin 4 ----------+[ 12 ohm ]+--------------+
                       |
                       +------------------------------- GND -

> Also, is there any danger that dicking around with a USB cable like this
> could cause corruption elsewhere in the USB bus?

While it is possible, I doubt you could do a lot of serious harm, still 
I'd unplug your other usb devices until you tested this and are happy 
with it. Worst you could do is reverse pin 1 and 4 and possible harm the 
"battery" driven device, or short circuit 1 and 4 in which case you 
might cause your computer power supply to trip into short circuit mode. 
Still it isn't like you are playing with high voltage here.

Lemme know how it goes, this isn't a bad idea, and could be a useful 
little cable to have kicking around.

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