Okay. So, I want to build something along the lines of this... http://www.instructables.com/id/Music-LED-Light-Box/
In that Instructable, the 6 LEDs are wired in series. I want to be able to wire them in parallel and replace the 12V DC adapter with a USB connector so that the box may be powered via my laptop's USB port. This way, I won't need another cable going off to a power point in the wall and things will be nice and tidy around my laptop.

Now, I know that the USB 2.0 ports (which is what my laptop has) are 5V @ 500mA.

The specifications for the LEDs I have are:

Emitted Colour : OCEAN BLUE
Size (mm) : 5mm T1 3/4
Lens Colour : Water Clear
Peak Wave Length (nm) : 465 ~ 470
Forward Voltage (V) : 3.2 ~ 3.8
Reverse Current (uA) : <=30
Luminous Intensity Typ Iv (mcd) : Average in 6000
Life Rating : 100,000 Hours
Viewing Angle : ±10°
Absolute Maximum Ratings (Ta=25°C)
Max Power Dissipation : 80mw
Max Continuous Forward Current : 30mA
Max Peak Forward Current : 75mA
Reverse Voltage : 5~6V
Lead Soldering Temperature : 240°C (<5Sec)
Operating Temperature Range : -25°C ~ +85°C
Preservative Temperature Range : -30°C ~ +100°C

So that's pretty much 6000MCD 3.8V LEDs @ 30mA, correct? And this guy here: http://www.youtube.com/watch?v=svWz3xQkkx4&feature=related has his version of the LED music light box USB powered. Would I be correct in saying that this USB powered version still needs the 3.5mm connection to the audio output, just as the 12V DC powered version does?

Seeing as my LEDs are 3.8V, I will need a resistor that will drop the 5V power supply down to 3.8V, correct? If this is correct, this resistor calculator http://led.linear1.org/1led.wiz?VS=5;VF=3.8;ID=30 tells me I need a 47 ohm resistor.

This calculator http://led.linear1.org/led.wiz when I put in my values 5V source voltage, 3.8V diode forward voltage, 30mA forward current and 6 LEDs in my parallel circuit, comes up with this:
+----|>|---/\/\/----+ R = 47 ohms
+----|>|---/\/\/----+ R = 47 ohms
+----|>|---/\/\/----+ R = 47 ohms
+----|>|---/\/\/----+ R = 47 ohms
+----|>|---/\/\/----+ R = 47 ohms
+----|>|---/\/\/----+ R = 47 ohms

So, in other words, I chuck the 12V DC adapter out and hook it up to a USB connector instead, stick a 47 ohm resistor in before each LED, then hook up the 6 LEDs in parallel and it goes. If any of what I said was correct, it should work, right ?

If anybody has any suggestions, comments or improvements, advice, please feel free to share!

 

I wouldn't use the LED at their absolute maximum rating for prolonged period of time @30mA. Also it says, forward voltage could be between 3,2 to 3,8V.

Check the datasheet to see at what current it has 3,2V forward voltage. For 3,2V the 47 ohm resistor would give you more than 30mA. So I'd suggest you increase the resistor a bit to let's say 68 to 75 Ohm.

I forgot: yes you will still need the audio output connection

 

Well, the good news is, you seem to have the available power for this circuit to operate.
As stated previously, you need to work with the nominal Forward Voltage (Vf) and nominal Forward Current (If). My recommendation is to use a Vf or 3.2V and an If of 20mA. This will insure that your LED's live long and prosper!! It also means that s series resistor for each LED of 100 Ohms would be recommended.

 

As iONic suggests, you're always better off to be conservative, or you may wind up releasing the magic smoke from your components.

The Vf (forward voltage) of LEDs can vary significantly for a given If (forward current), even from the same batch. Most of them (70%-85%) will fall in a pretty narrow range, but there will be some that have a much higher or lower Vf than the majority. This can result in some of the LEDs looking much more bright or dim than the others when you're using the same resistor values for all of them.

If you're not concerned with variations in the LED intensity, then don't bother testing them for Vf or relative brightness using the same resistor before installing them.

 

Sweet as guys, thanks heaps. Really helped.

Think I'm gonna use 68 ohm resistors for this project. Once I'm finished building it, I'll try to post pictures of the finished product.

Thanks again for all your help.

- SammY

 

Sweet as guys, thanks heaps. Really helped.

Think I'm gonna use 68 ohm resistors for this project. Once I'm finished building it, I'll try to post pictures of the finished product.

Thanks again for all your help.

- SammY

Good luck. Hope you don't burn your USB driver IC, It's so horrible to replace them...