USB powered LEDs that react to music - HELP ?

Discussion in 'The Projects Forum' started by SammYSkywalker, Jul 18, 2011.

  1. SammYSkywalker

    Thread Starter New Member

    Jul 18, 2011
    Okay. So, I want to build something along the lines of this...
    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: 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;VF=3.8;ID=30 tells me I need a 47 ohm resistor.

    This calculator 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! :)
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    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
  3. iONic

    AAC Fanatic!

    Nov 16, 2007
    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.
  4. SgtWookie


    Jul 17, 2007
    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.
  5. SammYSkywalker

    Thread Starter New Member

    Jul 18, 2011
    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
  6. praondevou

    AAC Fanatic!

    Jul 9, 2011

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