Resistor needed for LEDs?

Discussion in 'The Projects Forum' started by chuco61, Feb 2, 2015.

  1. chuco61

    Thread Starter New Member

    Jan 21, 2015
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    This is probably really simple.

    I have 20 Diffused 5mm Slow Fade Flashing RGB LED's that I would like to have operate at the same time. No programming, no micro controller. Just a simple 5v wall power, a pcb and LED array.

    Is this simply a matter of wiring them in parallel, adding a resistor at the + side and hooking the whole thing up to my 5v supply?

    If so, what resistor would i need to keep the whole thing safe?

    These are the ones i have:
    https://www.adafruit.com/product/679

    Along with powering the LED array, I need to power an Adafruit effects board that needs 5v of power.

    If i used the wall adapter below, with a split USB cord so that I can power the LEDs as well as the Adafruit 5v sound effects board. https://www.adafruit.com/product/501

    Do you think this would work?

    any opinions?
     
  2. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    About your LEDs:
    • 5mm diffused RGB LED
    • Two leads
    • Power with 3-3.4VDC
    • Current draw: 10-20mA depending on voltage and displayed color
    You have a 5 Volt supply so roughly we get Vsupply - Vled / Iled which works out to be about 120 Ohms series resistors. Each LED to have its own series resistor and the parallel them. Do not parallel the five LED and use a single resistor. The supply looks to be 5 VDC capable of 1 Amp so it should be fine unless beyond the LEDs your board has a very hign current draw.

    I based the 120 Ohms on Vf = 3.2 Volts and If = 15 mA.

    Ron
     
  3. chuco61

    Thread Starter New Member

    Jan 21, 2015
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    Reloadron,

    so i would put a 120ohm resistor for every 5 LEDs? Then all the groupings would go to a common power supply? Like this diagram, assuming that the battery is 5v and there are 5 LED's instead of 3.

    [​IMG]
     
  4. chuco61

    Thread Starter New Member

    Jan 21, 2015
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    or maybe this better describes it?

    [​IMG]
     
  5. #12

    Expert

    Nov 30, 2010
    16,278
    6,790
    No, you use a resistor for every single LED. There is not going to be a series connection of LEDs because 5 volts is not enough to light up 2 of your LEDs in series.
     
  6. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    ^^^^^^^^^^^^^^^^^

    What #12 said, one LED and One resistor in series with it. Then parallel the series LED / Resistor combinations.

    Ron
     
  7. chuco61

    Thread Starter New Member

    Jan 21, 2015
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    Something like the top left? So sorry, Im a visual learner when it comes to circuits

    [​IMG]
     
  8. #12

    Expert

    Nov 30, 2010
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    Yes, the one on top will work. The drawing on the bottom is a guaranteed smoke machine.
     
    RodneyB likes this.
  9. chuco61

    Thread Starter New Member

    Jan 21, 2015
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    Haha! thank you very much for your help! I'm off to get some 120ohm resistors.
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    For what it's worth, you could run those LEDs at 5mA and they'll be plenty bright as indicators. Bigger value resistors, less current drain on the battery.
     
  11. chuco61

    Thread Starter New Member

    Jan 21, 2015
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  12. wayneh

    Expert

    Sep 9, 2010
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    The heat they need to dissipate should be less than 1/8W. The calculation is P=I^2•R. If we're talking about 15mA: P=(0.015A)^2•120Ω = 0.027W, so those will be fine.

    Depending on your plans, you might want to consider an assortment such as this.
     
  13. chuco61

    Thread Starter New Member

    Jan 21, 2015
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    Thank you!!

    Does anyone have a link / resource to blank perf boads that I can use to wire / solder all these LED's? I have about a 5x5in work area to display these in a 4x5 grid. Frys may have some, I just havent looked closely.

    Thanks!
     
  14. wayneh

    Expert

    Sep 9, 2010
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  15. chuco61

    Thread Starter New Member

    Jan 21, 2015
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    Back with another question but along the same lines as the above posts.

    Is this schematic correct to create a simple sound activated LED circuit? I am looking to use some standard white LED's (and am assuming that a 120k resistor as mentioned above would keep the circuit safe?

    I can get the TiP31 from frys as well and will use 5v usb power. Thoughts?
     
  16. #12

    Expert

    Nov 30, 2010
    16,278
    6,790
    You are back to the, "guaranteed smoke machine".
    There were no 120k resistors mentioned.
    Whether it works at all depends very much on the amplitude of the signal. Too little and nothing happens, too much and the LEDs smoke. A bit more than that and the transistor breaks over backwards (voltage-wise).
     
  17. chuco61

    Thread Starter New Member

    Jan 21, 2015
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    hmmm, i think there wont be enough signal from my sound source, it's an old payphone handset that is providing the signal. I'll find an alternative.

    Thanks!
     
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