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Led circut

  1. Nov 19, 2004 #1
    hey guys,
    I have been looking to make a outdoor led washlight for a long time now. I am not very experienced with electronic circuts but I can understand the basics. I am planning to have 30 leds, 10 each of red green blue. I want to control the three different colors with a potentiometer or somthing like that. It would also be nice to be able to switch to something that can automatically vary the resistances slowly, makeing different color effects How do I set up the circut? Like I said I have been trying to work this out for a while.
    create somthing like this
    Also where would I get a fixure like this to house the circuts/leds.
     
  2. jcsd
  3. Nov 19, 2004 #2
    Do you have power supplies?
    It looks like 3 x 20 ma load and about 30 v.
    You better use series resistors or something or they will blow.
    Do you want to hand control with pots or electronically?

    I just want to hook up a bunch of white light LEDs for a night light. How cheap are they now?
     
  4. Nov 20, 2004 #3
    You could have three different transistors biased differently. As you turn the potentiometer, it would provide more and more voltage to the LEDs. Until, eventually, all the lights are glowing. To make the other lights dissipate, you could wire the output of the transistor on, to feed into the others, satturating them.
     
  5. Nov 20, 2004 #4
    white leds
    here is a good place to buy leds. He offers free shipping. I read on another fourm that they are goood quality.

    Would I use some type of circut board to mount the leds and other devices. (like i said I know very little about circut design). Im guessig I would need some sort of transformer to convert ac to dc.
     
  6. Nov 20, 2004 #5

    Cliff_J

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    Science Advisor

    Ok, lets be safe here, messing with 120V AC is dangerous. Using an existing power supply (the wall-wart) or some universal one is going to be cheap, easy, and safe. You can probably find one around the house from an old cordless phone or something.

    Now you need to find the current and voltage of the LED to figure out what kind of resistor to add in series, use this formula:

    (Total Voltage - LED Voltage) / LED Current

    Lets say you find a 9V DC power supply and want to use that to power the LED. I looked up the ones in the link and they said 3V and 20mA so that would be

    (9 - 3) / .020 = 300 ohms

    Nearest resistor value to that is 330 ohms and you would hook one to each LED in series with it - series means the voltage goes from positive through the resistor and through the LED to the negative. Each one of those resistor/LED pairs could then wired in parallel to the power supply (parallel means they all connect to the positive and negative).

    But lets play with the math more and see what we can come up with if it is one resistor and 2 LEDs all in series.

    (9 - (3 * 2)) / .020 = 150 ohms

    How about 3 LEDs in a row?

    (9 - (3 * 3)) / .020 = 0

    So with a 9V power supply the three 3V LEDs don't need a resistor in series, each one drops enough voltage to limit the current. Since you want 10 you can figure out how to divide them out and buy the correct resistors to limit the current to prevent burning out the LEDs.

    Now if you wire in a 5 Kohm potentiometer in series with the LEDs of each color that would allow you to turn each color up and down individually.

    Simple, cheap, safe. Ask if that isn't clear, but there's plenty of examples online on how to wire up the LEDs and current limiting resistors so you don't burn them out.

    Cliff
     
  7. Nov 21, 2004 #6
    Thank you vey much cliff that is exactly what I needed. Does any one know where to get the housing or fixture to
    hold the circut for outdoor lighting purposes.
     
  8. Nov 21, 2004 #7

    Cliff_J

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    Science Advisor

    Not sure what type of light you're trying to construct. If this was a small nightlight to use inside a camping tent to see what you're doing that's one thing, if you're trying to create a floodlight to light up a driveway you'll need many many more LEDs. It takes a dozen or so very bright LEDs to equal a automotive taillight bulb, something like a 60W household bulb would take a box of LEDs to equal.

    Anyways the generic circuit boards like you can find a radio shack would make this easy to solder up and get working. Just make sure to double-check the LEDs are inserted with the correct polarity before soldering it up! :smile: They also have small project boxes that you could drill a bunch of holes in and have the LEDs come through and then maybe epoxy in place to secure and make a good seal.

    Any time you have outdoors then water is an important consideration. The 120V AC side of the power supply needs to ALWAYS be dry. If it were to get wet that could be a short and if it isn't protected by a GFCI then it would be extremely dangerous too. Just a temperature change can cause condensation to form so always keep this in mind that you need to treat household voltage with plenty of respect. If you run the 9V or 15V DC or whatever power comes out of the power supply this is far less of a concern although if you short it out the power supply could burn out.

    Cliff
     
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