Noob: Simple RGB LED project. Potentiometers + suggestions?

Discussion in 'The Projects Forum' started by jorxster, Nov 4, 2010.

  1. jorxster

    Thread Starter New Member

    Nov 2, 2010
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    Hi everyone, I'm working on a simple RGB led project.
    My plan is to run this off my desktop computer's 5 volt wire.

    My plan is to run resistors accordingly for each colour (as Red is 2.0 volts, blue and green are both 3.6), followed by a potentiometer so each colour can be tweaked individually, and all the LEDs get wired into that in parallel.

    I've attached a simple easy to follow diagram.

    This is my first electrical plan- so what do you guys think? I'm wondering about milli-amps-- I contacted the manufacturer and they would only tell me that each RGB led takes 350 mA, didn't explain how it is split up between the different colours.

    Also, can you help me sort out the potentiometers for these colours? I'm aiming for a manual set mood-light, one little knob for each colour. I get confused when I browse this site for pots-- ( http://www.activecomponents.com/ ) as I'm seeing terms like "Trimmer Potentiometers 2.5Kohms". Could you help me know what Ohm rating I need for these guys?

    Any insight or suggestions you can give would be much appreciated!
    Cheers,
    Jordan
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Bill's Index

    LEDs, 555s, Flashers, and Light Chasers

    I think I can show you how to do this, but where is your variable voltage coming from?

    ******************************

    LEDs are not voltage operated devices, they are current controlled devices. You're going to need more than variable resistors to vary their intensity. Some circuitry required.
     
    Last edited: Nov 4, 2010
  3. wayneh

    Expert

    Sep 9, 2010
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    Not necessarily, depending on what he wants. A simple resistor+LED laid across a 5v supply works fine, and varying the resistor will vary the brightness, admittedly in unpredictable ways. Not as good as PWM, but it DOES work for simple stuff.
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    For the power he is looking at of 2-5Watts, it would be a bit more expensive than using a 555 sawtooth generator coupled to a comparator driving a MOSFET. High power potentiometers are not extremely common, and expensive when found.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Missed that. Makes a difference!

    Either way, the pots will likely be the most expensive part of this project. The OP needs to choose what he likes and design around them.
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Correct me if I am wrong.
    OP want to light 3 RGB led's. Which is 9 leds total. Each color is driven together to get a higher light output.
    So basically it is 3 (RGB) bright colors and brightness shud be individually adjustable
     
  7. thatoneguy

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    4 LEDs total, apparently to show the mix on the 4th, and the independent components on the other 3?

    Or all 4 being the same color, unsure why only 3 are numbered and one has a different label.
     
  8. Wendy

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    Mar 24, 2008
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    You are making a noob mistake, LEDs do not vary intensity very well with pots. It is not a linear process, unlike PWM. It really does not work like you think it does. Try it sometime. For RGB you need to be able to turn the LED off all the way for best effect, otherwise it is washed out in a major way.

    Read the link, it explains in detail.

    LEDs, 555s, Flashers, and Light Chasers

    There is a reason PWM is used with LEDs, especially RGB applications.
     
  9. R!f@@

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    Every body knows LED's do not work with pots.
    Dimming LED's can only be done by PWM.

    I really have no Idea what OP wants.
     
  10. Wendy

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    I do. We just need to work out the details.
     
  11. wayneh

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    Sep 9, 2010
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    C'mon guys, that's rubbish. I agree it's a poor approach but put a 5k pot in series with an LED on a 5v supply. On one end of the pot, it'll be fairly dim. At the other it'll blow up. In between, it's in between. Granted, poor and non-linear control, but something that might work if the needs are simple.

    How do you guys dial in LED brightness when you build a circuit? I just keep swapping resistors until it l get the current and brightness I want. That's not much different than turning a pot. But of course I'm looking at the 50-90% of full intensity range.

    If you have a fancy RGB LED and you need it to truly turn off as Bill notes, that's a different story. I completely agree a pot won't work well at the low end of intensity.
     
  12. R!f@@

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    Hah!! Put a pot with a led, and it will burn out sooner than u can say, son of .......
     
  13. wayneh

    Expert

    Sep 9, 2010
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    Yup, you absolutely must use an additional resistor in series, for when the pot is at 0 ohms. No matter how careful you are, you WILL turn it to zero at some point.
     
  14. Wendy

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    Mar 24, 2008
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    And with a pot it will likely stay lite to some extent 100% of the time, though I can think of configurations where this is not true.

    The main thing to remember, the thing that blows all assumptions out of the water, is these are high current LEDs operating off 5V. This means no buck pucks, only resistors and possibly solid state devices such as transistors. Wayneh is missing this point, very few pots can take 350ma. This is not an off the shelf item, it will require some construction to get around it. Pots control transistors nicely, and off the shelf transistors can handle 350 ma off the shelf.

    Having said that, if you are willing to buy rheostats this problem goes away. They are bulky and clunky, but could do this with one or two resistors. A rheostat is a specialty potentiometer designed for high current. I almost never use them, I lean toward solid state controls. A 100Ω rheostat would work nicely. They can be a little hard to find, and aren't off the shelf parts. I'm attaching a schematic of what I'm talking about. I hate this design. It would work, but it is not how I would do it, all the control would be in the top 1/8 to 1/4 of the pot range.

    [​IMG]

    I would be willing to draw a schematic that will do exactly what the OP wants, but before I spend a couple of hours on this I need to be asked. I dislike being taken for granted intensely, some acknowledgment of my time is required.

    A good solid state solution will require at least one IC (probably two), and a couple of transistors. For best operation some specialty logic MOSFETs, but it can be done with conventional BJT off the shelf transistors (6 of them, 2 per color). This is for a linear response out of a pot. If you don't care about linear it can be done with simple transistors, which will get very hot and need heat sinking. In this case the project will be much larger, as air flow is required, but the construction will be simpler. Here is something from my library that shows how to do this.

    [​IMG]

    Some modifications would be required to work for your application.

    I'll state it again, and I do know what I'm talking about, simple conventional resistance schemes will not work. Talk is cheap, schematics take time but address all questions.

    You can use the computer's 5V power supply, but you can not use the USB port. This project will pull around 1 amp total, which exceeds that source.

    To the OP: Do you want my specific help or not? I'm offering, the ball is in your court. I do a lot of artwork for the forum.
     
    Last edited: Nov 6, 2010
  15. jpanhalt

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    Jan 18, 2008
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    [​IMG]

    I do hope the OP will not be too discouraged and will return for real help.

    John
     
  16. R!f@@

    AAC Fanatic!

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    Hee hee...I have no idea what u are trying to say.
    Pls clarify the image. I am not tht well informed on such images and proverbs u guys used.

    But I do hope OP returns. Didn't mean to scare him.
    Just wanted to let him know dimming leds is not like dimming filament bulbs
     
  17. jorxster

    Thread Starter New Member

    Nov 2, 2010
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    Whoa! I'm back!
    Yeah I've had a crazy week- working overtime (till midnight) on an animated TV series, then was gone all weekend.
    Been entertaining studying up on all your guys' posts!!!
    In all honesty though, I'm quite lost!

    My plan was to make a very simple manually-controlled moodlight, with say - 4 RGB leds.
    For a first project, I do *not* care about linearity, or even if all the green diodes have the same brightness. ( at partial levels ) Not sure what "OP" stands for as you affectionately refer to me :p

    So is it not possible to have a simple dial pot that dials from no resistance to high enough to blank out the LEDs?

    PWM sounds great, modern, digital, but a little too advanced for me. I'll keep studying that article, thanks for the link Bill!
     
  18. wayneh

    Expert

    Sep 9, 2010
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    Yup, you're right. I had no idea we were talking about that kind of current. My bad. :eek: I was thinking 2-20mA, standard indicator dime-a-dozen LEDs.

    OP (original poster): Ignore what I said about using pots. Don't proceed with the pot-in-series strategy unless you fully understand what Bill et al have advised.

    John's colorful post shows that he appears to have not liked the comment on needing "only" PWM to control LED brightness. Perhaps the word "only" is a bit much. Anyway, I wonder if he made the same mistake as I.
     
    Last edited: Nov 7, 2010
  19. jpanhalt

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    Jan 18, 2008
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    Mistake about what? The power needs were clear. The topology suggested by Bill Marsden is simple and is used quite commonly to get around the power limitations of standard pots. High power pots are bulky. Also, when power needs get greater, it is easy to heatsink a transistor, compared to heatsinking a potentiometer.

    I think it is a good solution for this application by jorxster, considering mostly his desire to keep it simple. If he wants to go with PWM, less heat will be generated, but the complexity will increase greatly. Some people use relatively low frequencies for PWM dimming, such as 100 to 120 Hz. I think such low frequency would be a bad choice for a mood lamp.

    John
     
  20. Wendy

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    Mar 24, 2008
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    I can come up with something using rheostats, RadioShack has one for $3.99 (25Ω 3W, you need 3). Like or not LEDs are electronics, I'll need a datasheet. Where did you get them, and what is the part number?

    Part of the problem is they vary a lot. Each color drops a different voltage.
     
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