Understanding RGB Controllers and Tri Colour LED's

Discussion in 'The Projects Forum' started by RodneyB, May 15, 2013.

  1. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    I am trying to get an understanding of how A tri Colour LED works.

    The attached circuit is part of a circuit I built and have seen no benifit.

    By usig three out put from either a PIC or a 4029 IC switching on the diffarent colours should you get a mix of light?

    I thought that the capacitor on the base of the transistor was going to fade the colours in and out but have no idea or understanding how.

    I also want to know if there is a way of getting the colour to evenly distribute under a transparent surface. I have mounted my circuit on a mirror and behind some white plastic. I thought I would get a mix of colour. All I get is a very distinct dot fading off of a single colour.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    What is the model number of the LED?
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I've seen some LCD displays get a fairly uniform spread of light over a large (few sq inches) by first using a thick (approx 1/4 inch) white translucent plastic back, and shooting the LED light in from one side using 1 or 2 LEDs.

    One unit had RGB LEDs so it was simple to make several colors.

    To get anything beyond simple primary colors you would need to dim the LEDs, best way is with PWM techniques.
     
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  4. edwardholmes91

    Member

    Feb 25, 2013
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    Just need to clarify a few things... you mention both tri-colour and RGB leds. These are completely different things, here is a breakdown of the different types of LEDs that you can get:

    - Standard, one colour, two leads, an anode (+) and a cathode (-)
    - Bi-colour, two colours, two leads, essentially looks like a standard LED but will light a different colour depending on which way you connect it
    - Tri-colour, two colours, three leads, available with either a common anode or common cathode, either one of the colours can light or both at the same time
    - Full-colour, or RGB, seven colours or infinite colours with PWM, four leads, available with either a common anode or common cathode

    Your circuit diagram shows a standard LED labeled as a tri-colour.

    You can use a 555 timer and a potentiometer to provide a PWM (Pulse Width Modulated) output. Essentially you are creating a digital signal that turns on a off very rapidly. By varying the duty cycle (percentage of time that the LED is on) you can vary the brightness. The longer the on time the brighter the LED will appear to be. Ideally you want a fairly high speed pulse so that your eye can't detect it. Ever notice those LED signs in takeaways that are powered from AC and don't have a bridge rectifier to convert it to DC? Looking head on the LEDs appear normal but out the corner of your eye they have a really annoying flicker (this is the frequency of the mains turning them on and off 50-60 times a second).

    If you combine three of these circuits and my mentions full-colour or RGB LED, then you have yourself a simple controller with three variable brightnesses for each colour. You could even add a flash function which bypasses the PWM and provides full power. This will make that colour in the LED flash to full brightness when pressed.

    If you aren't really interested in all of the theory and messing around making things there are actually LEDs available that have three different colour elements inside and they have a tiny chip inside them that cycles gradually through the different colours: Rainbow Colour Cycling LEDs. These might interest you, they just require a power supply.

    Unfortunately there is no way of making the light spread differently in terms of the LED... although you could use some special, 'light gathering' Perspex which will glow around the edges.
     
  5. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
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    Thank you for the comprehensive reply.

    My LED is a full colour RGB 4 legs common Anode. I am sorry I dont have a model number as I just buy it from my local electronics shop


    I have managed to get the three colours Red Blue and Green and a sort of purple. I will continue working on the project and post my results.

    Am I correct that you drive each leg from the out put of a 555, This then will give the diffarent colours.

    I was thinking of using the 555 to clock a CD4029 and the BCD output would then be the on for each of the legs.

    Thanks very much

    Rodney
     
  6. edwardholmes91

    Member

    Feb 25, 2013
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    18
    My LED is a full colour RGB 4 legs common Anode. I am sorry I dont have a model number as I just buy it from my local electronics shop

    That's good to know then, it will give you the most flexibility, shouldn't need a datasheet for it but you may like to find out the forward voltages and currents of each element.

    I have managed to get the three colours Red Blue and Green and a sort of purple. I will continue working on the project and post my results.

    Connecting each one on it's own will give you Red, Green and Blue, red and green should give you Yellow, connecting red and blue should give Purple and connecting blue and green should give a Turquoise colour. Connecting all three should give white. Varying the brightness of each element will vary the overall colour.

    Am I correct that you drive each leg from the out put of a 555, This then will give the diffarent colours.

    I was thinking of having three 555 timers and have each PWM output to drive each element.

    I was thinking of using the 555 to clock a CD4029 and the BCD output would then be the on for each of the legs.

    That sounds like a good idea to me but you will get a sudden change in output colour as it cycles through, you won't be able to fade them easily like that.
     
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