Controlling LED intensity

Discussion in 'The Projects Forum' started by kballing, Oct 27, 2008.

  1. kballing

    Thread Starter Member

    Jun 18, 2007
    A project that I am planning is an automated glockenspiel (xylophone) that uses a serial controller and a bunch of soleniods to bang out the notes. The instrument has 37 notes and the shift registers in the circuit have 8 channels each making 5x8 = 40 channels, 40-37 = 3 left over. So I was thinking of what to do with the other three channels. I thought, why not add light to the project and include a channel for R, G, and B leds. The circuit only controls on/off via transitors without dimming, which seemed a bit boring to me.

    I'm not asking for a detailed schematic or long lecture on LED physics, but what are some ways I can control the intensity of each LED so I can get more colors?

    I have a few piezo pickups that I got really cheap. One idea I had was to use the audio signal to modulate the LED intensity, but don't know where to start there.

    I don't know much about LEDs, but I can figure it out. Any other cool suggestions are welcome.
  2. bertus


    Apr 5, 2008
  3. mik3

    Senior Member

    Feb 4, 2008
    Each leds illuminates only one colour,which depends on the material the led is contructed or more strictly on its badgap, unless it is a two or three colour led. You can vary the intensity (brightness) of the leds by varying the current through it but not its colour.
  4. StephenDJ

    Active Member

    May 31, 2008
    The original poster's project is very interesting (i'm a musician too). I'm assuming the shift registers are of the serial-in/parallel-out type. If it were possible to add an extra shift register, the 8-bit binary output of it could be feed into a resistor network to produce an analog voltage level that can vary the brightness of all LEDs that happen to be on at any one time. But you'll need a transistor operating in analog mode rather than switching mode to make this digital-to-analog scheme work, tho. Oh well, just a suggestion.
    Last edited: Oct 28, 2008
  5. kballing

    Thread Starter Member

    Jun 18, 2007
    Thanks Bertus, a light/color organ is exactly what I was thinking about (I just forgot). StephenDJ, interesting idea, but not what I'm going for.

    I guess I can just use an op-amp for 3 band-pass filters, and then I'll probably put the LED under a cool engraved plexi-glass bezel.

    maybe if I get really ambitious I'll add a RGB led for each note and use the extra three channels to control the on/off state of each color so that I can modulate color according to how many notes are played or something.

    This thread isn't closed, new ideas are still welcome.
  6. wy6k

    New Member

    Nov 4, 2008
    The accepted way to do this is to drive the LEDs with a pulse train. By varying the duty cycle, you can vary the intensity. The beauty is that you can do this totally digitally, under program control - no analog stuff.

    You can either send a pulse train on each of the three channels or you could build it to send a control word that specifies the duty cycle and then let the logic in the unit convert that control word to a pulse train. When you want to change the duty cycle, and therefore the intensity of each color, you send a new control word.