How to dim custom led lights

Discussion in 'The Projects Forum' started by fw2000, Mar 12, 2009.

  1. fw2000

    Thread Starter New Member

    Mar 12, 2009
    My project came about to increase visibility/safety on my motorcycle. Unable to find what I wanted I have built my own custom inserts for turn signals and brake lights. I am using 48-52 5mm leds with Vf=2 and If=20ma. I am wired into my bike's wiring and using either a 10v or 12v voltage regulator(my leds are wired in arrays) in each light assembly to smooth/restrict output of my charging system. As brake and t/s(5 lights in all) they run at 100% and work great. Now my problems: 1)Since leds are not linear how can I figure my resistors so these lights will run at 50%-60% brightness(running lights)? 2)Since leds are not dual filament when signal comes down brake and/or ts wires I think I need to disconnect the running light circuit(to prevent overdriving leds) and switch to 100% brightness(brake and ts lights). I need something simple and efficient I can can install in each housing to switch between these 2 circuits. Any suggestions?
  2. SgtWookie


    Jul 17, 2007
    What regulator(s) are you using, or planning on using?
    Keep in mind that the 78xx series regulators have a minimum 2v dropout from the input to the output terminal. In order to get regulated 12v out, you must put in >=14v.

    LM317 regulators have a 1.7v dropout; to get 12v out, you must put in >=13.7v.

    There are low-dropout regulators available; some are quite low dropout, some aren't.

    Why don't you post your schematic as it is now so that we have something to work with?
  3. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    Here's something to try, which both demonstrates....and takes advantage of...a human vision feature (or what some might call a "defect") If you PULSE the lamps at a high rate, where you can't actually see the flicker (a few hundred Hz), but adjust the DUTY cycle, the perceived brightness will change. I'd try a 555 circuit with two different duty cycles.

  4. Jassper

    Active Member

    Sep 24, 2008
    To calculate your dropping resistors,
    From there, simply increase your resistance value to get lower brightness.

    I wouldn't use a V regulator, Instead, calculate the dropping resistor to the max voltage you will put out, which should be close to 15 volts at a high RPM and under heavy charge. figure 13.5 to 14 volts nominal.

    This also may interest you, I have done this very successfully in the past for switch indicators.

    Calculate R2 so the LED is full brightness (100%), then calculate R1+R2 so the LED is about 2/3rds bright. Connect the break switch so when you apply the break, the LED is 100%, power is only going threw R2. The same can be done for the turn signals. Also if a Diode must be used anywhere to prevent feedback, be sure to figure in a 0.7v drop so you don't lose brightness.

    Otherwise, as suggested, Dimming of LEDs can be achieved via Pulse Width Modulation or PWM
    Last edited: Mar 12, 2009
  5. europium

    New Member

    Oct 16, 2008
    Eric writes:
    The so-called "fusion frequency" for most people is around 50 Hz. Now, if you look steadily at a light source which is flickering at this frequency, it appears as a steady glow, as you mentioned. But if you quickly move your eyes from side to side (or whatever's comfortable), you will see the light as a trail of individual flashes. You can use this to your advantage, actually, because it draws more attention to your light than if it were glowing steadily. You do want people to notice that your turn-signal/brake lights are suddenly switched on, after all!

    Automakers are beginning to incorporate this idea into new-car tail lights. You immediately notice which cars (at night) use this technique; as soon as you turn your head or look at a different point ahead of you, you see motion - subliminally, perhaps - but motion nonetheless, while your eyes are moving. Other cars' tail lights do not draw your attention so effectively. But those on that new Beamer or Caddie ahead of you most certainly do.

    If you're on a motorcycle, you absolutely want people to notice you - especially where they might otherwise not (let's face it: at night it's a bit harder for folks to see that your iron horse really is a vintage Harley Electro Glide).

    For my part, I would not run my PWM circuit at more than 100 Hz. Probably more like 60 to 80 Hz, but not one Hertz less. You don't want your circuit to blink at other drivers - like the emergency lights on a cop car - because the cops might wish to return the favor, to your loss.