dimming an led headlight

Discussion in 'FAQs' started by nachoman, Feb 7, 2007.

  1. nachoman

    nachoman Guest

    i have installed a white LED for a headlight on a narrow gauge steamer. The reason for using the LED was for longevity because replacing said bulb would be a real pain requiring removing many details.

    the problem is, the LED seems a bit too bright for what I would want. Any thoughts on how I could make it dimmer?

  2. MadHatter

    MadHatter Charging at full tilt.

    Just put a resistor on the positive side of the LED- maybe 1K Ohm.
  3. Squidbait

    Squidbait Recovering ALCO-holic

    Perhaps a yellow LED might not look so bright - or is the white one more-or-less permanent now?
  4. nachoman

    nachoman Guest

    the white is more or less permanent now. I have played around with several resistor values, but nothing seems to dim it much. It is either very bright or slightly less bright or off.

  5. MasonJar

    MasonJar It's not rocket surgery

    One of the guys at the local club has had success dipping the white LEDs in yellow paint to dim them, and give them a more appropriate colour for steamers. He simply dips the LED straight into the paint jar - without stirring the paint first. This "thinned" coat seems to do the trick.

  6. Squidbait

    Squidbait Recovering ALCO-holic

    That's a problem with LEDs, they require a minimum voltage to light (~5V), and once they're lit, increasing the voltage doesn't really increase the output.

    I'd say if you can't paint the LED, see if you can get a transparent paint to wash the lens with (I'm assuming it's in a headlight) and see if that dims/colours it a bit.
  7. baldwinjl

    baldwinjl Member

    LEDs are basically on/off devices. You could control the current, and hence decrease the brightness by tweaking the resistor, but it would take a fairly specific value, and it might change over time and/or temperature, so it really isn't a good plan. The other way to dim it (apparently) electrically is to flash it on and off (quicker than you can see), but that would require some electronics. So you are left with the mechanical means mentioned above.
  8. pgandw

    pgandw Active Member


    Inserting a small 1/4 watt potentiometer (tune with a plastic screwdriver type) in series with the LED will probably do the trick. I would try for a pot about 3X the resistance of the existing resistor. If circuit currently has a 1K resistor (typically 500 ohms-1K), add a 3K pot. Too much resistance in the pot (5K ohms) is better than too little (1K ohms) for experimentation with dimming.

    An LED typically is at full brightness at around 20ma (and should not exceed 20ma current), and begins to light around 5-10ma. There isn't all that much range for adjustment, but it can be done.

    Once you have established the additional resistance to dim the LED, you can add a fixed 1/4 watt resistor instead of the pot if you prefer, and the required resistance is commercially available.

    hope this helps
  9. nachoman

    nachoman Guest

    hey, thanks for the tips everyone. I've got drawers full of resistors to play around with. If I can't do anything with those, I will try coloring the bulb.

  10. kf4jqd

    kf4jqd Active Member

    Madhatter is right. Use a resistor. Here's an idea for finding out the value that would work for you. Use a varaible resistor. Ajust until you have the proper light you want. Then use an OHM meter to check the resistance. Once you got it. Then go to Radio Shack and buy that fixed value. Simple :p


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