PWM vs Constant (Direct) Switch - Headlight vs DRL

Discussion in 'Automotive Electronics' started by Nick V, May 17, 2018.

  1. Nick V

    Thread Starter New Member

    May 17, 2018
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    I would like to know if it is possible to take a circuit that is sometimes PWM (for daytime running lights (DRL) on low beam) or Constant 12V (regular low beam) and use that state to activate a switch (or relay)? Is this even feasible / possible?

    What I am trying to do is take my DRL and relocate them to the front parking lights, saving wear on the low beam headlights, but still have normal low beam headlight operation. I understand only the basics: such as solid state relay operation, choosing proper wire sizes, and I have even used a blocking diode, but that's about it.

    I hope this hasn't been covered, but i can't find it in a search.
     
  2. dendad

    Well-Known Member

    Feb 20, 2016
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    A lot will depend on the lamp type.
    LED lamps will be ok to PWM, but you will severely decrease the lamp life on some bulbs if you run them under the design temperature.
    I run my car's normal low beam lights all the time while driving, and find the lamps last at least a couple of years so is it even worth trying to save the lamps? Just carry a spare in the car if one pops.
    For switching headlights, a relay is probably the best option.
     
  3. olphart

    Active Member

    Sep 22, 2012
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    I'd go PWM at some duty cycle for DRL brightness, then a relay to jump the duty cycle to ~=100%.
    I did this on the bike pict as avatar, but for low vs high brightness.
    Those are driving lights, picked for style, not best function.
     
    Nick V likes this.
  4. Nick V

    Thread Starter New Member

    May 17, 2018
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    I understand exactly what you are saying. My low beam headlights are LED (aftermarket) and operate as the DRL.
    They don't seem to have any issue running on the PWM, but I would prefer not to have them on 24/7.
    My thought was to re-route the PWM power to my incandescent 7443 turn signal bulbs, which are much cheaper to replace and should take the PWM without issue, then if I could route the full strength power straight through to the low beam, thus saving on run time for the LED headlight and not exposing it to PWM.

    I would like to switch a relay based on the difference between PWM and full power, but I just have no clue if that is possible.
    I am guessing it would take some more advanced electronics to accomplish this, perhaps hardware alone can't do it? I'm sure a Raspberry Pi could make it happen (I have tinkered with those too), but using it in an automotive environment under the hood isn't very practical.
     
  5. dendad

    Well-Known Member

    Feb 20, 2016
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    The LED headlight will have no trouble with the PWM at all. It is some incandescent lamps that do not like running under powered. You will not harm LEDs running PWM.
     
  6. Nick V

    Thread Starter New Member

    May 17, 2018
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    We are kind of getting sidetracked, back to my original question:
    Can a circuit be designed that can pick out the difference between a PWM signal and full power, then use this variable to redirect the power?

    I added some specifics to what I was doing in hope that it would help visualize, but we seem to be focusing on preferences instead of circuit design.
     
  7. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Are you sure that the DRL are really the low beams? Many GM cars actually use the high beams for them. The quickest way to tell how to do this is to buy a Chiltons manual for your cars make and model, and look at the wiring schematics in it. Some makes use a relay to change between DRL and normal head lights. You do know if you start using the parking lights with the PWM input they will not be very bright, don't you? And they will burn out way faster than the headlight bulbs will. There was a reason they picked the head light over the parking lights for the DRL.

    Link to Chiltons, but most car parts places or even libraries have them. https://www.chiltondiymanuals.com/?gclid=EAIaIQobChMIlYngtsKe2wIVBkCGCh3jPQGwEAAYASAAEgJxbvD_BwE
     
  8. Nick V

    Thread Starter New Member

    May 17, 2018
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  9. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Just a Quick look , it seems that the DRL control is actually in the head light modules them selves. And that they seem to be using two bulbs on each side, upper and lower. Page 23 the lower half in 2 squares with dotted lines.
     
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