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How to dim DRLs at night

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  • How to dim DRLs at night

    How can I dim my DRLs at night automatically when the headlights turn on. I have a 12v regulator for each DRLs that switch off when signal lights turn on.
    I was thinking of adding another 12v regulator to power both sides when headlights are on but would need to cut current to the regulator. How would I do that???
    Or is there an easier way?

    12v regulator.jpg
    Attached Files

  • #2
    How about using a PWM (activated by the headlights) connected to the Disable pin of the regulator?

    There are a few kits available pretty cheap, just have to make sure they have a negative output. You can also easily make your own from components readily available at Radioshack or equivalent electronics store:
    • 555 timer
    • 2x 0.01uF capacitors
    • 47uF capacitor (for decoupling)
    • couple resistors
    • diode


    Check out Doctronics' extended duty cycle schematic (4.1): http://www.doctronics.co.uk/555.htm.
    Experiment with different resistor values to get the appropriate dimming level.

    You may need a resistor (4.7k maybe) on the 555 output so as to not interfere with the relay you have.

    Also, the diode you have from the "headlight negative relay" to pin 4 of the regulator may need to be reversed since the electricity is flowing from pin 4 to ground.
    Last edited by sprengBoard; February 19th, 2015, 10:55 AM.

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    • #3
      Not sure if I understand the 555 timer. All I want to do is dim the DRLs at night. How would I do this to the array. There's 50 leds in the array each side.
      What would I need to reduce the power of the 12v regulator?

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      • #4
        Originally posted by Madme89 View Post
        Not sure if I understand the 555 timer. All I want to do is dim the DRLs at night. How would I do this to the array. There's 50 leds in the array each side.
        What would I need to reduce the power of the 12v regulator?
        What the 555 timer does is act as a PWM signal to pulse the 12 V to dim the LED's. LED's require the full voltage to operate so you need to pulse the full voltage on and off really quickly. This is unlike regular halogen and incandescent bulbs that can operate on an actual reduced voltage like 9 V instead of 12.

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        • #5
          Ok so I would use that 555 timer to dim the LEDs. I just don't know how i would build it.

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          • #6
            If you know what the max current load of the LED's and their running voltage, LED supply.com has some great LED current drivers with dim options on them like these: http://www.ledsupply.com/buckblock-c...nt-led-drivers

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            • #7
              You can easily do this with a relay.
              ice-cube-iso-relay.jpg

              Wire terminal 30 to your fused 12v constant.
              ...terminal 85 to a good ground location.
              ...terminal 86 is your trigger, your headlight circuit
              ...terminal 87a is on when the trigger is off. Terminal 87 is off. This can be your brighter DRL circuit.
              ...terminal 87 will turn on once your trigger is on. Terminal 87a is now off. This can be your dim DRL circuit.

              You may want to use some diodes in the appropriate places.

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              • #8
                Originally posted by axipher View Post
                If you know what the max current load of the LED's and their running voltage, LED supply.com has some great LED current drivers with dim options on them like these: http://www.ledsupply.com/buckblock-c...nt-led-drivers
                Would a potentiometer work?
                I don't know much about LED drivers.
                I'm learning as I go.
                How does a LED driver work and how do I wire it?

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                • #9
                  Originally posted by Madme89 View Post
                  Ok so I would use that 555 timer to dim the LEDs. I just don't know how i would build it.
                  I'll try to elaborate as much as possible:

                  The 555 timer generates pulses of alternating bias (High/Low or Positive/Negative). Controlling the frequency and the ratio of high to low is known as pulse width modulation (you're modulating the width of the pulse). Let's look at the schematic I referred to on Doctronics' website:

                  doctronics_555_extended.PNG

                  The value of R1 and R2 control the ratio of high to low (known as duty cycle) and C controls the frequency of the pulses.

                  This is a very common method to "dim" LEDs. I put dim in quotes because it doesn't actually make the LEDs illuminate at a lesser intensity, it just flashes them so fast that the human eye averages the on and off pulses and tells your brain that they're dimmer. Connecting a PWM circuit (the pulse output on the above schematic) to the disable pin of your regulator simply pulses the regulator on/off which in turn pulses all the LEDs connected to it, effectively making your array appear dimmer.

                  You would build it following the above schematic. For C I'd use 0.01uF (10nF) which will guarantee a frequency high enough that you won't see any flashing/pulsing. R1 and R2 depend on how dim you want your LEDs to appear. Instead of the 680 Ohm resistor and the LED in the schematic you'd connect the pulse output (pin 3 of the 555) to the disable pins of your regulators. Only one PWM is needed since both sides will always dim together (not independently, like with turn signals). The "+3-15 V" input in the schematic would be from your headlights (positive input).

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                  • #10
                    For simplicity, this would be my approach:

                    1) Buy this style of PWM http://www.ebay.ca/itm/12V-24V-8A-Di...-/321457983256
                    2) Wire it into your existing 12V reg. The only thing you'll need to change to your existing circuit is remove the output (47uf) cap, and add a 2K resistor from the VREG pin 4 to battery +.

                    The PWM +input should be wired to your parking lights.

                    pwm-test-circuit_update.jpg

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                    • #11
                      What's the function of the added 2K resistor?

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                      • #12
                        Originally posted by yellow_cake View Post
                        For simplicity, this would be my approach:

                        1) Buy this style of PWM http://www.ebay.ca/itm/12V-24V-8A-Di...-/321457983256
                        2) Wire it into your existing 12V reg. The only thing you'll need to change to your existing circuit is remove the output (47uf) cap, and add a 2K resistor from the VREG pin 4 to battery +.

                        The PWM +input should be wired to your parking lights.

                        [ATTACH=CONFIG]40674[/ATTACH]
                        What kind of 2K resistor? I looked on Ebay for it and there's lots of diferent kind. And what is it's function?
                        And what is the R1 in the photo? is that a diode?

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                        • #13
                          R1 is the resistor for the series of LEDs behind it.

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                          • #14
                            The 2k Ohm resistor is a pull-up resistor for the disable pin of the regulator. It makes sure the regulator pulses accurately (it bring the disable pin high when the PWM low pulse is finished).

                            Any 1/4W resistor would be fine.

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                            • #15
                              ^^Yep.

                              The pull-up resistor value should be very flexible, I think any value from 1K - 10K will work.

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