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Please excuse the way I am this question as I do not have much of an electronics background...

I am trying to upgrade the headlights in my car to HID projectors. One of the issues I'm running into is that the daytime running lights circuit runs the low beam bulb at 6V when the system is active, and then 12V when the full power low beams are needed. I am trying to find a device that I can put between the power supply and HID ballast/ bulb so that it only turns on when 12V are available on the circuit. Does such a thing exist?

Thanks in advance!

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    \$\begingroup\$ I'm confused. When the 6V DRLs are on, is the 12V line that powers the normal headlights on? Or are you saying they are the same wire? \$\endgroup\$
    – Bort
    Jul 7, 2016 at 15:07
  • \$\begingroup\$ @Bort its probably the same wire. And this belongs on Mechanics.SE. Maybe. Update with your car information, and find a schematic or wire diagram. \$\endgroup\$
    – Passerby
    Jul 7, 2016 at 15:10
  • \$\begingroup\$ Also, have you tried just disabling the DRL system? lightsout.org/disable.html \$\endgroup\$
    – Passerby
    Jul 7, 2016 at 15:11
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    \$\begingroup\$ They are the same wire. Currently, the 6V supplied by the wire light the bulb to the point where it is dim. Then 12V once the low beams are turned on. So this basically means that the current low beam is on all the time albeit at lower power at times \$\endgroup\$
    – gilcontr
    Jul 7, 2016 at 15:12
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    \$\begingroup\$ See, that's important information to have in the question. Also, a capacitor? \$\endgroup\$
    – Passerby
    Jul 7, 2016 at 15:19

3 Answers 3

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A simple solution is to use a 5V relay, and put a zener with about 7V breakdown voltage in series with the coil. Something like 1N5342 or 1N5921 (6.8V) would be appropriate. This way, the relay will see 5.2V when there is 12V, and nothing when the voltage is only 6V (or anything under the breakdown voltage). This is a very simple circuit, which doesn't rely on some unspecified behavior.

Just put the zener the right way (cathode towards the positive supply). And size the relay appropriately (coil rated for 5VDC, contact specified for at least 20VDC or more, and check max amp capabilities against your projector's requirements). Better use some automotive-grade relay, by the way.

Schematic:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Keep in mind an auto system will see up to 14.5V on the regular. Not sure how much the 6V will vary. \$\endgroup\$
    – Passerby
    Jul 7, 2016 at 21:37
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You could use a relay which requires 12V to turn on. You will then connect your light to the relay so it is only connected to power when the relay is connected to 12V

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  • \$\begingroup\$ Are you saying to give 6V to the relay? What happens when you haven't supplied enough to the relay? Wouldn't it still waste a lot of current? I also wonder if enough shock/vibration could cause the relay to close with only "6V" (Which may be 7-8V). Or am I misunderstanding? \$\endgroup\$
    – Bort
    Jul 7, 2016 at 15:03
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Hi you can use a voltage comparator like this one : http://www.digikey.com/product-detail/en/intersil/ISL55141IVZ/ISL55141IVZ-ND/1976806

Then the circuit is super simple :

On the negative input you connect the 12V, on the positive input you connect the system voltage and the output goes to your HID.

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  • \$\begingroup\$ How do you power the comparator? \$\endgroup\$
    – Scott Seidman
    Jul 7, 2016 at 15:08
  • \$\begingroup\$ @ yanik Richard So then one end would have the 6V wire and a 12V wire from another power source (maybe the battery)? And then the output to the HID? Im assuming that there will be an output as soon as the 6V wire begins carrying 12V. Just trying to figure out if I'm understanding the inputs. \$\endgroup\$
    – gilcontr
    Jul 7, 2016 at 15:47
  • \$\begingroup\$ @yanik Richard would this work? amazon.com/gp/aw/d/B014H0U5IA/… \$\endgroup\$
    – gilcontr
    Jul 7, 2016 at 18:27
  • \$\begingroup\$ The comparator is supplied by 12V so it can output the desired 12V. For the part on amazon, it looks like what you want in a more compact way and not too expensive! so it's good. \$\endgroup\$
    – Yanik Richard
    Jul 8, 2016 at 18:52

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