Fixing the control scheme for an aftermarket headlight assembly

The headlight enclosures on [Bill Porter's] 2004 Passat had yellowed with age and were not outputting the kind of light they should. He decided to replace them with some aftermarket modules that also incorporated LED strips. When they arrived he was surprised at how easy there were to drop into place. But when testing he was certainly not satisfied with how they worked. The day-driving mode used the HID bulbs at full power, where the factory assembly had dimmed them during the day. He set out to alter the electronics to work as he prefers.

Always the mad scientist, [Bill] started off by making a truth table showing how the lights reacted to the various states of the ignition and headlight switches. What he came up with is an AND gate built from a relay and diode. It allows him to have the LEDs on as the running lights (without the HIDs on at all), and leaves the rest of the functionality unaffected.


  1. Chad B says:

    I would recommend a Euro switch :-)

  2. TheStig says:

    @ChadB: Ah, but that’s the easy way out.

  3. Chris says:


  4. fartface says:

    Looks like Bill will be buying a lot of HID bulbs and ballasts over the years….

    If the HID system is not designed for the Dimming, it will damage the system. Most specifically the low grade dog food that is the china aftermarket HID junk.

    • Bill says:

      I think you missed the result of the hack, now the HIDs never see the dimmed power source of the Daytime running lights, the relay keeps them disconnected and off.

      • ScottInNH says:

        @bill – Actually, you were punked into responding to our resident troll. @fartface -never- reads the articles he comments on, and only makes negative posts. I think he came to HAD because no one was noticing him on Slashdot.

  5. Eric says:

    That yellowing problem is easy and cheap to fix. Just wipe them with acetone (nail polish remover) and rinse well to protect the paint around the lights.

  6. CG says:

    Pretty cool. Usually on this site, I expect to see someone have to modify some firmware, download a bunch of crazy drivers, construct an arduino sheild, blah blah blah. Nice to see a hack on this level for a change.

  7. steve says:

    nice. Keep up the good work!

  8. Andrew says:

    factoid: fiancé is the guy, financée is the girl.

  9. Joe says:

    Well shit I snipped the yellow wires to the fuse box and grounded it on my 99 Silverado…now my lows stay on with my highs :)

  10. Black Soap says:

    Let us hope he adjusts the headlights correctly. The vast majority of people on the road with aftermarket headlight modifications end up being hazards, as they are poorly aimed, and end up blinding other drivers (not a good thing, it turns out.) Not to mention the common misconceptions about headlight color/ visual clarity.

    As for the electronics, it looks like he did a good job!

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s


Get every new post delivered to your Inbox.

Join 96,532 other followers