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Wiring lights to turn on with truck, but turn off with hi beams

Discussion in '1st Gen. Tacomas (1995-2004)' started by Colchicine, Aug 26, 2019.

  1. Aug 26, 2019 at 10:12 AM
    #1
    Colchicine

    Colchicine [OP] Well-Known Member

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    I have an LED light bar with a SPST relay and a switch, so the light can only be switched on when the truck is running. In order to pass state inspection, I have to have it wired so that it will also automatically turn off when the high beams of the headlights are on.

    In other words: I need the relay to be NO, and closed when the truck is running. I then need a second trigger, from the high beams, to then open the circuit.

    Doing searches on this topic will almost entirely give me results for people wanting accessory lights to be turned on WITH the high beams. I can figure out a way to do this with two relays, but I asked an electronics friend about this thinking they make a device that does this already. He suggested a DPDT relay, and I bought one.

    But I'm looking at it and I don't think it will do what I want. I don't see how it will accept two different triggers. Instead, it's best for reversing polarity.

    What do I need to do to have this light bar work with two different triggers?
     
  2. Aug 26, 2019 at 10:54 AM
    #2
    VE7OSR

    VE7OSR Well-Known Member

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    you want the lightbar to be on whenever the truck is running, and turn off, when the hi beams are switched on?
     
  3. Aug 26, 2019 at 10:55 AM
    #3
    Colchicine

    Colchicine [OP] Well-Known Member

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    Correct
     
  4. Aug 26, 2019 at 10:59 AM
    #4
    VE7OSR

    VE7OSR Well-Known Member

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    Easiest method would be to use the lowbeam circuit as a source of power for your switch to activate the LED light's relay. Your low beams are switched off when the hi beams are on. This would mean you would need to have at least your headlights low beam on in order to use your LED light.
     
    Colchicine [OP] likes this.
  5. Aug 26, 2019 at 11:01 AM
    #5
    VE7OSR

    VE7OSR Well-Known Member

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    can you look up your model's year electrical diagram and post a link? Can figure it out from there.
     
  6. Aug 26, 2019 at 11:01 AM
    #6
    Colchicine

    Colchicine [OP] Well-Known Member

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    I hadn't considered that! It would indeed be the simplest thing, since all I'd have to do is move the trigger wire to the low beams.

    I currently have it wired so the light bar can operate independent of the headlights. But really, I've put in too much time on this for it to be worth doing much more.
     
  7. Aug 26, 2019 at 12:30 PM
    #7
    jbrandt

    jbrandt ......LOADING

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    That's kind of a weird requirement. You'd think that, if anything, low beams would be the worst possible time to have the light bar turned on, because that's when you're likely driving around town, or trying not to blind on-coming cars.

    Of course I realize we're talking about government bureaucracy, and when has that ever made sense, lol...
     
  8. Aug 26, 2019 at 12:34 PM
    #8
    0xDEADBEEF

    0xDEADBEEF Where are we going? Why am I in this handbasket?

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    If you use the normally closed contact of the relay, and hook the relay coil to the high beam lights (instead of a switch), the lights will switch off whenever you turn on the highbeams and be on the rest of the time.
     
  9. Aug 26, 2019 at 12:43 PM
    #9
    Colchicine

    Colchicine [OP] Well-Known Member

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    I do have a switch on the dash just for that. The light bar will be off 99% of the time.

    I don't want that, and it would likely be illegal since the light is not DOT approved.
     
  10. Aug 26, 2019 at 12:48 PM
    #10
    0xDEADBEEF

    0xDEADBEEF Where are we going? Why am I in this handbasket?

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    Oooh. I feel like I misread your post. The most straightforward way is two relays in series - one driven by your switch through the NO contacts, and then the second driven by the high beam wire through the NC contacts.

    You're right that a DPDT isn't going to do what you want.
     
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  11. Aug 26, 2019 at 12:50 PM
    #11
    Colchicine

    Colchicine [OP] Well-Known Member

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    That's... disappointing. Since it come from an electrical engineer, although he hasn't been in the field for 20 years...
     
  12. Aug 26, 2019 at 12:52 PM
    #12
    0xDEADBEEF

    0xDEADBEEF Where are we going? Why am I in this handbasket?

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    He might have also misunderstood :) That said, relays are surprisingly elusive sometimes.
     
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  13. Aug 26, 2019 at 1:06 PM
    #13
    jbrandt

    jbrandt ......LOADING

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    I figured you did, I just mean as far as requiring the lights to only have the ability to be switched on with low beams. Just seems backwards...
     
  14. Aug 26, 2019 at 1:08 PM
    #14
    Colchicine

    Colchicine [OP] Well-Known Member

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    It's not ideal, but since I'm nearly a month overdue on my inspection I think I'll go the easy route for now and do the dual relays in series like
    0xDEADBEEF described.
     

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