Electrical gurus: AUX brake light wiring- Mtbr.com
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  1. #1
    K_W
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    Electrical gurus: AUX brake light wiring

    Here's one for the electrical gurus...


    Problem 1: I have a small car with a Swagman XTC2 rack on the back that blocks some of the rear lights with bikes installed (Center from direct rear and opposite turns from each side), and I want to minimize chances of being rear ended.

    Solution 1: I add an auxiliary center light above the bikes to signal something is happening.

    Problem 2: I only have a "4 flat" trailer connector to work with. 4 Flats do not have a dedicated "Stop" leg. Also the taillight wiring is only accessible from the trunk interior, and do not want to run any more wires out the back trunk wall due to leak risk.

    [4 Flat is Ground (-) , Tail +12v, Left +12v, Right +12v]

    Solution 2: mount 2 normal LED trailer lights in the center, without the Tail +12v... Lights all with brake, lights 1 of 2 for turns, Light both but flashes one for
    brake and turn. Fine, and good... but not ideal due to confined space. (~18 inches)

    So... Single light...

    If I were to simply combine left and right +12v the one light would light constant with brakes, constant with brakes and turn, and flash with any one turn signal.

    BUT it would back feed +12 to the vehicle on the dead leg. VERY BAD.

    Then...

    Problem 3: I need to isolate left and right +12v so as not to back feed my electrics, and have a light in the center that alerts to brake or turn.

    Solution 3a: Diodes.

    Simple to wire, one diode on each Left and Right +12v. Light is constant with brake, constant with brake and turn, and flashing with turn alone. This works for me, but flashing center may not be legal as turn signal.

    Solution 3b: "Relay logic"

    Wire standard 4 pin relay with the Right +12v as Trigger and Left as +12v switched, or vice versa. Light is constant with brakes, flashing with any turn with brakes, and dark with turn only. I can live with that too, if legal.

    Solution 3c: Solid state AND gate
    Same as "relay logic"? Neater and simpler wiring?

    Solution 3d: Somehow get a brake only signal from 2 separate and not always constant +12v legs... How?

    - Insert solution your here....

    (Not getting a roof rack, too $$$$$$ and I WILL eventually forget to take them off before pulling in the garage.)

  2. #2
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    People use those hitch mounted aux lights all the time. Most of which use the standard 4-pin flat connector. And they typically use the outer 2 wires of the connector. So, why not just wire it up like that? That gives you tail/brake and most likely right turn flashing, IIRC.
    Or find/build a dual element setup that will plug n pray with the 4-pin flat?

  3. #3
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    Do you already have a flat 4 in the car? What type of tail lights does your car have, dedicated brake lights or dual brake/turn? Does your car have a CHMSL?

    If my car had a CHMSL or dedicated brake lamp elements, I would tap into that to drive a single LED brake light (trailer light or something else). If you only have dual purpose elements, an AND gate is probably your only easy option if you only want a brake light.

    I'm mechanical, not electrical, so I can't quote AND gate device options from memory. I'm sure most of them work at 5V or less so you will have to regulate the voltage into the device at each input then use an additional transistor to handle the "high" power 12V output to the brake light.

  4. #4
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    Possible cheap AND gate
    CD4081BE Texas Instruments | Mouser

    You will need to figure out adapting voltage levels, circuit protection/filtering, and output power.


    You could also build an AND gate with relays. If you are worried about cycle life, use transistors instead of relays.

  5. #5
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    Quote Originally Posted by RetroGrouchNJ View Post
    If my car had a CHMSL ...
    +1

    Tapping into that wire and adding a ground would be the easiest.. use some kind of 2-pin connector pair from the local auto parts store.

    For reference, I made this set to plug into a flat-4 and velcro it on my rack when I carry 4 bikes.



    --------------

    [WTB] 1987 Cannondale SM800, 20", Pink with airbrushed graphics.

  6. #6
    K_W
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    I was directed to a product by Tecniq that is the same form factor as my current brake light I used for my old car, but works with 4 flat.

    I was unaware that these trailer lights could have all functions in one single light. I had assumed all lights were high/low only.

    It is an ID bar, Brake light, and Left/Right turn in one 15 inch bar.

    USA Made Tecniq Red Hi Mount Center Brake Turn ID Bar 11 LED Light Trailer Truck | eBay

  7. #7
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    Maybe something like this could be adapted?

    https://www.etrailer.com/Hitch-Cover.../CR-007XL.html

    --------------

    [WTB] 1987 Cannondale SM800, 20", Pink with airbrushed graphics.

  8. #8
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    Over the years I've wired a few trailers and camper shells (etc) The kit J_Westy assembled would be the easiest, and in some states a requirement if the auto lights are obscured by (in this case, bicycles). I don't know if it's universal, but of those vehicles I've done this on I was able to find a connector under the driver side by the rear bumper you could separate, and a 4 wire pig tail harness (from U-haul or Pep Boys) would plug in. No issue with warranty or proper functioning of the vehicle lights.

    Use magnets, spring clamps, cloths-pins, Velcro or just screw trailer lights direct to the bike mount.

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