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Thread: Question for the electrical knowledgable - RT

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    Be Thou My Vision dduelin's Avatar
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    Question for the electrical knowledgable - RT

    I'm not sure this is the sub forum to post in as it pertains to my BMW R1200RT. I'm sure mods will move it to where it needs to be,

    I bought a Feniex T3 strobe light head to augment the brake lighting on the RT. My ST and NC have a similar strobe head installed in the brake light circuit. The BMW has a Canbus computer controlled electrical system that requires a different hook up. The single filament brake light bulbs double as running lights - as running lights the computer sends the bulbs about 30% power or about 3.7 volts and they glow appropriately as running lights. When the brakes are applied the computer sends the full 12.6 to 13.6 volts to the bulbs and they glow at 100%. When I installed the Feniex in the brake light circuit the strobe flashed constantly at the power of the bulb which is 30% all the time and then goes to 100% when the brake is applied.

    I'm thinking there must be a way to wire into the brake lights and still prevent the strobe from coming on until the brakes are applied. Can I install a device like a diode or resistor that stops 3.7 volts from passing through the wire to the Feniex but allows 12+ volts to flow when the brakes are applied. I thought about using the brake light to trigger a relay but I read that automotive-type 30 amp relays will remain energized with as little as 1 or 2 volts so would not release when the brakes are released.


    Dave

    Honda ST1300
    Honda NC700XD
    BMW R1200RT

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    STOC #687 dwalby's Avatar
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    Re: Question for the electrical knowledgable

    Given what you've said so far, you could use a string of diodes in series to drop the 3.7 volt mode down to zero or near zero. Each diode provides about 0.7 volts of drop while conducting. If you use the diodes to control the voltage applied to a relay trigger, dropping that down to near zero so the relay won't energize without the brakes applied, you should be good. Then when the brakes are applied the relay trigger will have about 10v, which should easily trigger the relay and turn your brake strobe on.

    I thought about it as long as it took to type this, so there may be something I overlooked, but I think the basic concept will work.

    The other question is if your premise that the computer drops the voltage while in running light mode is correct, as opposed to using some other method like lowering the duty cycle. Not familiar with how that system works, so you need to verify your original assumption too.

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    spiderman302's Avatar
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    Re: Question for the electrical knowledgable

    I would try at least 7 diodes, mouser still has 1n4003 diodes (200 volt, 1 amp) that would run your relay.

    https://www.mouser.com/productdetail...69AR2%2FCIU%3D

    It is most likely dc otherwise they would create an EMI problem.... The question is will be if adding the relay causes a Canbus error.....
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    240Robert ST Gui's Avatar
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    Re: Question for the electrical knowledgable - RT

    What about taking the power off of a brake light switch itself. Tap into the side that becomes energized with the switch is closed. Wouldn't that get 12V when closed and an nada when open? Use a diode to isolate the branch (and a relay?) and feed that to the strobe.

    I know 0 about CanBus. If the computerized voltage conversion takes place downstream from that side of the switch then never mind. But a DMM should tell you what you need to know about a/the brake light switch.
    Last edited by ST Gui; 01-12-2018 at 07:14 AM.
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    240Robert ST Gui's Avatar
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    Re: Question for the electrical knowledgable - RT

    Quote Originally Posted by dduelin
    Feniex T3 strobe light head
    Holy moly!

    3x4W LEDs
    AL base
    13 patterns
    5yr warranty
    Brakelight function 70% of
    rated brightness
    $29!

    That's some deal.

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    SMSW's Avatar
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    Re: Question for the electrical knowledgable - RT

    I think you might not be the first person doing this. Why not head over to a BMW forum and ask the same question? Accessories can be added to a canbus system so there must be a readily available interface module.

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    Be Thou My Vision
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    Re: Question for the electrical knowledgable - RT

    Quote Originally Posted by SMSW View Post
    I think you might not be the first person doing this. Why not head over to a BMW forum and ask the same question? Accessories can be added to a canbus system so there must be a readily available interface module.
    Yes, I've been reading those forums to cross-eyed.

    Of course there is a way to plug and play into a BMW but first you have to pay the roundel tax. Skene is about $150 to do this, Hyperlights is only about $100. Neither use a strobe though so add the cost of a Feniex or Whelen to get what I want.

    I'm playing to the DIY team here of knowledgeable forum members.

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    Site Supporter T_C's Avatar
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    Re: Question for the electrical knowledgable - RT

    Quote Originally Posted by dduelin View Post
    about 30% power or about 3.7 volts and they glow appropriately as running lights. When the brakes are applied the computer sends the full 12.6 to 13.6 volts to the bulbs and they glow at 100%.
    Are they filaments or LED?Just wonderinig if it is analog thinking in a digital world.
    LED's do not respond with variable brightness very well to voltage changes, they work on the current. To efficiently dim them it's easier to do PWM (Pulse width modulation). Essentially turning the light on/off faster then the human eye can detect. You could do current control, but it's a little more work to regulate it. Doing the PWM at a relatively high frequency and the the smoothing function in most multi-meters will interpret it as a reduced voltage.

    If you have a frequency counter on your meter, try that setting and see what you get at the lights when in marker light mode. Or... try setting your meter to A/C and measure.
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    Re: Question for the electrical knowledgable - RT

    Quote Originally Posted by T_C View Post
    Are they filaments or LED?Just wonderinig if it is analog thinking in a digital world.
    LED's do not respond with variable brightness very well to voltage changes, they work on the current. To efficiently dim them it's easier to do PWM (Pulse width modulation). Essentially turning the light on/off faster then the human eye can detect. You could do current control, but it's a little more work to regulate it. Doing the PWM at a relatively high frequency and the the smoothing function in most multi-meters will interpret it as a reduced voltage.

    If you have a frequency counter on your meter, try that setting and see what you get at the lights when in marker light mode. Or... try setting your meter to A/C and measure.
    OK thanks. The OEM taillight bulbs are incandescent. The Feniex uses 3 four watt LEDs. The voltage of <4VDC >12VDC at the taillight is what my multimeter did read so I'll try AC.

    Still, how does a store bought controller handle plugging into the harness and regulating LEDs in the Feniex? I don't necessarily need theory to accomplish the task.
    Last edited by dduelin; 01-12-2018 at 08:19 AM.

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    mlheck mlheck's Avatar
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    Re: Question for the electrical knowledgable - RT

    Is there a top box plug available? This may have a dedicated brake light wire for a top box brake light. This is what I used to run the Whelen on my K1600.

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