Question for the electrical knowledgable - RT

dduelin

dduelin

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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.
 
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.
 
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.
 
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dduelin said:
Feniex T3 strobe light head
Click to expand...

Holy moly!

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

That's some deal.
 
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.
 
SMSW said:
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.
Click to expand...
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.
 
dduelin said:
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%.
Click to expand...

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.
 
T_C said:
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.
Click to expand...

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.
 
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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.

Sent using Tapatalk
 
T_C said:
To efficiently dim them it's easier to do PWM (Pulse width modulation).

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.
Click to expand...

Yeah, that's what I was wondering with my reduced duty cycle comment earlier. Dunno if the same dimming method could be used for incandescent bulbs as well. A scope would be best to see exactly what's going on with the voltage supplied by the Canbus because if it is some low duty cycle PWM then using the diodes probably won't help much. With your multimeter in AC mode you just have to make sure its measuring voltage from the pulses and not just some small ripple voltage that its picking up off the DC circuit in general. I think with the alternator running there's always some small AC component on the DC anyway, but I've never measured it to be sure. Not home right now or I'd go out to the garage and measure the AC component on my battery to see what level to expect for that.
 
mlheck said:
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.

Sent using Tapatalk
Click to expand...

The Hexheads use incandescent taillights and they never offered an LED light top box like the 1600s have. So, no, that is not an option on 2005-2013 RTs.
 
Have you tried a zener diode? Find one that has about a 10v breakdown threshold and use that.
 
I like Vinny's idea about the Zener diode.

Also, regarding using your multimeter in AC mode, I measured the AC component on my battery and it was in the 60-70mV range with the bike idling. The reason I mentioned this before is you'll probably want to calibrate your voltage drops with the bike running, since that's the voltage the brake circuit will be seeing in normal operation with the alternator charging.
 
dduelin said:
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.
Click to expand...

Are you sure the 3.7ish volts isn't really system voltage that is pwm'd? Dropping voltage with diodes probably wouldn't work as expected. Maybe an RC to smooth it and a comparator to trigger the LED running from a constant 12 volt source. Or maybe a missing pulse detector?
 
st1300r said:
Are you sure the 3.7ish volts isn't really system voltage that is pwm'd? Dropping voltage with diodes probably wouldn't work as expected. Maybe an RC to smooth it and a comparator to trigger the LED running from a constant 12 volt source. Or maybe a missing pulse detector?
Click to expand...
You are correct. The tail light is 12 volts DC switched off 70% of the time and the brake light is 12v 100% on. I talked to a couple of guys at two of the places that sell canbus compatible brake lighting and came to the conclusion while it can be done it's better for me to just buy a plug and play controller.


vinny said:
Dave, did you get this issue worked out?
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Not yet Vinny. Riding the heck out of it though. I've put about 3500 miles on it since Thanksgiving.
 
Ok so I'm unclear as to how this is going to play out. But I have a question about my 'Wing buddy's 2016 GS1200R:

Is there a trick or anything special to adding a 12V auxiliary LED tail light to his bike? No brake light no strobe just a basic LED light in addition to the stock taillight.
 
ST Gui said:
Ok so I'm unclear as to how this is going to play out. But I have a question about my 'Wing buddy's 2016 GS1200R:

Is there a trick or anything special to adding a 12V auxiliary LED tail light to his bike? No brake light no strobe just a basic LED light in addition to the stock taillight.
Click to expand...
I'd suggest visiting the HyperLite and Skene web sites. They both make bike specific light kits and components that plug and play.
 
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