LED taillight question

Tegger wrote:
> N8N > wrote in
> :
>
>> On Jan 12, 3:46 pm, Tegger > wrote:
>
>>> I'll bet it's cheaper to use PWM rather than put two LEDs in the
>>> cluster. Funny how Cadillac apparently wants to do things on the
>>> cheap rather than do them better. Aren't they supposed to be a
>>> high-end marque?
>>>
>>>
>>>
>> I would say less "cheap" and more "aesthetically appealing." Likely
>> if there were more LEDs, such that some were dedicated to the parking
>> light function and the remainder for the braking function, the cluster
>> would appear "grainy" when the parking light were on, as it is the
>> Caddy taillights still present, from a distance at least, the
>> appearance of a smooth solid light on the parking light function as
>> opposed to some pointillistic mess. Except, of course, the frequency
>> used for the PWM is such that at least some people can detect it.
>>
>> nate
>>
>
>
>
> Now I'm wondering how it is that BMW manages to achieve the
> "smooth solid light" look without stutter.
>
>

Again this is just a WAG but I'm guessing higher frequency PWM.

nate

--
replace "roosters" with "cox" to reply.
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Nate Nagel > wrote in
:

> Tegger wrote:
>> N8N > wrote in
>> :
>>
>>> On Jan 12, 3:46 pm, Tegger > wrote:
>>
>>>> I'll bet it's cheaper to use PWM rather than put two LEDs in the
>>>> cluster. Funny how Cadillac apparently wants to do things on the
>>>> cheap rather than do them better. Aren't they supposed to be a
>>>> high-end marque?
>>>>
>>>>
>>>>
>>> I would say less "cheap" and more "aesthetically appealing." Likely
>>> if there were more LEDs, such that some were dedicated to the parking
>>> light function and the remainder for the braking function, the cluster
>>> would appear "grainy" when the parking light were on, as it is the
>>> Caddy taillights still present, from a distance at least, the
>>> appearance of a smooth solid light on the parking light function as
>>> opposed to some pointillistic mess. Except, of course, the frequency
>>> used for the PWM is such that at least some people can detect it.
>>>
>>> nate
>>>
>>
>>
>>
>> Now I'm wondering how it is that BMW manages to achieve the
>> "smooth solid light" look without stutter.
>>
>>
>
> Again this is just a WAG but I'm guessing higher frequency PWM.
>


Must be awfully fast. I flick my eyes back and forth as fast as I can, even
on an angle relative to the horizon, and I can detect no stutter at all.
Smooth like incandescent.


--
Tegger

In article >,
Tegger > wrote:

> Nate Nagel > wrote in
> :
>
> > Tegger wrote:
> >> N8N > wrote in
> >> :
> >>
> >>> On Jan 12, 3:46 pm, Tegger > wrote:
> >>
> >>>> I'll bet it's cheaper to use PWM rather than put two LEDs in the
> >>>> cluster. Funny how Cadillac apparently wants to do things on the
> >>>> cheap rather than do them better. Aren't they supposed to be a
> >>>> high-end marque?
> >>>>
> >>>>
> >>>>
> >>> I would say less "cheap" and more "aesthetically appealing." Likely
> >>> if there were more LEDs, such that some were dedicated to the parking
> >>> light function and the remainder for the braking function, the cluster
> >>> would appear "grainy" when the parking light were on, as it is the
> >>> Caddy taillights still present, from a distance at least, the
> >>> appearance of a smooth solid light on the parking light function as
> >>> opposed to some pointillistic mess. Except, of course, the frequency
> >>> used for the PWM is such that at least some people can detect it.
> >>>
> >>> nate
> >>>
> >>
> >>
> >>
> >> Now I'm wondering how it is that BMW manages to achieve the
> >> "smooth solid light" look without stutter.
> >>
> >>
> >
> > Again this is just a WAG but I'm guessing higher frequency PWM.
> >
>
>
> Must be awfully fast. I flick my eyes back and forth as fast as I can, even
> on an angle relative to the horizon, and I can detect no stutter at all.
> Smooth like incandescent.

It doesn't need to be all that fast, really - If the frequency of the
pulse is above (roughly - varies per individual) about 80 Hz, it's
unlikely that anyone will see any flicker. Take the pulse frequency into
the 100+ Hz range, and practically nobody will see a flicker. By
comparison, typical "mid-range" computer monitors run at 75-90 Hz, which
almost totally eliminates flicker for pretty close to everybody. "Cheap"
monitors frequently run in the 60Hz range, and flicker is *VERY* evident
from them for many people. Same basic concept applies to LED lighting.

With PWM, jacking up the frequency can be as simple as putting a
different capacitor in the circuit. And since a higher frequency
*GENERALLY* wants a smaller cap, the cost isn't likely to change
significantly.

--
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or the subject of the message doesn't contain the exact text "PopperAndShadow"
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Don Bruder > wrote in
:

> In article >,
> Tegger > wrote:
>

>>
>> Must be awfully fast. I flick my eyes back and forth as fast as I
>> can, even on an angle relative to the horizon, and I can detect no
>> stutter at all. Smooth like incandescent.
>
> It doesn't need to be all that fast, really - If the frequency of the
> pulse is above (roughly - varies per individual) about 80 Hz, it's
> unlikely that anyone will see any flicker. Take the pulse frequency
> into the 100+ Hz range, and practically nobody will see a flicker. By
> comparison, typical "mid-range" computer monitors run at 75-90 Hz,
> which almost totally eliminates flicker for pretty close to everybody.
> "Cheap" monitors frequently run in the 60Hz range, and flicker is
> *VERY* evident from them for many people. Same basic concept applies
> to LED lighting.
>
> With PWM, jacking up the frequency can be as simple as putting a
> different capacitor in the circuit. And since a higher frequency
> *GENERALLY* wants a smaller cap, the cost isn't likely to change
> significantly.
>



Then why doesn't GM do this?

Don't tell me nobody in GM's vast and vaunted R&D hierarchy didn't notice
the stutter their taillights make.



--
Tegger

"Nate Nagel" > wrote in message
...
> Tegger wrote:
>> N8N > wrote in
>> :
>>> On Jan 12, 3:46 pm, Tegger > wrote:
>>
>>>> I'll bet it's cheaper to use PWM rather than put two LEDs in the
>>>> cluster. Funny how Cadillac apparently wants to do things on the
>>>> cheap rather than do them better. Aren't they supposed to be a
>>>> high-end marque?
>>>>
>>>>
>>> I would say less "cheap" and more "aesthetically appealing." Likely
>>> if there were more LEDs, such that some were dedicated to the parking
>>> light function and the remainder for the braking function, the cluster
>>> would appear "grainy" when the parking light were on, as it is the
>>> Caddy taillights still present, from a distance at least, the
>>> appearance of a smooth solid light on the parking light function as
>>> opposed to some pointillistic mess. Except, of course, the frequency
>>> used for the PWM is such that at least some people can detect it.
>>>
>>> nate
>>>
>>
>>
>>
>> Now I'm wondering how it is that BMW manages to achieve the
>> "smooth solid light" look without stutter.
>>
>>
>
> Again this is just a WAG but I'm guessing higher frequency PWM.

It is possible that some manufacturers may also use several banks of LEDs
which are switched on different phases so when one LED is off, another one
is one, so that unless you get very close, the LEDs merge together and
appear to be on all the time.

One other thing: you can't simply feed an LED with a lower voltage and
reduce its brightness: LEDs are all-or-nothing: as you reduce the voltage,
the brightness will remain the same until you get down to a threshold below
which the LED will go out altogether.

I've seen the same stuttering effect with some cats-eyes (reflective road
studs) which emit light from LEDs when a car's headlight shines on a sensor,
as opposed to simply reflecting it off a glass bead. The active cats-eyes
are used because they are brighter and can light the route further ahead -
especially useful when the road is fairly straight but with a bend in the
distance.

On Jan 11, 3:21*pm, "Steve W." > wrote:
> Tegger wrote:
> > Incandescent bulbs have a steady glow. When at night you're behind cars
> > with incandescent taillights and you make your eyes scan quickly from one
> > side to the other, the taillights will appear to leave a solid streak in
> > your field of vision.
>
> > I've noticed that there are certain makes of cars with LED taillights where
> > a quick eye-scan from side to side creates a sort of "stutter trail" of
> > widely-spaced red dots instead of a streak. This is obviously due to the
> > fact that LEDs pulse on and off in operation.
>
> > But what I don't understand is why it's only certain models that do this.
> > I've noticed the "stutter" with Cadillacs, Lincolns, Nissans and VWs, but
> > not with Hondas/Acuras and BMWs. Honda and BMW LED taillight design makes a
> > nearly solid streak on a quick eye scan.
>
> > Anybody know why the difference?
>
> On the Caddy at least what you are seeing is the effect of PWM on the
> LED. They use pulsed DC to "dim" the LED level to match a typical
> incandescent taillight level. The PWM is removed and full power restored
> when the driver hits the brakes.
>
> Probably the same thing you're seeing on the others as well.
>
> The last Prius I looked at used two different LEDs in each segment.
> Those work more like the conventional lights by turning on the brighter
> segment while turning off the dimmer segment while braking.

I thought the Prii used LEDs for braking only and the parking light
was an incandescent in a different segment. Or have you examined the
new Prius (which I haven't seen yet?)

nate

Tegger wrote:
>
> Don Bruder > wrote in
> :
>
> > In article >,
> > Tegger > wrote:
> >
>
> >>
> >> Must be awfully fast. I flick my eyes back and forth as fast as I
> >> can, even on an angle relative to the horizon, and I can detect no
> >> stutter at all. Smooth like incandescent.
> >
> > It doesn't need to be all that fast, really - If the frequency of the
> > pulse is above (roughly - varies per individual) about 80 Hz, it's
> > unlikely that anyone will see any flicker. Take the pulse frequency
> > into the 100+ Hz range, and practically nobody will see a flicker. By
> > comparison, typical "mid-range" computer monitors run at 75-90 Hz,
> > which almost totally eliminates flicker for pretty close to everybody.
> > "Cheap" monitors frequently run in the 60Hz range, and flicker is
> > *VERY* evident from them for many people. Same basic concept applies
> > to LED lighting.
> >
> > With PWM, jacking up the frequency can be as simple as putting a
> > different capacitor in the circuit. And since a higher frequency
> > *GENERALLY* wants a smaller cap, the cost isn't likely to change
> > significantly.
> >
>
> Then why doesn't GM do this?
>
> Don't tell me nobody in GM's vast and vaunted R&D hierarchy didn't notice
> the stutter their taillights make.
>
> --
> Tegger

The "stuttering" is actually a safety feature as it subtly catches your
attention far better than an ordinary non "stuttering" tail light. I've
seen the same type of operation subtly on some LED truck turn signal
modules (the grommeted type on commercial trucks) where the turn flash
cycle goes something like Off - 25Hz flicker - Solid on - Off and really
catches your attention.

In article >,
Tegger > wrote:

> Don Bruder > wrote in
> :
>
> > In article >,
> > Tegger > wrote:
> >
>
> >>
> >> Must be awfully fast. I flick my eyes back and forth as fast as I
> >> can, even on an angle relative to the horizon, and I can detect no
> >> stutter at all. Smooth like incandescent.
> >
> > It doesn't need to be all that fast, really - If the frequency of the
> > pulse is above (roughly - varies per individual) about 80 Hz, it's
> > unlikely that anyone will see any flicker. Take the pulse frequency
> > into the 100+ Hz range, and practically nobody will see a flicker. By
> > comparison, typical "mid-range" computer monitors run at 75-90 Hz,
> > which almost totally eliminates flicker for pretty close to everybody.
> > "Cheap" monitors frequently run in the 60Hz range, and flicker is
> > *VERY* evident from them for many people. Same basic concept applies
> > to LED lighting.
> >
> > With PWM, jacking up the frequency can be as simple as putting a
> > different capacitor in the circuit. And since a higher frequency
> > *GENERALLY* wants a smaller cap, the cost isn't likely to change
> > significantly.
> >
>
>
>
> Then why doesn't GM do this?

Don't ask me... I ain't got the first clue.

>
> Don't tell me nobody in GM's vast and vaunted R&D hierarchy didn't notice
> the stutter their taillights make.

Beats me. Maybe they did and decided it wasn't worth it. Maybe they just
never looked at the things outside of the circuit design program. Maybe
they like it better as-is. Maybe they <pick whatever reason you happen
to like best>.

Is it necessary for me to say "I Am Not A GM R&D Person"?

--
Don Bruder - - If your "From:" address isn't on my whitelist,
or the subject of the message doesn't contain the exact text "PopperAndShadow"
somewhere, any message sent to this address will go in the garbage without my
ever knowing it arrived. Sorry... <http://www.sonic.net/~dakidd> for more info

Tegger wrote:
> Incandescent bulbs have a steady glow. When at night you're behind cars
> with incandescent taillights and you make your eyes scan quickly from one
> side to the other, the taillights will appear to leave a solid streak in
> your field of vision.
>
> I've noticed that there are certain makes of cars with LED taillights where
> a quick eye-scan from side to side creates a sort of "stutter trail" of
> widely-spaced red dots instead of a streak. This is obviously due to the
> fact that LEDs pulse on and off in operation.
>
> But what I don't understand is why it's only certain models that do this.
> I've noticed the "stutter" with Cadillacs, Lincolns, Nissans and VWs, but
> not with Hondas/Acuras and BMWs. Honda and BMW LED taillight design makes a
> nearly solid streak on a quick eye scan.
>
> Anybody know why the difference?
>


Its the switching rate of the LED controller. Lighting engineers have
found that you can make a LED vehicle light appear much brighter by
rapidly switching the LEDs between off and an overdrive state than you
can by simply running the LEDS continuously at the maximum rated
intensity. Same is true for emergency vehicle LED light-bars. A higher
switching rate "fuses" the image better in your eye than a low switching
rate.

Tegger wrote:
> wrote in
> :
>
>
>> Yes, the LEDs are pulsed on and off to reduce the brightness and if
>> the pulsing is too slow, you can see it when you scan your eyes just
>> as you desribed.
>
>
>
> So I wasn't too far off with my first guess. Seems those LEDS are on solid
> when braking, and pulsed when showing taillights.


Actually they are ALWAYS pulsed, and brightness is controlled by varying
the duty cycle.