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Car LED tail lights strobe rate too slow

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Spehro Pefhany

Jan 1, 1970
0
On Fri, 30 Jun 2006 13:46:03 +0000 (UTC), the renowned
See if it is actually that bad. The amount of energy the phosphor over
an LED chip stores is so low and it emits at such a high rate (when the
LED is glowing at least) that I suspect it won't glow much for more than a
few microseconds after the LED turns off.

Don, do you have any actual measurements on this?


Best regards,
Spehro Pefhany
 
P

Peter Pan

Jan 1, 1970
0
One point that hasn't been made is how the current can be modulated. In
a PWM you can do it through different ways. You can have a fixed pulse
width and modulate the off time (or the inverse). You can can have a
fixed frequency and modulate the ratio between on and off.

Further you can work at different current levels, specially if you are
working at short pulse widths. Since you can work at a much higher
current level.

My guess it that maybe the switching frequency isn't to high, but when
you have a long off period it could appear as a low frequency strobe.
Does any one have a scope?
 
C

Chris Jones

Jan 1, 1970
0
Mark said:
Anybody else notice that the strobe rate bothers them on the newer cars
with LED taillights.

At night, when the tail lights are dimmed, (brakes not on) and you
look at them and blink or turn your head, the after image is very
strange, you see dashes of light. I find it very annoying and
distracting and I hope that this problem is recognized and corrected
before every car on the road is like this.

Mark

I think it's a deliberate choice to make it like that. If they ran it at a
higher frequency, you wouldn't notice that the car has "New LED Taillights"
and the marketing value would be lost.

Chris
 
D

Douglas G. Cummins

Jan 1, 1970
0
I do, and I have tested hundreds of automotive LED rear combination
lamps. If the LEDs were modulated, it was by PWM (pulse width
modulation) with a constant peak intensity at a frequency usually
greater than 200 Hz (can't remember them all, but it seems that they
were never near the flicker frequency range). The pulse width depended
on the function being activated (Stop versus Tail) and, for the more
advanced ones, the input voltage so that no matter the input voltage the
output light was the same. This is very important in trucking
applications where the voltage at the cab may be around 13.5V but only
12.0V at the end of a couple of linked trailers.
 
D

Don Klipstein

Jan 1, 1970
0
Don said:
Plenty of times. As I said "for the most part". Phosphor persistence has
a lot to do with it. And of course now the pixel data is latched anyway
on the LCDs and whatnot.

I have rolled my eyes and looked at plenty of CRT monitors and TV sets,
and consistently see quite a lack of phosphor persistence - downright
dropping something like 90% well within a millisecond.

(about to shoot photos with my digital camera with a fast exposure time)

(Shoot a 1/250 second exposure of my CRT color TV and my Dell Trinitron
monitor catching roughly the middle of the screen being scanned)

Results are at until July 15 2006:

http://www.misty.com/~don/pper1.jpg (my monitor in 800*600
non-interlaced mode, vertical scan rate 60 Hz or close to that)

http://www.misty.com/~don/pper2.jpg (my TV on an NTSC broadcast signal,
during a Toyota commercial)

Both show all three color phosphors largely coming on and dying out
within a millisecond, with the green and blue phosphors doing so much
faster and red having perhaps so much as about .4-.5 millisecond halflife.
Overall brightness fades a majority of the way as fast as just a
couple lines get scanned.

- Don Klipstein ([email protected])
 
C

Clive Mitchell

Jan 1, 1970
0
Don Klipstein said:
I have rolled my eyes and looked at plenty of CRT monitors and TV
sets, and consistently see quite a lack of phosphor persistence -
downright dropping something like 90% well within a millisecond.

Try looking at a traditional CRT monitor with it turned upside down or
looking backwards with your head between you legs. (Well _I_ can do
it...)

We've adjusted to ignore the top to bottom scan flicker, but not in the
opposite direction.
 
M

Mark

Jan 1, 1970
0
Don said:
I have rolled my eyes and looked at plenty of CRT monitors and TV sets,
and consistently see quite a lack of phosphor persistence - downright
dropping something like 90% well within a millisecond.

(about to shoot photos with my digital camera with a fast exposure time)

(Shoot a 1/250 second exposure of my CRT color TV and my Dell Trinitron
monitor catching roughly the middle of the screen being scanned)

Results are at until July 15 2006:

http://www.misty.com/~don/pper1.jpg (my monitor in 800*600
non-interlaced mode, vertical scan rate 60 Hz or close to that)

http://www.misty.com/~don/pper2.jpg (my TV on an NTSC broadcast signal,
during a Toyota commercial)

Both show all three color phosphors largely coming on and dying out
within a millisecond, with the green and blue phosphors doing so much
faster and red having perhaps so much as about .4-.5 millisecond halflife.
Overall brightness fades a majority of the way as fast as just a
couple lines get scanned.

- Don Klipstein ([email protected])

in an NTSC display..the interlaced scanning reduces the visability of
the flicker ..
even so, if you look at a TV with your periphrial vision, it is easy to
see the flicker.

Mark
 
M

Mike

Jan 1, 1970
0
Clive Mitchell said:
Agreed. The new buses with LED lights here in the UK have a fairly low
PWM rate.

On West Midlands Travel buses, they also seem to have one LED within the
cluster that stays on longer than the rest -- just noticeable.

What's the deal with that?

[And yes, they do flicker when dimmed]
 
C

Clive Mitchell

Jan 1, 1970
0
Mike <[email protected]> said:
On West Midlands Travel buses, they also seem to have one LED within
the cluster that stays on longer than the rest -- just noticeable.

What's the deal with that?

If the cluster of LEDs doesn't have an exact multiple of the series
circuits number, then the odd LED/s out will have their own circuit with
a higher value resistor, but will also have a lower forward voltage than
the rest of the circuits. As the power supply to the light runs down
after disconnection the last remaining light lit will be that circuit.
Because there's only a single circuit lit at that point it may trail off
very slowly as the capacitor discharges.
 
M

Mike

Jan 1, 1970
0
If the cluster of LEDs doesn't have an exact multiple of the series
circuits number, then the odd LED/s out will have their own circuit

That makes sense, but it's definitely just one LED. Sounds like it would've
been simpler to just use one less LED :)

Off by one errors, don't you love 'em?
 
S

Simoc

Jan 1, 1970
0
Don said:
Old digital displays, however, often had each die receiving a couple mA
average while the die had higher efficiency at higher instantaneous
current. That was a secondary benefit of "strobing" digital displays -

Do you really mean that the multiplexing frequency was intentionally
"too low" in order to make them more efficient? I've always thought
that it would be just a "bug" that "visibly low" freq is used, just
like in the case of these flickering PWM'd taillights...
the primary purpose of strobing them was to have a multiplexing system to
reduce the number of wires going to the display.

Isn't multiplexing used anymore? Or did you mean that higher freq is
used nowadays? (you said "old digital displays"). At least my friend
has a Panasonic microwave with a LED-7seg display, bought in 2001, and
its display has very visible multiplexing flicker.
 
D

Don Klipstein

Jan 1, 1970
0
Do you really mean that the multiplexing frequency was intentionally
"too low" in order to make them more efficient? I've always thought
that it would be just a "bug" that "visibly low" freq is used, just
like in the case of these flickering PWM'd taillights...

No, I was talking about why digital displays were strobed rather than
having each LED die being fed steady DC. I made no mention of what
frequency to strobe them at.
Isn't multiplexing used anymore? Or did you mean that higher freq is
used nowadays? (you said "old digital displays"). At least my friend
has a Panasonic microwave with a LED-7seg display, bought in 2001, and
its display has very visible multiplexing flicker.

Most digital displays are now LCD and I don't know how those work and
they may be multiplexed. In any case, LCD responds very slowly and I
suspect it will smooth out any multiplexing if multiplexing is used.

Meanwhile, unlike the old digital displays, modern LEDs in tail/brake
lamps (used as brake lamps), traffic signals and flashlights will
generally have little or no or downright negative gain in efficiency if
they are pulsed with the same average current or the same average power at
which they are operated steadily. Many but not all taillights use PWM
because the efficiency of the red LEDs there will be either reduced or
unreliable (varying excessively from one production run to another) with
steady low current.

- Don Klipstein ([email protected])
 
C

Clive Mitchell

Jan 1, 1970
0
Simoc said:
Isn't multiplexing used anymore? Or did you mean that higher freq is
used nowadays? (you said "old digital displays"). At least my friend
has a Panasonic microwave with a LED-7seg display, bought in 2001, and
its display has very visible multiplexing flicker.

Multiplexing flicker can sometimes increase dramatically in old
equipment due to the increased ESR of the PSU capacitors creating enough
ripple to cause a beat effect in conjunction with the multiplexing
frequency.

Ask if it's always flickered so strongly.

Then again, the older processors ran slower and multiplexing speed was
compromised by other processing tasks.
 
M

Michael A. Terrell

Jan 1, 1970
0
Paul Hovnanian P.E. said:
Hmm. Subliminal messages in the taillights?


Like, "Back off, idiot"? ;-)


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
M

Michael A. Terrell

Jan 1, 1970
0
Jim said:
[snip]
10 years ago, the engineers of BMW used modern technology and
significant effort to serve the aggressive drivers of their cars
to annoy the drivers of any other manufacturers car,
by installing xenon front lights with damn increased light output,
ignoring that on wet streets light is shed directly into the eyes
of the driver of an oncoming car, because the water on the streets
acts like a perfect mirror.
[snip]

And the drivers are such rude shitheads that they don't dim for
approaching vehicles.

A friend countered by installing aircraft landing lights in his grill.
He can only fire them up for 15 seconds at a time because the cables
start smoking ;-)

...Jim Thompson


Fair Radio used to have new surplus strobe tubes made for aircraft
wing tip markers. Think what a dozen of them would do, mounted in the
grill.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
G

Goshawk1

Jan 1, 1970
0
Jim said:
[snip]
10 years ago, the engineers of BMW used modern technology and
significant effort to serve the aggressive drivers of their cars
to annoy the drivers of any other manufacturers car,
by installing xenon front lights with damn increased light output,
ignoring that on wet streets light is shed directly into the eyes
of the driver of an oncoming car, because the water on the streets
acts like a perfect mirror.
[snip]

And the drivers are such rude shitheads that they don't dim for
approaching vehicles.

A friend countered by installing aircraft landing lights in his grill.
He can only fire them up for 15 seconds at a time because the cables
start smoking ;-)

...Jim Thompson
 
G

Goshawk1

Jan 1, 1970
0
Jim said:
[snip]
10 years ago, the engineers of BMW used modern technology and
significant effort to serve the aggressive drivers of their cars
to annoy the drivers of any other manufacturers car,
by installing xenon front lights with damn increased light output,
ignoring that on wet streets light is shed directly into the eyes
of the driver of an oncoming car, because the water on the streets
acts like a perfect mirror.
[snip]

And the drivers are such rude shitheads that they don't dim for
approaching vehicles.

A friend countered by installing aircraft landing lights in his grill.
He can only fire them up for 15 seconds at a time because the cables
start smoking ;-)

...Jim Thompson
 
G

Goshawk1

Jan 1, 1970
0
Jim said:
[snip]
10 years ago, the engineers of BMW used modern technology and
significant effort to serve the aggressive drivers of their cars
to annoy the drivers of any other manufacturers car,
by installing xenon front lights with damn increased light output,
ignoring that on wet streets light is shed directly into the eyes
of the driver of an oncoming car, because the water on the streets
acts like a perfect mirror.
[snip]

And the drivers are such rude shitheads that they don't dim for
approaching vehicles.

A friend countered by installing aircraft landing lights in his grill.
He can only fire them up for 15 seconds at a time because the cables
start smoking ;-)

...Jim Thompson
 
G

Goshawk1

Jan 1, 1970
0
Jim said:
[snip]
10 years ago, the engineers of BMW used modern technology and
significant effort to serve the aggressive drivers of their cars
to annoy the drivers of any other manufacturers car,
by installing xenon front lights with damn increased light output,
ignoring that on wet streets light is shed directly into the eyes
of the driver of an oncoming car, because the water on the streets
acts like a perfect mirror.
[snip]

And the drivers are such rude shitheads that they don't dim for
approaching vehicles.

A friend countered by installing aircraft landing lights in his grill.
He can only fire them up for 15 seconds at a time because the cables
start smoking ;-)

...Jim Thompson
 
J

Jim Thompson

Jan 1, 1970
0
Jim said:
[snip]
10 years ago, the engineers of BMW used modern technology and
significant effort to serve the aggressive drivers of their cars
to annoy the drivers of any other manufacturers car,
by installing xenon front lights with damn increased light output,
ignoring that on wet streets light is shed directly into the eyes
of the driver of an oncoming car, because the water on the streets
acts like a perfect mirror.
[snip]

And the drivers are such rude shitheads that they don't dim for
approaching vehicles.

A friend countered by installing aircraft landing lights in his grill.
He can only fire them up for 15 seconds at a time because the cables
start smoking ;-)

...Jim Thompson
[snip]

Goshawk1, Are you having a stuttering problem, that's four identical
non-content "retorts" now, or are you just a disgruntled Beamer ?:)

...Jim Thompson
 
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