On 5 Jan 2005 17:07:51 -0800, "Cecil Moore" wrote:

|Slightly off topic - I've seen a couple of new automobiles with
|time-multiplexed LED taillights. What would be the advantage of such a
|design in an automobile?

What the hell is "time-multiplexed"?

Cecil Moore wrote:
Wes Stewart wrote:

On 5 Jan 2005 17:07:51 -0800, "Cecil Moore" wrote:
|Slightly off topic - I've seen a couple of new automobiles with
|time-multiplexed LED taillights. What would be the advantage of such a
|design in an automobile?

What the hell is "time-multiplexed"?


In this case, on half the time and off half the time at about
a 100 Hz rate. Maybe square-wave power? Maybe pulse-width
modulation, different from night to day? I thought it was a
simple question to which someone had a simple answer.

"Time-multiplexed" means that they're turned on in sequence, without all
being on at once. That would be as opposed to simply pulse modulated all
at once.

My guess is that they're pulse modulated to provide easy and efficient
control of the brightness, and possibly compensation for voltage and/or
temperature. If they're time-multiplexed as well, that would even out
the total current drain rather than its being in the form of big current
pulses.

Are they really time-multiplexed, or are they all pulsed at once?

Roy Lewallen, W7EL

Wes Stewart wrote:

On 5 Jan 2005 17:07:51 -0800, "Cecil Moore" wrote:
|Slightly off topic - I've seen a couple of new automobiles with
|time-multiplexed LED taillights. What would be the advantage of such a
|design in an automobile?

What the hell is "time-multiplexed"?

In this case, on half the time and off half the time at about
a 100 Hz rate. Maybe square-wave power? Maybe pulse-width
modulation, different from night to day? I thought it was a
simple question to which someone had a simple answer.
--
73, Cecil http://www.qsl.net/w5dxp


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Roy Lewallen wrote:
"Time-multiplexed" means that they're turned on in sequence, without all
being on at once. That would be as opposed to simply pulse modulated all
at once.

The most trivial case of binary time multiplexing would be a
single LED being turned on for a logic 'one' and off for a logic
'zero'. That "binary time multiplexing" concept is a carry over
from 40 years as a digital engineer, 13 of them at Intel.

Are they really time-multiplexed, or are they all pulsed at once?

It appeared that they were all pulsed at once. Perhaps I should
have used the word "pulsed" to avoid confusion in the RF analog
(non-digital) world.
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil Moore wrote:
Roy Lewallen wrote:

"Time-multiplexed" means that they're turned on in sequence, without
all being on at once. That would be as opposed to simply pulse
modulated all at once.


The most trivial case of binary time multiplexing would be a
single LED being turned on for a logic 'one' and off for a logic
'zero'. That "binary time multiplexing" concept is a carry over
from 40 years as a digital engineer, 13 of them at Intel.

Are they really time-multiplexed, or are they all pulsed at once?


It appeared that they were all pulsed at once. Perhaps I should
have used the word "pulsed" to avoid confusion in the RF analog
(non-digital) world.

Yeah, digicats have their own language. In analog parlance, multiplexing
basically means sharing. As in telephony, where you share one line among
many users by connecting each to the line for a short time in sequence
(time division multiplexing or TDMA - time division multiple access),
breaking the spectrum into pieces and giving each user his own channel
(frequency domain multiplexing), or more complex methods such as spread
spectrum code domain multiplexing, or CDMA (code division multiple
access). There's no sharing when there's only one user, channel, or LED
involved, so no multiplexing. Except, I guess, for digital types who can
share an LED between a 1 and a 0.

Just out of curiosity, what does your revered IEEE Dictionary have to
say about it?

Roy Lewallen, W7EL

Cecil,

Here's the deal.

I will assume the low intensity tail light and the high intensity brake
light share the same LEDs. In a typical system there will be 10's to
even 100's of LEDs. The required light output from the tail light
function and brake light function is quite different. I have seen
numbers from ranging from 7:1 up to 15:1. Let's say that the brake light
requires 10X the output required from the tail light.

Contrary to what was posted earlier in this thread, the light output
from LEDs is roughly linear with forward drive current over several
orders of magnitude. This is true up to nearly the point where the
device explodes from overheating.

So why not just change the DC current to get the varying output required?

The problem is device matching. As you well know all semiconductor
devices are subject to significant performance variation depending on
exact process conditions, materials, phase of the moon, or whatever.
LEDs are typically specified at one nominal current, with a nominal
light output.

When used at the nominal conditions a batch of LEDs can be selected to
match light output performance so that no obvious visual artifacts
appear. However, if these same matched LEDs are driven at significantly
different conditions, the light output will have wider variation,
perhaps detectable to the ordinary observer.

Therefore the recommended method for reducing light intensity from a
matched array of LEDs is to operate at nominal electrical conditions,
but at a reduced duty cycle.

I do not know the details of the vehicle you observed, but it is likely
that the brake light function uses DC or a high duty cycle pulse, while
the tail light uses a low duty cycle pulsed activation.

There are other issues such as temperature variations and dealing with
non-functioning LEDs, but that is enough for now.

73,
Gene
W4SZ




Cecil Moore wrote:


It appeared that they were all pulsed at once. Perhaps I should
have used the word "pulsed" to avoid confusion in the RF analog
(non-digital) world.

Roy Lewallen wrote:
There's no sharing when there's only one user, channel, or LED
involved, so no multiplexing. Except, I guess, for digital types who can
share an LED between a 1 and a 0.

A two-wire serial ASCII RS232 line is time-multiplexed between
marks and spaces, i.e. you can't have a mark and a space occurring
at the same time. :-)

Just out of curiosity, what does your revered IEEE Dictionary have to
say about it?

I'm at the GED office presently. I'll check that when I get home.
--
73, Cecil http://www.qsl.net/w5dxp


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Gene Fuller wrote:
I do not know the details of the vehicle you observed, but it is likely
that the brake light function uses DC or a high duty cycle pulse, while
the tail light uses a low duty cycle pulsed activation.

Thanks very much, Gene, for that information. I only observed
the pulsed taillight function and not the braking function so
the above explanation didn't occur to me. Makes a lot of sense
now. Thanks again.
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil Moore wrote:
Roy Lewallen wrote:
Just out of curiosity, what does your revered IEEE Dictionary have to
say about it?

I'm at the GED office presently. I'll check that when I get home.

Well, looks like I win. The IEEE Dictionary agrees with you and you
have said in the past that the IEEE Dictionary is wrong. :-)
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil Moore wrote:

Well, looks like I win. The IEEE Dictionary agrees with you and you
have said in the past that the IEEE Dictionary is wrong. :-)

Ah, binary again. I disagree with all entries or I disagree with none,
with no other possibilities. Spoken like a true digital engineer.

Indeed, I often disagree with that book, and don't feel that it does a
very good job of reflecting usage among working engineers. But
apparently on this issue I do agree with it -- a possibility only in my
analog world but seemingly not in your digital one.

Roy Lewallen, W7EL