Some tail light LED found distracting

Leddies answer
Hi Dennis, here is what I could find.

There seems to be a general problem of distraction affecting drivers tailing a few of new car models, NOT just Cadillac. It mostly happens whe you try looking at a bunch of tail light LED from the corner of you eye. As your eye will be strongly attracted to the light you will see a kind of a strobing effect through the involuntary saccadic movement of your eyes.


The story of tail light LED for Cadillac is quite interesting too.
Since their 2000 model (DeVille) Cadillac has introduced LED light units for tail and then brake lights. The assembly called "SnapLED" was the result of HP and Philips collaboration LumiLeds Lighting. What you'd actually love about it is its ingenuity. The thin metal strip connector serves 3 roles at once. (1) Structural support, (2) electrical conductors, and (3) heat sinks. Because of this there are no wire leads. Fixing or taking apart is nut much more than a Snap on or off. This reduces the likelihood of open or short circuits.

Speaking of Cadillac, another model lines to include provisions for LED taillight were: Seville, SLS, STS, Catera, CTS, DHS, DTS, Escalade, Eldorado, and Fleetwood. (you could have seen any of these)


As to blinding brightness:
If it really bugs you on a neigbours car, and want to do something about it -- well, one way to solve the problem is 'simply' to remove it from the offending car, which of course I don't advocate ...
http://www.ledfix.com/brakelightremoval.html


But seriously, tail light LED - often combined with a center high-mounted stoplight - is supposed to be safer giving the driver behind (you if I am not mistaken) more time to brake. That's because LEDs reaction time to light up is heaps faster than slow incandescent bulbs or halogens or HIDs. It runs counter to this concept if the tail light LED in fact blinds you due to its concentrated and very intense brightness. I suspect what happens here is a combination of two-three things:


The first thing will be more tough to break and is about your saccadic
persistence of vision. Find more ont this here
http://www.fun-led-light.com/led-strobe-light.html


The other is about LEDs focus. It can be finetuned much sharper then
most other light source save LASER. Manufacturers might have overdone this a wee bit to make the effect more brilliant, more spectacular.


And some suggest their is third, perhaps more important problem: Manufacturers don't eliminate flickering by not filtering the power delivered past the PWM that controls the duty cycles to the tail light LED units. Either that, or they could use a higher frequency PWM. And the very reason for the PWM appears to be a use of a cheap on-off transistor after which you need pulse width modulation to drive top brightness from the LEDs.


We will be on their tail (so to speak :wink:) to post your concern to a broader audience at fun-led-light. Because if substantiated, this is quite a serious traffic hazard, indeed.

Have a great weekend
Cheers, Leddie

**************************************************
Reply from Dennis
Thanks for looking into this. Yes, if this affects other people in the same way it is definitely a safety hazard.
 

**************************************************
**************************************************

Interestingly, same problem, But this time from a DIY-selfer perspective

... it became no longer a hobby. I recently found and purchased a model police car identical to the ones I make, however the individual had done something I wanted to do for years.. placed 3mm LED's into this 1:18th scale model. Here's my question: why do some of the LED's have a srtobe effect? This car seems to have a small circuit board attached that runs off a 9v battery that I don't want to play with because wiring has never been a strong point of mine. It does not seem very difficult, however was hoping maybe you could point me in the right direction. ... Thanks, Jeff

**************************************************
Reply from Leddie
Hi Jeff, thanks for the line ;-)
Those models of  yours can be pretty cool. I like your dedication, can relate to that. See, I have been fascinated about LED from the time they weren't more than just indicator lights. ... Send me photo as jpg attachment and a few words, and if useful for other readers I will see if I can feature at the DIY section. It is something I'd like to. But because your problem is a common one, so I will feature it referring to you...
I have my theory for your problem shared by others who are more knowledgeable -- I checked around. And if it turns out to be the case, that is the same reason why some auto tail light LEDs are so uneasy. Same problem. As the common wisdom goes, the reason for the strobing you see is that it is a cheap solution to drive higher brightness from the LED. This means that PWM also has to be added. The reason you see strobing because the PWM frequency is too low.
The better solution I understand could be (1) varying the amount of current but that would use more power and drain the battery faster. Or (2) eliminate PWM with a better (likely more expensive) transistor. But at the very least, the higher the PWM frequency, the less likely you will see annoying strobing. -- I think I have it in my FAQ page already, what is PWM. Hope this helps
Cheers, Leddie

**************************************************
Reply from Jeff
Good thread,
this is exactly what i'm looking to try to do. thanks again, jeff


Jeff's excuisite police car model with LED lights

**************************************************
**************************************************

United Pacific LED taillight problem

Jon from Wexford, Pennsylvania wrote
You seem to be someone who understands LEDs well, so I'm hoping you can help me.  I purchased LED taillight units for my '31 Model A ford (12volt). I bought three units so I could fabricate a matching 3rd brake light for the rear window.  The lens/light units are made with upper and lower sections which are wired together to work as one.  The turn signals are not part of the system.  After installing them I found that the park lights work the way they should and the brake lights also. What I noticed was that with all switches off, there are three LEDs in the lower section of all three lights which glow dimly....The same three on each of the lights. All the other LEDs are off.  How or why would these be lit with the switches controling them open? The United Pacific Company could offer no answers. Is this anything you've seen before? Thank you for any information you might provide.

**************************************************
Reply from Leddie
Hi Jon -- honestly, I don't know the answer, could be a circuit problem. Passed on your message in my sci.electronics.design newsgroup here...

Leddies google groups usenet thread

We'll see what comes up. -- Leddie
**************************************************
Reply from Paul
Just a guess: First of all, does this vehicle feed +12V battery to the +
side of the brake lights at all times and ground the brake - to
illuminate the lights?

If so, I'd look for a sneak circuit that allows a small current to flow
from the brake light - side to ground. Something like a car alarm would
do it.

I'm guessing that the 3 LEDs are used as crude voltage references in a
regulator circuit that drives the remaining LEDs. The forward voltage
across an LED is roughly constant for a wide current range. The LEDs in
each light are broken up into 3 groups with one regulator (and LED) for
each. The sneak circuit current is sufficient to cause these three to
illuminate dimly, but not enough to turn on the others controlled by the
regulator circuits.

**************************************************
Reply from Leddie
Jon, hope the answer from Paul helps...
Let me know if this answers your question so it will help others with similar problem. Thank you -- Leddie

**************************************************
Reply from Jon
Leddie,

Paul is exactly right. The circuit is designed just that way.  I was able to
locate someone who has had experience with just this problem.  He related
the following..
Every stoplight switch ever made will bleed a certain amount of energy.
(enough to energize an LED)  There are two easy solutions.  One is to wire a regular incadescent socket and lamp into the circuit. This provides the
correct resitance to stop the bleed.  The other is to install both a 10 ohm
resistor and a 10 uf capacitor in the hot wire.  I did not try that solution
so I can't say for certain that it will work.  The first solution was
easier, so I  installed the socket and hid it up under the car.  The problem
is solved.

**************************************************
Reply from Leddie
Hey Jon, that was easy. Your incandescent lamp has a ballast resistor to protect the circuit for when burning hot wolfram wire resistance goes dangerously low. It proved to be clever to use it just as a resistor.
And I also thanked Paul on your behalf.
Glad we could help.

**************************************************
**************************************************

And the Disturbing News is ...

Now, be prepared that various LEDs may age differently. That's because no two light diode crystals are exactly the same inside. So as a result, over time they may not just fade a bit, but the strobing effect might also come back to haunt -- even if it appeared to be under control in a brand new model. I can only hope that manufacturers decide to do something about the obvious problems of tail light LED. Meanwhile, try not look at them from the CORNER of your eye.