OT : LED Brake lights, why do they strobe ?

Buzby

Senior Member
#1
Hi All,

I was talking with a colleague whilst he drove back from the airport, and we got talking about LED dimming. I said that LED brake and tail lights are PWM'ed, and if you can move your eyes 'just right' the strobing can be seen.

He said I was talking rubbish, as he couldn't see any effects on lights of the cars in front of us. He then said 'Why should they be PWM'ed, there is no need to dim them ?'.

The only reason I could think of was that it might save the power dissipation from a linear regulator, but I'm not sure.

Do any of the forum members know the real reason ?.

Cheers,

Buzby
 
#3
Yes, the modulated LED rear nights are very visible and distract my poor brain.
I don't know if there is a minimum (legal regulation, not voltage regulation) frequency but I wish they'd turn it up a bit.
And this applies also to the white LED front running lights which seem to arranged in pretty patterns.


Often the LEDs for side-lights and brake lights are the same.
So, modulated (dimmer/cooler) for side and full on for brake, which are intermittently used.

If they'd used a switched/mode method (like many ICs you can buy) they'd have got rid of the flashy-flashy.
But the 'car world' likes to save pennies so there you have it.
I have seen some newer cars which don't seem so strobie so maybe...

If your friend can't see strobing lights then he should go to an optician.

Maybe someone can Google and be an expert?
 

techElder

Well-known member
#4
The only thing that I can think of offhand is that the vehicle was designed to provide just so much power to each lamp assembly. By multiplexing, the lamp designer was able to include more LEDs in the lamp assembly than would be allowed otherwise.

The reason the lamp assembly needs more power than is normally allowed is that each LED is strobed at a much higher current than its steady state current for brightness.

So there it is ... a more visible lamp and more efficient use of the power available for the lamp assembly.
 

Goeytex

Senior Member
#6
Being able to "move your eyes just right" to see the strobing, suggests a rather low strobe frequency.

Not everyone has the same eye persistence. A CRT display monitor with a 60 Hz refresh rate will drive me crazy. However, my daughter does not notice it at all.
 
#8
Some people do experience visual "problems" particularly with their peripheral vision with flashing/strobing lights distracting them.
Even fluorescent lamps can cause them visual discomfort.

Hence why some can see vehicle lights "strobe" and others may not.
 
#9
At a company I once worked for we had a conference room set up with both standard transformer ballasts and our new solid-state ballasts on the fluorescent lights. We could flip a switch between them. We had used a light meter to balance the lighting to exactly the same intensity between the two different ballast types.

We demonstrated the system for our customers by switching between the two and asking them which they preferred. We did not tell them which was which, but they always preferred our solid-state ballast switching at 30 kHz over the 60 cycle ballasts even if they didn't know exactly why. The customers would say things like "It just seems quieter," or "It's more relaxing," or "It's more comfortable."

Even when people don't consciously "see" the strobe effect of lighting, their eyes and brain are responding to it. And some people "see" more than others do, just as some people hear or taste more than others do.
 
#11
BTW, you can probably take a photo with a digital camera as you are panning across the tail-lights, (use a very slow "shutter-speed,") and get proof that they are being pulsed, although it was easier to do back in the day of film cameras. The data gathering patterns of digital detectors can make for very odd images of pulsating light sources, especially when you are panning across them.
 

Hemi345

Senior Member
#14
It's because the brake and running lights (LED) must be 'dimmed' for night time use
The nighttime setting must have been broken on a Prius I was following the other night. When the person stepped on the brakes, I could feel my retinas burning out from how bright they were.
 
#15
It seems to me that the old tail-light bulbs were about 15 Watts. With a modern 3W LED likely putting out about the same amount of light, I can easily see how an array of 15 (or 50,) of the 3W or even just 1W LED's would be blinding to the driver behind. Some of the cars I've seen in the last few months have an amazing number of LED's in their lighting system, and I very much doubt if any of them are the old style 20mA LED's.

Now, if they can just give me enough light to find what I dropped on the floor I'll be happy. I've never driven a car that did, not even a Cadillac or Mercedes.
 

Buzby

Senior Member
#17
I never though this thread would generate so many replies.

The video idea I tried during the drive from the airport. A few seconds of it showed a slow pulsation of the tail lights, due to some sort of phasing effect between the LED strobing and the camera's frame rate. This was enough to show my colleague that I wasn't talking rubbish !.

I've not a chance to talk with him about it since then.

I think the idea that it is done to ensure a constant brilliance on varying supplies is probably the reason.

If you get some 'identical' LEDs and 'identical' resistors, on a very variable 12v, you will not get 'identical' brilliances, but if you drive the LEDs hard with a high current, then strobe to fine tune, the apparent brilliances are much closer.

I might get a chance to find out more about this as my new job is with a company that manufactures expensive LED lights for military airfields. I'm not on that development team, I do software for controlling the landing light patterns, not the stuff inside the light units themselves.

On an aside, in reply to John West, "Now, if they can just give me enough light to find what I dropped on the floor I'll be happy.", my local council is replacing our street lamps with LEDs, and the warehouse at work has LED floodlights.

When I first saw those tiny red LEDs in the early 70's, never in a million years would I have envisaged LEDs would be bright enough to light a road or a theatrical stage !!!

Cheers,

Buzby

Edit : Just read Dippy's attachments. One is PWM, the other is pure DC, so that explains why some LEDs strobe and others don't.
 
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premelec

Senior Member
#18
I think it's important to note that you don't _have_ to strobe LEDs to vary intensity efficiently - you can make a constant current switching power supply. I use an LED white head light a lot and it strobes the LEDs in the dimmed positions - very annoying - and also amusing watching various rotating objects and things like water streams...
 

Haku

Senior Member
#19
It does bother me that so many rear lights on cars now strobe at a visable frequency. Those lights go to full brightness with no strobing when they brake.

I always assumed they strobed because the manufacturer only went as far as required in regards to lighting requirements, that making them not visably strobe would only cost them more money.

I've also seen the light on some pedestrian crossing button boxes strobe at visable frequencies.
 

MPep

Senior Member
#20
I must admit that I have never noticed any strobing, although I am certain that I'll notice it from now on. Thanks Guys!!!;)

There does not appear to be any requirement in NZ for dimming lights at night. The whole idea of a STOP indicator is to let you know the vehicle in front is slowing down, so the brighter the better. I must also admit that some vehicle manufacturers appear take this to the extreme! :cool:

When I first saw those tiny red LEDs in the early 70's, never in a million years would I have envisaged LEDs would be bright enough to light a road or a theatrical stage !!!
Apparently they call that progress :rolleyes:
 

tmfkam

Senior Member
#21
I'd assumed it was related to efficiency and LED lifetime.

Some LED's can be pulsed at a higher short-term current than they can at a continuous current, making them brighter for a given lifetime? If an LED is only on for 30-40% of the time, it is only using current for the time it is on, saving power during the off time. (Drive circuit losses ignored there...)

Some of the latest traffic lights are LED and these too appear to be strobed, presumably for similar reasons?
 
#22
Regardless of the reason for dimming the LEDs via PWM, I believe the decision to use of a low PWM frequency is because of EMI concerns.
They must have used a frequency as low as possible, which still will appear as continuous to a majority of the population.
 

Goeytex

Senior Member
#23
When I first saw those tiny red LEDs in the early 70's, never in a million years would I have envisaged LEDs would be bright enough to light a road or a theatrical stage !!!
Things have come a long way. Here is a pic of a light head that I built back on 2007 as part of a portable military tactical lighting system. It has 24 Cree 3 watt LEDs. 12 are spots and 12 are floods. There are also 8 5-watt infrared LEDs where 4 are spots and 4 are floods. This thing can light up a football field.

LED-light-head-2007.jpg
 
#24
Regardless of the reason for dimming the LEDs via PWM, I believe the decision to use of a low PWM frequency is because of EMI concerns.
They must have used a frequency as low as possible, which still will appear as continuous to a majority of the population.
Inasmuch as PWM is done with sharp-edged pulses, the EMI from odd-order harmonics is bad no matter the fundamental frequency. I'd say the most likely reason to use a low PWM frequency is to make the switching device, (probably a MOSFET,) run cooler and thus save the cost of a heat-sink. Car manufacturers spend a lot of research dollars trying to make their electrics and electronics as cheaply as possible.
 
#25
I've a suspicion that this is just CANBUS related, and a function of the control command update rate over the bus to the light unit. Virtually all newer cars use CANBUS controlled electrical systems, and IIRC the command update rate is only a few tens of times a second. The lights probably just stay on after a command then goes off momentarily before the next command sequence is due to be transmitted, telling them to stay on or turn off. I can't see any merit in using low frequency PWM to limit dissipation, as it just doesn't make any sense. All the PWM LED drivers I've ever seen and used have been fine working at a few tens of kHz.
 
#26
It's because the brake and running lights (LED) must be 'dimmed' for night time use
Not if they are to remain legal. s.41 of the RTA makes no mention of any requirement to dim rear lights at night, to do so would render them non-compliant with the visibility requirements implicit in the E marking.
 
#27
All the PWM LED drivers I've ever seen and used have been fine working at a few tens of kHz.
Yep, if indeed they do use drivers designed for the job, instead of taking shortcuts to save a few cents. If everyone used a good FET driver for their PWM LED projects we would have a lot fewer folks on the forum asking why their FET's get hot.
 
#28
It's because the brake and running lights (LED) must be 'dimmed' for night time use
Often for LED lamp arrays there is no separate brake and back light circuit. Just one set of LEDs usually arranged in multiple parallel strings

Dimmed for back light function and brightened for brake function and also depending on whether it's daytime or night time so that they don't "burn your retinas out" !
 
#29
Often for LED lamp arrays there is no separate brake and back light circuit. Just one set of LEDs usually arranged in multiple parallel strings

Dimmed for back light function and brightened for brake function and also depending on whether it's daytime or night time so that they don't "burn your retinas out" !
Yes, but that's just done via a CANBUS command to the light unit, for those lights that do use the same LEDS for both the brake light and rear light function. If so, then the light E marking and approval will be based on this.

There is no requirement for dimming any external lights on a car at night, though, (only internal lights) and to do so would contradict the C&U Regs I referred to earlier (s.41 of the RTA).

I remain convinced that this is just a function of the CANBUS polling frequency on the lighting bus, and not related to any PWM control of the LEDs themselves. If I get a chance at the weekend I'll stick a bus monitor on one of the rear light controllers on my car this weekend and see how often the light on/off command is transmitted.
 
#30
But the blindingly obvious thing to do with Canbus+timeout would be to have the "stay lit" time say 10% longer than the time you expect the next command ... cost free and flicker free. It really wont matter if it takes the light 100mS or 110mS to go off when you "switch" it.

Don't UK Construction and Use regs mandate proper tungsten bulbs, but are overidden by whole-vehicle type approval for LED equipped vehicles? (where I would guess the testers measure the actual effect and don't care about the methodology)
 
#31
But the blindingly obvious thing to do with Canbus+timeout would be to have the "stay lit" time say 10% longer than the time you expect the next command ... cost free and flicker free. It really wont matter if it takes the light 100mS or 110mS to go off when you "switch" it.
I agree, but if they follow the methodology that some car manufacturers have adopted then it may be that they just don't have any intelligence in the passive CANBUS switches, to reduce cost. Perhaps they just turn on when they receive a correctly addressed command and then off again until receipt of the next correct address, only turning on again if they receive and process the "on" command word after the address that tells the module whether it should turn on or off. This could be also be related to fail off type thinking by the designers, perhaps, with the light modules just switching off if they don't see their address.

In terms of time, then I think for brake lights the manufacturers probably don't want to add any delay (despite the fact that tungsten lamps are inherently very slow), for fear of someone using this programmed delay to argue (almost certainly spuriously) that it changes the available reaction time for any driver following behind.

All just a guess until I get some time to stick a bus monitor on the rear lights of my car and see what seems to happen. Not much chance of that today, though, as it's tipping it down at the moment.




Don't UK Construction and Use regs mandate proper tungsten bulbs, but are overidden by whole-vehicle type approval for LED equipped vehicles? (where I would guess the testers measure the actual effect and don't care about the methodology)
Hence why I wrote this:
If so, then the light E marking and approval will be based on this.
 
#32
I had to do a search on the expression "tipping it down," to see what it meant. It's about to "tip it down" here in Boulder Colorado, as well. We are getting flash flood warnings from the National Weather Service. I suspect there will be a lot of PWM automotive lighting flickering today.
 

geoff07

Senior Member
#33
You certainly wouldn't want any delay to the lighting of brake lights. But if led lights can light in say 5mS (probably much less with a simple drive circuit), and incandescents light in 100mS, that is 8 feet for a car travelling at 60mph, or half a car's length. A highly desirable benefit I would say. If I could figure out how to remove the rear light clusters on my car I might fit some leds.
 
#34
If I could figure out how to remove the rear light clusters on my car I might fit some leds.
Law of unexpected consequences; if you fit LEDs to reflector/lens designed for incandescent bulbs, you may alter the angle of view characteristics, possibly for the worse.
 

geoff07

Senior Member
#35
Well of course any such attempt would be an experiment, with all that implies. But brake light reflectors are there to turn a spherical radiation pattern into a directional one, which leds have by design.
 
#36
Yes, have try with some combinations.
Obv you may have a reflector/lens combination so don't forget the position of LED source versus filament.

The pukka regs require (amongst other things) certain things such as colour, intensity and dispersion angles so it may be a job for a camera and light-meter to be a bit more confident.
PC Plod and the MOT are unlikely to have the equipment to test it, but MOT can be a little subjective depending on the tester, so don't give them any opportunity(excuse) to make a fuss and possibly lighten your wallet.

Does your vehicle have bulb-fail (non CAN) sensing?
Mine does, so if yours does it'll be interesting to see if there any problems ... before I do it myself :)
 

geoff07

Senior Member
#37
No fancy failure detection on my eight year old Espace, I think that would be a show-stopper. But first I have to get the clusters off and that is non-trivial. I bought some led lamps in a French motorway service station a few weeks ago for experimentation. Helpfully labelled "LED lights shall not be used on ordinary roads" in tiny type. Not entirely sure what an "extra-ordinary" road is.
 

Hemi345

Senior Member
#38
Does your vehicle have bulb-fail (non CAN) sensing?
Mine does, so if yours does it'll be interesting to see if there any problems ... before I do it myself :)
You'll probably need to add a resistor across the LED to increase the load a bit so the blinkers work correctly and the bulb-fail sensing doesn't think the standard element has burnt open. In a car club I was in years ago, we had this issue with the early aftermarket LED lights. Adding a a resistor across them usually fixed it, but the light output was pretty poor and very directional. The taillights that have an array of LEDs for both rear and side visibility are more 'legal'.... at least they are here in the states.
 
#39
My Cadillac had fiber optics from the bulbs to the instrument panel to let me know if any lights were burned out. No electronics at all. That seems to be about as reliable as one can get, but I suppose some sort of current sense would be less expensive.
 

Circuit

Senior Member
#40
As these are safety-critical elements of the car, I understand that some insurance companies may regard such a modification as a reason to invalidate the policy. Policies usually require the declaration of any modifications to the car and the advice that I have had is that the replacement of incandescent lamps with LED lamps would need insurance company approval. i.e. don't go there...
 
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