LED taillight question

This is sheer speculation on my part, but the DC on the car electrical bus is not filtered, it is pretty noisy. LEDs can really modulate at fairly high frequency. Incandescents do not, because the thermal inertia of the bulb filters out all except VERY low frequency. I wonder if we are seeing the noise from the alternator and regulator, and maybe even hash induced by motors.

Here comes my speculation:

I believe they are intentionally modulated to achieve lower-than-full brightness; I've seen rear lights that first appear as "stutter rail" in the eye, then when the driver applies break the lights go much brighter and non-stuttering. So it would seem same lights are used both as standard rear lights and break lights. I think it'd be better (or at least nicer to the eye) to just adjust the voltage, but maybe it's cheaper to the manufacturer to modulate the power.

Almost forgot, I've seen the same phenomenon on some cars' dashboard lights. When full brighness is used the light is constant, but in all lower brightness settings the light stutters.

P.V.

"P.V." wrote in news:tMqal.222$ snipped-for-privacy@read4.inet.fi:

But why only on certain makes? Are some manufacturers better able to condition their electrical systems than others?

That was my first thought as well.

I was guessing that the "stutter" ones altered pulse frequency rather than voltage to the LEDs. To wit: foot off the brake, the LED pulses /less/ frequently; foot on the brake, the LED pulses /more/ frequently, the eye perceiving the increased pulse frequency as increased brightness.

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 suspect but have no proof that the LEDs in the taillight cluster are wired up so as to provide full brightness when fed a steady 12V (e.g. if a single LED has a voltage drop of 1.3V they wire every 10 of them in series) and then the "dim" (parking light) functionality is provided by PWM.

nate

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.

It is very distracting and I hope they correct the design in newer cars.

Mark

snipped-for-privacy@yahoo.com wrote in news: snipped-for-privacy@b38g2000prf.googlegroups.com:

So I wasn't too far off with my first guess. Seems those LEDS are on solid when braking, and pulsed when showing taillights.

Distracting is /definitely/ the word. I hate them. Imagine the spark shower you'd have in front of you at night if all cars' rear lamps operated that way.

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

N8N wrote in news: snipped-for-privacy@r10g2000prf.googlegroups.com:

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

Nate Nagel wrote in news: snipped-for-privacy@news2.newsguy.com:

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

Don Bruder wrote in news:gkh57q$vnp$ snipped-for-privacy@news.motzarella.org:

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.

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.

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

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.

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

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 .

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

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.

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