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Comment by abainbridge | Hacker News Reader

What gets me are side-lights on cars that flicker. Cars have DC electric systems in the first place. Where did the Audi engineers find 100 Hz to run the LEDs from?

I guess there's a switch mode power supply to convert down from 12 V to something lower that the LEDs need. But AFAIK, switch mode power supplies don't switch at 100 Hz.

I'm pretty sure you could show by experiment that flickering car side-lights are distracting. For example, in a situation where you are pulling out of a T junction in the dark, you're less likely to see a motorbike when it is in the same part of your visual field as a car with flickering lights. Particularly when you are moving your head from left to right, as you do when at a T junction.

I'm not sure how to do the experiment though, since using TV screens, monitors or projectors wouldn't work.

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I suspect its not the PSU, its the PWM for the dimming.

compared to actually lowering the voltage/current of a LED/light/other its far simpler and cheaper just to turn it off and on again at varying duty cycles/ frequency.

Correct me if I'm wrong, but isn't it just the de facto way of dimming LEDs? Since as opposed to incandescent bulbs, they require certain voltage/current to be lit, if you pass them less, they simply won't emit light.

You can feed it DC and vary the current, for example by filtering the pwm.

They require a certain voltage (to a first approximation). Their light output is (again, to a first approximation) directly proportional to the applied current.

PWM control is just a cheap way of reducing the current.

Good point. But there's no reason for the PWM to be in the 100 Hz range either. Even the crappest possible microcontroller using timer interrupts could handle 1000 Hz. Hardware PWM peripherals (that can switch at 500 KHz) have been trivially cheap additions to microcontrollers for decades.

It's likely not for dimming but for reducing power usage without reducing apparent brightness. See my comment sibling to yours.

May not be for dimming. PWM control is used also to reduce the current (and therefore power, and therefore heat) required to achieve a particular apparent brightness. This utilizes the persistence of vision effect.

My guess is that, above a certain frequency, persistence of vision no longer applies, and the light is in fact seen simply as being dimmer (which is undesirable in this case). Hence ~100 Hz.

Another technical consideration would be to have grid-based scanning of the LEDs. That is you have some sort of row output, one row at a time, and the columns are multiple parallel outputs. This inherently pulses the LEDs as the rows aren't all lit simultaneously. The persistence-of-vision effect is a great side effect and can be capitalized on. The grid is best for limiting the number of wires to access a substantial number of LEDs (like 16 or so) in a group. I don't think a grid for the turn signals of a car would be a great idea, at least not at the level of the individual turn signal diodes. Perhaps within a single corner's turn signal assembly.

Many rear car LED lights flicker because their full brightness is used as brake lights, and they are dimmed via pulse width modulation for normal lighting. This is simply a bit cheaper, which is ridiculous on an expensive car.

I don't actually see the flicker normally, but when they quickly move accross my visual field I sometimes see a line of dots from the flicker.

Cadillacs are even worse. I could swear their brake lights run at 20Hz.

That is seriously annoying. Not only do they flicker, they are slightly blue as well. I have mistaken an Audi for an emergency vehicle in my rear view mirror on multiple occasions. I am sure that this is their plan. Such lights should be outlawed.

Now I have found that some mirrors on cars and even motorcycles can distort the reflected light to give blue points. combine with polarizing glasses and it becomes even more difficult to find the exact cause.

now if viewing straight on without another element to distort the light, like polarized lenses; I would be interested if the reflector built into the headlamp assembly can cause an issue.

I've found the colour to be too close to emergency-vehicle blue as well, particularly if the car behind hits a bump and momentarily brings their headlights into my mirrors, it's almost exactly like the flash of emergency lights. I shouldn't have to be second-guessing whether there's an emergency vehicle behind me, I should be following my first instinct which is to ensure I'm not obstructing traffic. Not clearing out of the way for an Audi pulling a tonne to rocket past me.

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