5 ms·

How about taillight brightness? I'm not sure if this has gotten better in recent years, but there are a bunch of high-end vehicles (looking at you AUDI) with e

by jmole 2y ago

How about taillight brightness? I'm not sure if this has gotten better in recent years, but there are a bunch of high-end vehicles (looking at you AUDI) with extremely bright red LED taillights that are just painful to sit behind at a red light.

alamortsubite 2y ago
Yeah, but it's not just the brightness. It's also the strobing, which is highly distracting and nausea inducing.
- Zak 2y ago
  I'm assuming it saves $0.47 per car to use slow PWM to set brightness rather than a linear regulator.
  - marssaxman 2y ago
    Is it possible to control LED brightness with a linear regulator? I am surprised to hear it - when I was making light effects with LEDs years ago, I understood PWM to be the only practical mechanism, due to the strongly non-linear conductivity curve. But maybe it's possible, if you just want to fix your LEDs at a specific brightness which never changes, that you could find exactly the right current level...?
    - kedikedi 2y ago
      You can but it isn’t efficient. You lose (V_in - V_out)*I_led Watts to heat if you use a linear regulator. You might ask why not connect them in series and get the voltage difference as small as possible. But the “forward voltages” of LEDs are highly temperature dependent and car battery voltage is (somewhat) engine rpm dependent (might swing between 12 and 13.8V between no rpm and some rpm. It’s kept flat at 13.8V)
    - deleted 2y ago
      [deleted]
    - SAI_Peregrinus 2y ago
      Linear regulators can do it easily, if used in a constant-current driver configuration. That constant can be variable, so regulated-current might be a better name. Your classic LM317 linear regulator can do it pretty easily, with a potentiometer instead of a fixed resistor to set the output current. Section 9.3.3[1] of the datasheet describes the circuit, though in practice you might add a couple of capacitors for stability. There are also a number of discrete-transistor circuits. A current mirror with a rheostat is probably the simplest, though not generally ideal performance. [1]https://www.ti.com/lit/ds/symlink/lm317.pdf?ts=1734395381792#%5B%7B%22num%22%3A391%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C0%2C464.4%2C0%5D https://www.ti.com/lit/ds/symlink/lm317.pdf?ts=1734395381792...
      - marssaxman 2y ago
        Interesting! I have used plenty of LM317s with LEDs, but I never thought of using them to regulate brightness - I would just pick whichever resistor corresponded to the forward current specified in the emitter datasheet. Didn't realize that there was enough room on the curve to use current control as a meaningfully configurable brightness setting.
    - Zak 2y ago
      As a flashlight nerd and reviewer, I know a number of ways to control the brightness of an LED, and linear regulators are a fairly common one. It's important to note that a linear regulator can be designed to target a specific voltage, or a specific current. LEDs should be regulated with current for the reason you mentioned. One that I think would work well in cars is a fixed-current linear regulator like the AMC7135. There's no need to do anything fancy to control it; put a 7135 in the circuit and the LED gets 350mA. Two in parallel and it gets 700mA, and so on. More fancy is a linear regulator with variable current output. Linear regulators are a low-end option for flashlights, with switched-mode power supplies being preferred for their superior efficiency. A car has a lot more energy to work with and more mass with which to sink and radiate heat.
  - ssl-3 2y ago
    Does slow PWM save money? I first noticed slow PWM in factory tail lights on Cadillacs somewhere around the turn of the century. I pointed it out to my dad one night when we were out for a walk, and told him how to see it on a car that was disappearing down the road. It took a few seconds for him to understand and to observe it himself, and then he said "I can accept that you're annoyed by that, but maybe they just build them that way because they think it looks cool." Ever since that night, I have always assumed that slow PWM was used as a deliberate visual effect: After all, General Motors isn't really trying to squeeze a few pennies out of a new Cadillac [that's what Chevrolet is for!]. (I also think it's a dick move on the manufacturer's part, whether it is to save forty-seven cents, or if it is to look cool, or even if it is both.)
    - Zak 2y ago
      This prompted me to go look for a better answer, and I found a good candidate: pulsed current at a relative low frequency (~60hZ) allows higher perceived brightness from smaller/cheaper LEDs. From my perspective, it is undesirable to increase the brightness of car taillights relative to what was typical with incandescent bulbs or to make them flicker. https://electronics.stackexchange.com/questions/164078/why-is-pwm-used-to-control-leds-even-when-theyre-only-used-on-or-off https://electronics.stackexchange.com/questions/164078/why-i...
      - neuralRiot 2y ago
        The PWM function is built-in into the electronic switches that drive the light outputs, so you can programmatically control the brightness, it also accepts different switching frequencies so it’s just a matter of increasing the frequency for the “strobe effect” to be unnoticeable. The advantage of these switches over analog are several, including output diagnostic and lower power dissipation.
        Zak 2y ago
        Keeping in mind that it's on a moving vehicle, and road users often pan their eyes around rapidly, the frequency would need to be higher than for many other situations. Or they could use a variable linear regulator with no flickering at all.
- gruez 2y ago
  >strobing I've never seen a strobing tail light. The most "strobing" I've seen are some brake/tail lights that flash for 1-2 seconds right after they're turned on (to alert drivers better? still annoying imo).
  - spaceisballer 2y ago
    I know some cars have the blinking when you are applying sudden pressure to the brakes. To alert people behind them they may be stopping quickly. People use their hazards in some states for the same reason.
  - smcameron 2y ago
    Almost all LED taillights strobe, but too fast for most people to see the strobing due to persistence of vision. But if you move your eyes side to side while taillights are in your field of view you can catch a glimpse of the strobing.
  - toast0 2y ago
    That blinking is usually a dealer installed add-on that can be hard to get removed (and usually involves modifying the factory wiring, thanks), generally on the high mount brake light (CHMSL). I personally believe these lights violate the law (and I think this letter supports that position [1], if it's still current?), but dealers gonna deal. But I think the poster is complaining about the higher frequency flickering that's supposed to be imperceptible... if you don't perceive it, that's better for you. Unfortunately, I regularly see it especially when moving my heads or eyes and find it pretty distracting. There's circuit design techniques to reduce the flickering, but saving pennies here and there adds up, so there you go. [1] https://www.nhtsa.gov/interpretations/20288ztv https://www.nhtsa.gov/interpretations/20288ztv
    - amluto 2y ago
      I doubt this even saves pennies. These lights are driven by an IC, and the current is regulated because the supply voltage is nowhere near stable enough to get away with an unregulated drive circuit. The mention of the TLE4242 chip upthread quite clearly indicates that there’s an entire wire from the ECU to the driver to control it. All that’s needed is for the IC to support some way to change the current regulator set point instead of flashing the LED. Sure, this would add a handful of transistors and maybe a capacitor or two inside the IC, and it will cost some R&D, but it won’t affect the unit cost in any meaningful way. (This could be as simple as a filter that turns the duty cycle into a current target. This feature is nearly ubiquitous in off-the-shelf “dimmable constant current” line-voltage-input LED drivers from basically every supplier (e.g. Mean Well). The tricky bit would be to get the thing to turn on quickly enough without annoying brightness swings, so some actual care in the filter design and frequency selection would be required. Or one could use 0-10V control (also ubiquitous) or have just two states plus off or have digital control… there’s no shortage of perfectly fine solutions.)
cbm-vic-20 2y ago
Are these rear fog lights? They tend to be bright, and lots of people always drive with their fog lights on for some reason.
- _yb2s 2y ago
  This- a lot of European cars have rear fog lights, which people sometimes leave on all of the time, but really should only be used in low visibility weather.
sans_souse 2y ago
There are a lot of oblivious drivers operating their vehicles with their tow-lamps enabled, who are not towing. There's a button by the light cluster, for example on the Audi Quattro; you may wonder what it does - or maybe you thought it was a do-nothing button. It makes your tail lights equal to the brightness of your brake lamps. CEASE USE.
switch007 2y ago
And nobody bothers putting on the handbrake any more (helped by the proliferation of automatics). I often use the visor when stationery behind a car

by jmole 2y ago

alamortsubite 2y ago

Yeah, but it's not just the brightness. It's also the strobing, which is highly distracting and nausea inducing.

Zak 2y ago
I'm assuming it saves $0.47 per car to use slow PWM to set brightness rather than a linear regulator.
- marssaxman 2y ago
  Is it possible to control LED brightness with a linear regulator? I am surprised to hear it - when I was making light effects with LEDs years ago, I understood PWM to be the only practical mechanism, due to the strongly non-linear conductivity curve. But maybe it's possible, if you just want to fix your LEDs at a specific brightness which never changes, that you could find exactly the right current level...?
  - kedikedi 2y ago
    You can but it isn’t efficient. You lose (V_in - V_out)*I_led Watts to heat if you use a linear regulator. You might ask why not connect them in series and get the voltage difference as small as possible. But the “forward voltages” of LEDs are highly temperature dependent and car battery voltage is (somewhat) engine rpm dependent (might swing between 12 and 13.8V between no rpm and some rpm. It’s kept flat at 13.8V)
  - deleted 2y ago
    [deleted]
  - SAI_Peregrinus 2y ago
    Linear regulators can do it easily, if used in a constant-current driver configuration. That constant can be variable, so regulated-current might be a better name. Your classic LM317 linear regulator can do it pretty easily, with a potentiometer instead of a fixed resistor to set the output current. Section 9.3.3[1] of the datasheet describes the circuit, though in practice you might add a couple of capacitors for stability. There are also a number of discrete-transistor circuits. A current mirror with a rheostat is probably the simplest, though not generally ideal performance. [1]https://www.ti.com/lit/ds/symlink/lm317.pdf?ts=1734395381792#%5B%7B%22num%22%3A391%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C0%2C464.4%2C0%5D https://www.ti.com/lit/ds/symlink/lm317.pdf?ts=1734395381792...
    - marssaxman 2y ago
      Interesting! I have used plenty of LM317s with LEDs, but I never thought of using them to regulate brightness - I would just pick whichever resistor corresponded to the forward current specified in the emitter datasheet. Didn't realize that there was enough room on the curve to use current control as a meaningfully configurable brightness setting.
  - Zak 2y ago
    As a flashlight nerd and reviewer, I know a number of ways to control the brightness of an LED, and linear regulators are a fairly common one. It's important to note that a linear regulator can be designed to target a specific voltage, or a specific current. LEDs should be regulated with current for the reason you mentioned. One that I think would work well in cars is a fixed-current linear regulator like the AMC7135. There's no need to do anything fancy to control it; put a 7135 in the circuit and the LED gets 350mA. Two in parallel and it gets 700mA, and so on. More fancy is a linear regulator with variable current output. Linear regulators are a low-end option for flashlights, with switched-mode power supplies being preferred for their superior efficiency. A car has a lot more energy to work with and more mass with which to sink and radiate heat.
- ssl-3 2y ago
  Does slow PWM save money? I first noticed slow PWM in factory tail lights on Cadillacs somewhere around the turn of the century. I pointed it out to my dad one night when we were out for a walk, and told him how to see it on a car that was disappearing down the road. It took a few seconds for him to understand and to observe it himself, and then he said "I can accept that you're annoyed by that, but maybe they just build them that way because they think it looks cool." Ever since that night, I have always assumed that slow PWM was used as a deliberate visual effect: After all, General Motors isn't really trying to squeeze a few pennies out of a new Cadillac [that's what Chevrolet is for!]. (I also think it's a dick move on the manufacturer's part, whether it is to save forty-seven cents, or if it is to look cool, or even if it is both.)
  - Zak 2y ago
    This prompted me to go look for a better answer, and I found a good candidate: pulsed current at a relative low frequency (~60hZ) allows higher perceived brightness from smaller/cheaper LEDs. From my perspective, it is undesirable to increase the brightness of car taillights relative to what was typical with incandescent bulbs or to make them flicker. https://electronics.stackexchange.com/questions/164078/why-is-pwm-used-to-control-leds-even-when-theyre-only-used-on-or-off https://electronics.stackexchange.com/questions/164078/why-i...
    - neuralRiot 2y ago
      The PWM function is built-in into the electronic switches that drive the light outputs, so you can programmatically control the brightness, it also accepts different switching frequencies so it’s just a matter of increasing the frequency for the “strobe effect” to be unnoticeable. The advantage of these switches over analog are several, including output diagnostic and lower power dissipation.
      - Zak 2y ago
        Keeping in mind that it's on a moving vehicle, and road users often pan their eyes around rapidly, the frequency would need to be higher than for many other situations. Or they could use a variable linear regulator with no flickering at all.
gruez 2y ago
>strobing I've never seen a strobing tail light. The most "strobing" I've seen are some brake/tail lights that flash for 1-2 seconds right after they're turned on (to alert drivers better? still annoying imo).
- spaceisballer 2y ago
  I know some cars have the blinking when you are applying sudden pressure to the brakes. To alert people behind them they may be stopping quickly. People use their hazards in some states for the same reason.
- smcameron 2y ago
  Almost all LED taillights strobe, but too fast for most people to see the strobing due to persistence of vision. But if you move your eyes side to side while taillights are in your field of view you can catch a glimpse of the strobing.
- toast0 2y ago
  That blinking is usually a dealer installed add-on that can be hard to get removed (and usually involves modifying the factory wiring, thanks), generally on the high mount brake light (CHMSL). I personally believe these lights violate the law (and I think this letter supports that position [1], if it's still current?), but dealers gonna deal. But I think the poster is complaining about the higher frequency flickering that's supposed to be imperceptible... if you don't perceive it, that's better for you. Unfortunately, I regularly see it especially when moving my heads or eyes and find it pretty distracting. There's circuit design techniques to reduce the flickering, but saving pennies here and there adds up, so there you go. [1] https://www.nhtsa.gov/interpretations/20288ztv https://www.nhtsa.gov/interpretations/20288ztv
  - amluto 2y ago
    I doubt this even saves pennies. These lights are driven by an IC, and the current is regulated because the supply voltage is nowhere near stable enough to get away with an unregulated drive circuit. The mention of the TLE4242 chip upthread quite clearly indicates that there’s an entire wire from the ECU to the driver to control it. All that’s needed is for the IC to support some way to change the current regulator set point instead of flashing the LED. Sure, this would add a handful of transistors and maybe a capacitor or two inside the IC, and it will cost some R&D, but it won’t affect the unit cost in any meaningful way. (This could be as simple as a filter that turns the duty cycle into a current target. This feature is nearly ubiquitous in off-the-shelf “dimmable constant current” line-voltage-input LED drivers from basically every supplier (e.g. Mean Well). The tricky bit would be to get the thing to turn on quickly enough without annoying brightness swings, so some actual care in the filter design and frequency selection would be required. Or one could use 0-10V control (also ubiquitous) or have just two states plus off or have digital control… there’s no shortage of perfectly fine solutions.)

cbm-vic-20 2y ago

Are these rear fog lights? They tend to be bright, and lots of people always drive with their fog lights on for some reason.

_yb2s 2y ago
This- a lot of European cars have rear fog lights, which people sometimes leave on all of the time, but really should only be used in low visibility weather.

sans_souse 2y ago

There are a lot of oblivious drivers operating their vehicles with their tow-lamps enabled, who are not towing. There's a button by the light cluster, for example on the Audi Quattro; you may wonder what it does - or maybe you thought it was a do-nothing button. It makes your tail lights equal to the brightness of your brake lamps. CEASE USE.

switch007 2y ago

And nobody bothers putting on the handbrake any more (helped by the proliferation of automatics). I often use the visor when stationery behind a car