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Combating LED Headlight/Taillight Strobing

Discussion in 'Model S: Interior & Exterior' started by JaredBanyard, Mar 7, 2014.

  1. JaredBanyard

    JaredBanyard Member

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    Ever wondered how Tesla combats the problem of LED refresh rates?

    Looks like they individually address them and spin them around:
    DSC_0724_zpse55b58b7-MOTION.gif
     
  2. bsd

    bsd Member

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    Huh, I didn't know LEDs had a refresh rate.
     
  3. pgiralt

    pgiralt Active Member

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    #3 pgiralt, Mar 7, 2014
    Last edited: Mar 7, 2014
    Has the nice side-effect of keeping the current draw relatively steady as well.

    @bsd, To get an LED to "dim", you usually turn it on and off at a specific duty cycle to reduce the amount of light it produces. For example, if you want it dimmed to 50%, you make sure that within a given period of time, it is on 50% of the time and off 50% of the time. The faster the refresh rate, the less noticeable it is. This page has a nice diagram showing the duty cycle:

    ATmega168A Pulse Width Modulation PWM - Tutorials

    What Tesla has done is instead of just turning the whole thing on or off, they are cycling the LED's so that at any given time, there is always something on.
     
  4. Gizmotoy

    Gizmotoy Active Member

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    Neat. Is there a purpose for doing it this way? To the eye, they just look solid on. To look cool in video?
     
  5. tokuro

    tokuro Member

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    Many LED devices (alarm clock etc) does the same. that is to reduce power and heat from LED. It also reduces numbers of components needed to build complex display.

    If you have a decent camera, try taking a picture of LED clock at shutter speed of 1/250 or higher. You will see that only portion of LED is "on".
     
  6. JPP

    JPP Active Member

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    At night, look in your Model S rear camera at some recent production vehicles with front LED lighting (when they are following you), and watch the different light shows--pretty interesting. I guess the frame rate of the rear camera somehow catches the rotation rate of the lights (...maybe like old movies and stagecoach wheels). VFX--we need expertise here...:)
     
  7. Doug_G

    Doug_G Lead Moderator

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    The effect is called "aliasing". It's exactly the same effect that makes wagon wheels appear to turn backwards in old western movies. The sampling rate of the movie or video is insufficient to keep up with changes in the scene.
     
  8. Gizmotoy

    Gizmotoy Active Member

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    Sure, but I kind of meant "why make the refresh circle around rather than just flash on/off simultaneously like other automakers"

    Maybe the OP is wrong? I could see the LEDs being in separate banks that have slightly different refresh rates. What I don't get is why they'd go through the effort to make them refresh circularly ("spin them around").
     
  9. markb1

    markb1 Active Member

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    You can certainly dim LEDs without PWM. Just drop the voltage. PWM is typically used to dim LEDs because it's easy to control from a digital circuit. But if you just need a constant level, the easy thing to do is put a resister in series with the LED to drop the voltage. I suppose they would use PWM for power efficiency. (Though presumably it's possible to make LEDs that achieve the desired brightness at 100%.)
     
  10. scaesare

    scaesare Active Member

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    Often times multiplexing banks of LED's on a PWM circuit simply is cheaper, in that you can round-robin them such that they eye can't tell, but a single PWM circuit is handling all of them. Less components = less $$$.
     
  11. Doug_G

    Doug_G Lead Moderator

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    Yes but the resistor would burn a lot of power and get hot. Complete waste of energy and a potential failure mode if it gets too hot. PWM is really the sensible solution. It has very high efficiency because the driver transistor is either all the way on or all the way off, and this means it doesn't waste any power. It is also trivially easy to control the brightness by adjusting the duty cycle, no D/A converter required. Human eyes can't see the flicker. Low cost, efficient, flexible, reliable, effective. That's why everyone does it.
     
  12. captain_zap

    captain_zap Electron tamer

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    Doug is right. I spent two years running an LED driver design team for a major semiconductor company. Certain applications, such as TV studios and theaters can't tolerate the flicker and must use linear dimming. Super high quality lighting requirements, like museum lighting, also opt for linear dimming. There are also studies that show people's ability to perceive flicker varies widely. LED PWM frequencies tend to be between 300-1000 Hz. Some people can see up to 1000 Hz flicker and others can't perceive 60 Hz.

    - - - Updated - - -

    I will also point out that the MS DRLs are multiplexing between banks of LEDs as well as PWM dimming (when a turn signal is active). Multiplexing allows one driver to drive several banks of LEDs, further saving cost. White LEDs are so bright these days that they are not always needed to run at full brightness. This allows cost saving tricks such as multiplexing.
     
  13. markb1

    markb1 Active Member

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    But I did mention power. :)
     
  14. JaredBanyard

    JaredBanyard Member

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    Glad you guys enjoyed it. Don't suppose someone could get high speed footage of the effect? I would guess we would need somewhere around 240fps? Maybe more?
     
  15. scaesare

    scaesare Active Member

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    That sounds familiar... oh yeah:

    ;)
     
  16. tom66

    tom66 Member

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    They might be using PWM to vary the DRL intensity depending on conditions, maybe making them brighter when it is brighter outside.

    Multiplexing like this is clever, it means they will be able to reduce input ripple current, which can allow for smaller conductors, minimal capacitance on the LED driver board, and also reduces electrical interference. If it is done really well, it will look like the module consumes a fixed current.
     
  17. LMB

    LMB Member

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    @captain_zap -- Sounds like you would know better, but decades ago I seem to remember that LEDs were most efficient (light per watt) at power levels too high for steady-state operation so PWM was the most efficient way to run them.
     
  18. tom66

    tom66 Member

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    In general, from my experience, LEDs are most efficient in the middle of their operating curve, they drop off at either end of the curve.
     
  19. captain_zap

    captain_zap Electron tamer

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    LEDs are more efficient in lumens/watt at low to medium drive levels. They are more efficient in lumens/dollar at high drive levels. So cheap LED bulbs tend to use fewer LEDs, drive them hard and run them hot, which is what shortens their lifetime, and why some cheap LED bulbs will not last 'forever'.
    PWM drive and multiplexing lowers average current and die temperature, beneficial to LED lifetime.
     

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