Do LEDs strobe?

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  • Do LEDs strobe?
  • Just curiosity really.
    Riding in the rain, I noticed the drops of water flying off the front wheel back towards me looked they were in a strobe light, in that I could see each drop as multiple drops about 10mm apart, not one continuous moving drop.
    I did wonder if it was the drops tumbling and only showing up as they were orientated a particular way, but then I got the same effect working under a vehicle wearing a head torch and watching dust fall down off the chassis.
    As I understand it, any sort of diode will only work with DC, so this isn’t the old record turntable 50Hz AC light bulb trick.
    Do LEDs work off some sort of pulsed DC?

    scotroutes
    Member

    When you run an LED at different output levels you can see the frequency of the strobing change

    smiffy
    Member

    All depends on how it’s driven. if it just has a current control resistor in series it will glow constantly, if there is an electronic driver circuit is will vary the mark-space ratio to control power consumption, eg it is not on all of the time.

    Premier Icon JAG
    Subscriber

    Yes they do strobe – watch slow-motion video of any car with LED lights. Usually see it best on the tail lights during some video from Top Gear.

    Premier Icon scaredypants
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    yeh, it’s great when you’re riding in rain and it looks like tracer bullets 😀

    peachos
    Member

    yeh, it’s great when you’re riding in rain and it looks like tracer bullets

    or like hitting warp speed!

    Premier Icon richmars
    Subscriber

    Mine strobes, you can use it as a speedo, as it freezes the tire thread at a certain speed.

    footflaps
    Member

    Do LEDs work off some sort of pulsed DC?

    No, DC driven. However, you can switch them on and off very quickly….

    Premier Icon hot_fiat
    Subscriber

    LEDs on cars strobe as the 12v DC isn’t really DC, either its generally a half (badly) rectified AC, or the DC circuit voltage contains canbus data on carrier signal which would again lead to pulsing. As the response time of an LED is so quick they get to switch on and off in between pulses.

    Bike LEDs strobe at different output settings as the drivers use Pulse Width (PWM) controllers so for a low output it might turn the light on & off 70 times per second, whereas a higher output it might go to 140 times per second, full power is usually just “on” with no interference from the controller.

    andyl
    Member

    All good LED drivers will be DC PWM.

    It not only helps control the brightness much more effectively than voltage or current limiting but it is also used to reduce heat build up so even on full power your LED might be under PWM as the manufacturer has set a PWM level that gives a good brightness but keeps the heating and battery drain under control.

    Premier Icon phiiiiil
    Subscriber

    If your bar light is shining over the knobbles on knobbly tyres you get the usual strobing effects like the wheel going round the wrong way.

    Premier Icon jairaj
    Subscriber

    yeh, it’s great when you’re riding in rain and it looks like tracer bullets

    or like hitting warp speed!

    Oh What! You mean physics doesn’t really break down at night time and I’m not really going faster than the speed of light? Its just a trick of the lights? 😥

    Premier Icon aracer
    Member

    All good LED drivers will be DC PWM.

    Er, no. Quite a lot of poor quality LED drivers use DC PWM (along with some of those in quite expensive lights), the good ones use a current regulated DC convertor – one of those does have a PWM signal in, but it’s controlling a switch mode convertor which will have a bit of variation in current as the inductor charges and discharges, but the LED doesn’t switch on and off. Significantly more efficient to do that as LEDs get more efficient at lower current levels – if you use a higher current switched with PWM to achieve the same brightness you’ll use more power.

    Premier Icon Scapegoat
    Subscriber

    And the strobe effect will knack the readings on a wirelss computer. Used to get some really weird and fanciful readings on night rides, but eh best was when I caught the Aldi Bikemate computer doing 78 mph on the breadbin thanks to the LED downlighters in the kitchen.

    andyl
    Member

    Er, no. Quite a lot of poor quality LED drivers use DC PWM (along with some of those in quite expensive lights)

    you are reading things into my post that I didn’t write or imply

    I am simply stating that high quality ones won’t be some simple potential divider or variable resistor and will have some PWM control. Cheap ones can have it too of course as PWM circuits are not exactly expensive.

    Premier Icon aracer
    Member

    you are reading things into my post that I didn’t write or imply

    You wrote DC PWM – is that not what you meant?

    rocketman
    Member

    If your bar light is shining over the knobbles on knobbly tyres you get the usual strobing effects like the wheel going round the wrong way.

    ^^ this

    Dibbs
    Member

    The answer to the OP is yes, why does it need 19 posts to argue about it?

    Gary_C
    Member

    The answer to the OP is yes, why does it need 19 posts to argue about it?

    Well, this is STW after all…

    Premier Icon aracer
    Member

    The answer to the OP is yes, why does it need 19 posts to argue about it?

    Because the answer is actually, yes some but not all of them 😉

    CountZero
    Member

    The little red figures on pedestrian crossing controls strobe too, if you look at them in the dark and quickly look away you get a string of little red figures trailing after each other.
    Well, I do, anyway… 😀
    Also, sometimes, if you put an LED flashlight down on a hard shiny surface while turned on, like a loo cistern, you get a high-pitched buzzing.

    Premier Icon molgrips
    Subscriber

    My headtorches definitely strobe on lower power settings. I don’t think my bike lights do though.

    globalti
    Member

    On my Petzl head torch with three LEDs, presumably all three would be strobing simultaneously?

    CountZero
    Member

    If you’ve got a small LED flashlight, try holding it between your teeth, it’ll buzz like fury! 😀

    Premier Icon ampthill
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    I was a bit surprised and disappointed to find that my SMART LED rear strobes even on full power

    [url=https://flic.kr/p/puckGY]painting with light-1[/url] by John Clinch, on Flickr

    Premier Icon thisisnotaspoon
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    On my Petzl head torch with three LEDs, presumably all three would be strobing simultaneously?

    Maybe or maybe not, unlikely to strobe separately.

    Or they’re 3 cheap LED’s (i.e. not that bright) being run directly from the battery.

Viewing 27 posts - 1 through 27 (of 27 total)

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