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?

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

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.

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.

http://en.wikipedia.org/wiki/Pulse-width_modulation

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

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

or like hitting warp speed!

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

Do LEDs work off some sort of pulsed DC?

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

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.

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.

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.

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? 😥

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.

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.

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.

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

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

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

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

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

Well, this is STW after all…

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 😉

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.

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

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

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

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

painting with light-1 by John Clinch[/url], on Flickr

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.