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Old 08-08-12, 03:35 PM   #1
Allen
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Science question--LED tail lights strobe to me

The family and I were riding home the other night and I was complaining about how LED lights strobe when I look away from them. The rest of my family couldn't see it.

What's going on here?


^^^
What my family sees.


^^^
What I see.

It's just LED lights that strobe too. Incandescent don't. Some makes of cars are worse than others as well. Cadillac's SUVs are especially bad.

The trails are strobes as well, not streaks. Do LEDs power on and off like fluorescent lights?

No, I am not under the influence of any psychedelic drugs. I come by my insanity naturally.
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Old 08-08-12, 03:48 PM   #2
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My eyes do that too. I notice it more when I wear my glasses at night. Now I have to go to youtube and watch a Cory Hart video.
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Old 08-08-12, 03:53 PM   #3
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I have no clue, but I feel your pain... I have to wear sunglasses when driving at night sometimes... seriously.
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Old 08-08-12, 03:55 PM   #4
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In order to make them brighter and yet not burn out from too much heat, some LEDs ARE pulsed... and you happen to be able to see the pulses. Some folks can, some folks cannot.

I too can see the pulses, just as I can see fluorescent lights flickering and incandescent lights blink, the latter under certain surge conditions (which "nobody" seems to hear/see). I don't believe we are insane or alien (but who knows).
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Old 08-08-12, 04:07 PM   #5
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In the flashlight world people will often want to know if certain lights use PWM (Pulse Width Modulation) for the dimmer modes. It's very visible to certain people and can be annoying when using the flashlight.

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

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PWM
Pulse Width Modulation. A technique of varying the brightness of a LED by flashing it on and off very quickly, making it appear to the eye that the LED is dimmer. Good PWM is at such a high rate that you can't notice it. But if you wave your hand in front of the light and get a strobe effect, the PWM is getting too low. Some use current regulation which makes the light dimmer without PWM and is also more efficient.
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Old 08-08-12, 04:29 PM   #6
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Too much scotch with dinner?
I know I get monkey-eyed after a few on Friday nights.
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Old 08-08-12, 05:27 PM   #7
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In the flashlight world people will often want to know if certain lights use PWM (Pulse Width Modulation) for the dimmer modes. It's very visible to certain people and can be annoying when using the flashlight.

http://en.wikipedia.org/wiki/Pulse-width_modulation
Thank you.
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Old 08-08-12, 07:38 PM   #8
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I believe we are insane or alien.
This.
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Old 08-08-12, 07:42 PM   #9
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The family and I were riding home the other night and I was complaining about how LED lights strobe when I look away from them. The rest of my family couldn't see it.
All LED's strobe. Find an old LED calculator and shake it in front of your eyes. You'll see the digits strobe.

It's either that or your family is trying to drive you insane. Oh, wait...
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Old 08-08-12, 08:56 PM   #10
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unless there direct driven they strobe
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Old 08-08-12, 09:02 PM   #11
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My understanding of LED's is that they operate in a somewhat narrow voltage range- too little voltage and they don't work, too much voltage and they are damaged. If you can't provide a constant voltage of the right amount then you have to have some form of electronics to step up or step down the voltage, and the simpler circuits that don't use transformers do some form of pulsed modulation. This is relative to flashlights, I don't know if taillight modules and such do it.
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Old 08-09-12, 11:08 AM   #12
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Quote:
Originally Posted by AllenG View Post
The family and I were riding home the other night and I was complaining about how LED lights strobe when I look away from them. The rest of my family couldn't see it.

What's going on here?


^^^
What my family sees.


^^^
What I see.

It's just LED lights that strobe too. Incandescent don't. Some makes of cars are worse than others as well. Cadillac's SUVs are especially bad.

The trails are strobes as well, not streaks. Do LEDs power on and off like fluorescent lights?

No, I am not under the influence of any psychedelic drugs. I come by my insanity naturally.
A given LED puts out a constant amount of light. If you want to make it appear less than full brightness, you do it with the duty cycle (i.e., shut it off, then turn it back on very rapidly). If it's rapid enough, the eye doesn't pick it up. Some people are more sensitive to the blinking than others. On car tail lights, if the same LEDs are used as marker lights and brake lights, they use the duty cycle modulation to make them appear dimmer in marker light mode.

I took some pics a couple weeks ago of riders going by trees with LED lighting. I moved the camera to track the rider, so the lights in the background are streaks, but they are not continuous; they look like dotted lines.
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Originally Posted by bragi "However, it's never a good idea to overgeneralize."
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Old 08-09-12, 11:10 AM   #13
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Quote:
Originally Posted by AllenG View Post
The family and I were riding home the other night and I was complaining about how LED lights strobe when I look away from them. The rest of my family couldn't see it.

What's going on here?


^^^
What my family sees.


^^^
What I see.

It's just LED lights that strobe too. Incandescent don't. Some makes of cars are worse than others as well. Cadillac's SUVs are especially bad.

The trails are strobes as well, not streaks. Do LEDs power on and off like fluorescent lights?

No, I am not under the influence of any psychedelic drugs. I come by my insanity naturally.
A given LED puts out a constant amount of light. If you want to make it appear less than full brightness, you do it with the duty cycle (i.e., shut it off, then turn it back on very rapidly). If it's rapid enough, the eye doesn't pick it up. Some people are more sensitive to the blinking than others.

I took some pics a couple weeks ago of riders going by trees with LED lighting. I moved the camera to track the rider, so the lights in the background are streaks, but they are not continuous; they look like dotted lines.

Oh, and this is also why some LED headlights interfere with some wireless bike computers- the computer receives the pulse from the pickup on the wheel, and also the radiated pulse from the light.
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Originally Posted by bragi "However, it's never a good idea to overgeneralize."
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Old 08-09-12, 11:27 AM   #14
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In order to make them brighter and yet not burn out from too much heat, some LEDs ARE pulsed... and you happen to be able to see the pulses. Some folks can, some folks cannot.

I too can see the pulses, just as I can see fluorescent lights flickering and incandescent lights blink, the latter under certain surge conditions (which "nobody" seems to hear/see). I don't believe we are insane or alien (but who knows).
Not alien. Fairies are of this world. I'd guess you are about 1/32nd fairie. The fair folk are of this world.
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Old 08-09-12, 01:00 PM   #15
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This

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Old 08-09-12, 10:18 PM   #16
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Ah, some of the pics I was talking about.... notice the lights in the trees:





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Originally Posted by bragi "However, it's never a good idea to overgeneralize."
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Old 08-11-12, 01:25 PM   #17
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Traditional analogue regulation of LED power was done through voltage-dividers and linear-regulators to present a fixed-voltage to the LED based upon its Vf and current-handling capacity. LED output is directly related to the current going through it and these specs are available in the datasheets.

Nowadays with digital APICs, the voltage doesn't matter as much. These controllers use PWM (like a fuel-injector circuit) that provides a variable duty-cycle signal to regulate current through the LED. A 7.2v battery-source may have a 10-20% duty-cycle signal to get maximum-output from a certain LED. With a 14.4v source, it may be 5-10% duty-cycle. A 4.2v battery may have 20-40% duty-cycle going to the LED when fresh, which increases to 40-80% as the battery is drained. At some point, the controller will do straight passthrough of the battery-voltage for 100% duty-cycle when the battery is close to depleted.

However, the frequency of the duty-cycle signal is typically in the 100khz range, I don't think the persistence of the retina can distinguish that fast of a signal. It's more related to the draining of chemicals in the rods & cones of the retina when the are presented with a certain high-strength signal. Kinda like the trails you see when you look at a bright-light on a dark background and you move your eyes (or looking at the sun). It takes some time for the chemicals to refresh to send another signal. When you move your head quickly, the spots will appear to be spaced further apart. When you move slowly, the spots are closer together.
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