# Do all LED christmas lights have a strobe-like flicker? **URL:** \ **Category:** Factual Questions\ **Created:** [October 25, 2010, 9:07pm UTC](https://boards.straightdope.com/t/do-all-led-christmas-lights-have-a-strobe-like-flicker/558333 "2010-10-25T21:07:14Z")\ **Posts on this page:** 14\ **Page:** 2
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [October 26, 2010, 11:12am UTC](https://boards.straightdope.com/t/do-all-led-christmas-lights-have-a-strobe-like-flicker/558333/21 "2010-10-26T11:12:50Z")
> [@elfkin477](#): > > Is it common for people to not notice that they flicker? I complained about this to a few people I know last winter, and no one else noticed that they flickered, even while looking at them. LED flicker (for me, at least) is far more noticeable in peripheral vision and when my eyes are moving across them. ---
**Author:** ![Fridgemagnet](https://avatars.discourse-cdn.com/v4/letter/f/22d042/32.png) [@Fridgemagnet](https://boards.straightdope.com/u/Fridgemagnet)\ **Post date:** [October 26, 2010, 11:19am UTC](https://boards.straightdope.com/t/do-all-led-christmas-lights-have-a-strobe-like-flicker/558333/22 "2010-10-26T11:19:34Z")
> [@elfkin477](#): > > Is it common for people to not notice that they flicker? I complained about this to a few people I know last winter, and no one else noticed that they flickered, even while looking at them. Yeah, some people are more sensitive to light flicker than others, and even incandescent light bulbs will flicker a little. Curiously, the brain can train itself to ignore certain flickers, such as those due to the limited refresh rate of a TV screen. US citizens used to a 60 Hz (after interlacing) refresh rate might notice flicker on a European TV system running at 50 Hz, whereas a European native wouldn’t see a problem. Even more curiously, when viewing a cathode ray tube (CRT) TV the brain is used to the raster scan going from top to bottom, and if the screen is inverted then the image may start flickering quite visibly as the scan direction is effectively reversed. Try it - if you’ve got a CRT TV, put it on and stand on your head. Alternatively, watch TV exclusively upside-down for a few weeks, then try it the right way up. ---
**Author:** ![standingwave](https://avatars.discourse-cdn.com/v4/letter/s/9de0a6/32.png) [@standingwave](https://boards.straightdope.com/u/standingwave)\ **Post date:** [October 26, 2010, 11:20am UTC](https://boards.straightdope.com/t/do-all-led-christmas-lights-have-a-strobe-like-flicker/558333/23 "2010-10-26T11:20:37Z")
> [@elfkin477](#): > > Is it common for people to not notice that they flicker? I complained about this to a few people I know last winter, and no one else noticed that they flickered, even while looking at them. In another life, I designed CRT monitors (when we still made such things in this country) and yes, sensitivity to flicker varies widely. Some people won’t even detect a flickering source that would given others a migraine. I myself am not particularly sensitive but trained myself to see it by learning to look for it with my peripheral vision. Like usiing [averted vision](http://en.wikipedia.org/wiki/Averted_vision) on a telescope. ---
**Author:** ![pyromyte](https://avatars.discourse-cdn.com/v4/letter/p/d26b3c/32.png) [@pyromyte](https://boards.straightdope.com/u/pyromyte)\ **Post date:** [October 26, 2010, 1:57pm UTC](https://boards.straightdope.com/t/do-all-led-christmas-lights-have-a-strobe-like-flicker/558333/24 "2010-10-26T13:57:09Z")
> [@engineer\_comp\_geek](#): > > LEDs are very often pulsed. There are two reasons for this. > > 1. It makes them appear brighter. > > If you have an LED that is rated for 30 mA, you can drive it with 50 mA pulses which exceed the maximum current for the LED. However, as long as the pulse frequency is pretty fast and the average current (averaging 50 mA and 0 mA for when the pulses are on and off) doesn’t exceed 30 mA then the LED will be just fine. > > Your eyes are kinda quirky. Even though both LEDs are putting out the same average amount of light, the one that is being pulsed will seem brighter to your eyes than the one being driven by a constant current. > > Not only do Christmas lights take advantage of this, but other LED devices like LED clocks do this as well. > [@jz78817](#): > > automotive tail lights too. Don’t they do this to make the dimmer too? I am thinking of one of the Cadillacs with LED tail lights. If i remember correctly, I see the flicker when the tail lights are dim, but when the brakes are applied the flicker goes away. I had just assumed they drove the LEDs with constant freq. square wave and just adjusted the duty cycle (kind of like PWM) to adjust the brightness. So when the brakes are on, the duty cycle = 100%. The reason I am asking is because the flicker is slightly distracting and I wondered if they could eliminate it by increasing the frequency, but keeping the duty cycle (and thus the brightness) the same?? ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [October 26, 2010, 3:28pm UTC](https://boards.straightdope.com/t/do-all-led-christmas-lights-have-a-strobe-like-flicker/558333/25 "2010-10-26T15:28:41Z")
you could hang a phosphorescent glow in the dark thing on the pull chain. not to electrical code for the room but better than nothing. leave a flashlight at the door way. use a 40W bulb in the fixture to prevent over brightness. if the fixture is mounted to wall or ceiling you might be able to place a screw-in PIR (infrared) switch which switches on when detecting movement and can switch off after a few minutes. ---
**Author:** ![ZipperJJ](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zipperjj/32/211_2.png) [@ZipperJJ](https://boards.straightdope.com/u/ZipperJJ)\ **Post date:** [October 26, 2010, 3:34pm UTC](https://boards.straightdope.com/t/do-all-led-christmas-lights-have-a-strobe-like-flicker/558333/26 "2010-10-26T15:34:07Z")
Hey **xanthous** …is there a plug inside the bathroom? Not sure if you are plugging the string of lights inside the bathroom or outside… But if there is a plug inside, you can get a motion sensor night light for not too much money. It will pop on when you walk towards the bathroom and stay on while you’re in there - then turn off when you leave. I got one at Wal Mart. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [October 26, 2010, 3:53pm UTC](https://boards.straightdope.com/t/do-all-led-christmas-lights-have-a-strobe-like-flicker/558333/27 "2010-10-26T15:53:28Z")
> [@](#): > > Don’t they do this to make the dimmer too? I am thinking of one of the Cadillacs with LED tail lights. If i remember correctly, I see the flicker when the tail lights are dim, but when the brakes are applied the flicker goes away. yes. see, the thing is LEDs shift color when you change the forward voltage, and the actual shade of red for tail lamps/brake lights is a regulatory requirement. So in cars the LEDs are driven by a pulse-width-modulated (PWM) power supply, and to dim them they simply reduce the duty cycle. Which has the effect of making the flicker more noticeable. When the brakes are applied, the PWM duty cycle is increased (though I don’t think it’s ever 100%) and the flicker is less perceptible. ---
**Author:** ![kayaker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kayaker/32/441_2.png) [@kayaker](https://boards.straightdope.com/u/kayaker)\ **Post date:** [October 26, 2010, 4:02pm UTC](https://boards.straightdope.com/t/do-all-led-christmas-lights-have-a-strobe-like-flicker/558333/28 "2010-10-26T16:02:32Z")
[Clap on. . .clap off. . .](http://www.asseenontv.com/prod-pages/theclapper.html) ---
**Author:** ![Shadowfyre](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/shadowfyre/32/9972_2.png) [@Shadowfyre](https://boards.straightdope.com/u/Shadowfyre)\ **Post date:** [October 26, 2010, 5:57pm UTC](https://boards.straightdope.com/t/do-all-led-christmas-lights-have-a-strobe-like-flicker/558333/29 "2010-10-26T17:57:22Z")
So, couldn’t they reverse every alternating LED so it is coming on just as the other one is turning off? Would that even out the flicker? Would that make the lights seem dimmer? ---
**Author:** ![hst101rox](https://avatars.discourse-cdn.com/v4/letter/h/ee7513/32.png) [@hst101rox](https://boards.straightdope.com/u/hst101rox)\ **Post date:** [May 24, 2015, 2:18am UTC](https://boards.straightdope.com/t/do-all-led-christmas-lights-have-a-strobe-like-flicker/558333/30 "2015-05-24T02:18:19Z")
> [@jz78817](#): > > automotive tail lights too. Is that why when you see angel eyes on newer cars on a Youtube video (filmed), they appear to be flickering? I have some aftermarket brake light LEDs that don’t flicker through a video camera. [![](https://img.youtube.com/vi/EdMe5o3Qgx0/maxresdefault.jpg "Ferrari 488 GTB & 488 GTS Spider in Maranello!") ](https://www.youtube.com/watch?v=EdMe5o3Qgx0) ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [May 24, 2015, 9:25am UTC](https://boards.straightdope.com/t/do-all-led-christmas-lights-have-a-strobe-like-flicker/558333/31 "2015-05-24T09:25:28Z")
Surely the simple (and in-code) answer is to install a PIR. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [May 24, 2015, 11:08am UTC](https://boards.straightdope.com/t/do-all-led-christmas-lights-have-a-strobe-like-flicker/558333/32 "2015-05-24T11:08:52Z")
You want no flicker, get a 12 volt DC wall wart, and run the LEDs in parallel groups of 4 with a current limiting resistor. That’ll get you the forward voltage you need and flicker free operation. Be certain the wall wart is rated for the wattage of however many LEDs you use. You _could_ use a 24 or even 120 volt DC supply to put in more LEDs per circuit, but those can be hard to find, and _dangerous_ at the higher voltages. ---
**Author:** ![Zach29](https://avatars.discourse-cdn.com/v4/letter/z/e0b2c6/32.png) [@Zach29](https://boards.straightdope.com/u/Zach29)\ **Post date:** [January 23, 2016, 1:37am UTC](https://boards.straightdope.com/t/do-all-led-christmas-lights-have-a-strobe-like-flicker/558333/33 "2016-01-23T01:37:36Z")
The cheap LED string lights have terrible 60 Hz flicker. It’s an annoying pulsating effect, distracting and maybe even slightly nauseating. Better LED string lights are bridge rectified, so have 120 Hz flicker. This is mostly not very noticeable, but when you suddenly dart your eyes across from one end of the room to the other, there is a perceivable strobe effect. Some people find this very slightly annoying. There have been all sorts of studies trying to examine whether 120 Hz flicker in work environments could affect people subconsciously, possibly causing more difficulty concentrating, but the effect is very small. When it comes to LED bulbs, if you were curious, about a third of them being sold now have 120 Hz flicker and the rest essentially have no flicker. They usually do not bother putting in the electronics to eliminate flicker in LED Christmas lights because it would be more expensive to do so (remember all these string lights are made in China), and string lights are usually more for decorative purposes so it is assumed that the light is not as important. ---
**Author:** ![fubar.gr](https://avatars.discourse-cdn.com/v4/letter/f/898d66/32.png) [@fubar.gr](https://boards.straightdope.com/u/fubar.gr)\ **Post date:** [January 25, 2016, 2:18am UTC](https://boards.straightdope.com/t/do-all-led-christmas-lights-have-a-strobe-like-flicker/558333/34 "2016-01-25T02:18:13Z")
> [@engineer\_comp\_geek](#): > > The voltage has to be fairly precise when you drive LEDs this way. At 2.2 volts per LED, the LEDs may appear very dim (maybe half of their max brightness) and at 2.6 volts the LEDs could overheat and go _POOF_! > > When you are designing your lights though, because you have so many LEDs, you can fairly precisely determine the voltage across each by the number of LEDs that you use. Adding one more LED to your 50 LED chain would reduce the voltage across each from 2.4 to 2.35 volts, so by adding or subracting LEDs you can get the fairly precise voltages required to drive LEDs this way. Actually when you are stringing LEDs in series like that, the VI curve of the resulting string will be much shallower than that of each individual LED, so you end up with a wider useful voltage range. Let’s suppose that each LED can be driven from 2.3 to 2.5 volts. That’s a 0.2 Volt range. Add another LED in series. Now the drive voltage can be anywhere from 4.6 to 5 Volts. That’s a 0.4 Volt range. String 10 of them in series and now you have a very healthy 2 Volt range to play with. Adding a small value resistor in series will make the VI curve even shallower and more manageable. [Previous page](https://boards.straightdope.com/t/do-all-led-christmas-lights-have-a-strobe-like-flicker/558333.md?page=1)