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If you carefully look at LED in flashlights, they are blinking at fast rate despite the batteries are DC. It's even more obvious on flashlights with separate R,G,B LED (to allow the control of color) when you move it fast. Why is it so ?

( It's also the case for "power on" indicators in many devices, but here it could just be a low quality DC low voltage from AC 220V. )

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The basic problem is that the power source is not matched to produce exactly the right voltage and current that the LED takes to put out the light you want. Some conversion is required between the power source and the LED.

Converting DC at one voltage and current to another voltage and current is done most efficiently with pulses. It isn't too hard to smooth out these pulses by the time they get to the LED, but it does take a few extra parts and cost a few extra cents.

In high volume mass market products, those few extra cents add up. People are used to LEDs flashing, so the manufacturers don't feel much push to prevent the flashing. Put another way, way more people are going to buy on price than whether the light flashes a bit. Most people don't even think of looking at the latter, and it's rarely featured, so it's not a consideration in purchasing.

The extra components not only add a little cost, but also weight and they take space. All around, it's just not worth it when selling to the average consumer.

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The reason LED's are blinking is because they use Pulse Width Modulation (PWM) power the LED. Using PWM, the LED are switched on for a specific period of time and switch off rest of the time. The reason as suggested is Efficiency.

  • Over a period of time LED used less power, or in other words less average current
  • The LED dissipates less heat, thus will degrade slow.

There reason that you are notice the blinking is because the duty cycle in your flashlight might be pretty low. Chances are if the LED is switched on around 200Hz, most humans will not notice blinking. To learn more I suggest investigating Persistence of vision and Flicker fusion threshold

Here is an example of a 50% duty cycle PWM signal

PWM Signal

References:

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  • $\begingroup$ Thanks! all this is very interesting, even way beyond my initial question. There is still one point that puzzles me: ok for the cheap adaptation to voltage + the cheap modulation of brightness. But when in comes to flashlight trying to provide the maximum visibility for the user, do you mean that human visual system accuracy really take advantage of the peak brightness, and not the average ? $\endgroup$ – Fabrice NEYRET Oct 9 '16 at 8:14
  • $\begingroup$ Sorry I don't understand your comment. I think the human visual system adapts to the situation. $\endgroup$ – Mahendra Gunawardena Oct 9 '16 at 10:30
  • $\begingroup$ Then were is the efficiently, beside the simplification of the electronics ? In his answer, Andrews raises something about perceptual efficiency, human eye perceiving the flashing light as brighter than just the constant average would. It's the part I'm curious about... if you agree with this effect. $\endgroup$ – Fabrice NEYRET Oct 9 '16 at 12:40
  • $\begingroup$ PWM, uses less current, thus longer battery life, also low current mean lower heat dissipation, so the electronics will last longer, thus is the efficiency. I suggest you read the second link Lower Power consumption of additional detail. $\endgroup$ – Mahendra Gunawardena Oct 9 '16 at 13:51
  • $\begingroup$ I did. For comparison, "efficiency" makes sense for equal perception. (Otherwise we dimm even more the light and save even more :-) ) The point is : how is perceived a 100mA lighted 10% of the time ? same as a 10mA lighted 100% of the time ? or more ? in the latter case, how much, and why ? $\endgroup$ – Fabrice NEYRET Oct 9 '16 at 17:06

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