LEDs are very often pulsed. There are two reasons for this.
- 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.
- You can make Christmas lights really really cheap by driving them with AC.
Generally, you don’t want to drive LEDs from a voltage source. As the voltage increases, LEDs tend to go from too dim to see to being too bright to the point where the LED self destructs over a very narrow voltage range. Instead, LEDs are generally driven from a current source. Batteries and your AC outlet are both voltage sources. If you want to have a current source, you have to make one out of a voltage source with a current driving circuit. This adds expense.
For Christmas lights, someone got the bright idea that if you string a bunch of LEDs in series, the voltage across each LED will be the total voltage divided by the number of LEDs. With a large voltage and a large number of LEDs, you can fairly precisely determine the voltage across each LED by adding or subtracting LEDs to your strand. This maybe isn’t the best way to drive LEDs since if one LED happens to fail short, the rest of the LEDs in the strand could also be damaged due to the overvoltage across them. However, it does mean that you can make LED Christmas lights using LEDs and absolutely no external driving circuitry, which makes them very cheap to make.
Your Christmas LEDs only conduct current in one direction, so they flicker at the power line frequency (60 Hz in the U.S., 50 or 60 Hz elsewhere depending on where you are). The LEDs will be on during the half of the AC cycle where the LEDs end up with a forward polarity across them, and will be off during the second half of the AC cycle when the voltage across them will be reversed.