PWM Christmas icicle lights, how do they work? - Page 1
Forum section: electronics / projects

author: Tracker
author_rank: Regular Contributor
timestamp: December 06, 2021, 01:10:56 pm
content: Hi Folks,Ti's the season for Christmas lights.I was looking at how LED icicle lights operate.The transformer unit provides a 60v p-p PWM.This gets sent along two wires to the 'array' of led lights.50% duty cycle results in 'all on' mode.Other duty cycles give various flashing cycles.The Array of LED are wired in some weird arrangements which I have yet to figure but I suppose it must be something like this...I would think that there are series groups of LED 's which are across the 60v p-p, but there myst also be some delay line function at play here, otherwise all groups would illuminated at once.There is certainly enough wire in the array.Has anyone got more insight?Just curious to know how they work.Ps, there is no circuit board or components in the array, just yards and yards of wire and LED's.

author: BradC
author_rank: Super Contributor
timestamp: December 06, 2021, 02:28:47 pm
content: Are you sure it’s PWM DC? Some I’ve seen are broken into two groups with reversed polarity, so PWM AC allows independent brightness control over each group.

author: Tracker
author_rank: Regular Contributor
timestamp: December 06, 2021, 04:48:09 pm
content: Yes Brad you could be right.The wall wart device says 30v, but I measured 60v p-p on my scope... but that was in AV mode... so quite likely its positive and negative PWM.The bit I'm amazed about is that 30 ft of wire can act as a delay line to target specific LED groups, very clever!

author: thm_w
author_rank: Super Contributor
timestamp: December 07, 2021, 12:07:38 am
content: Photos/video?Clive seems to have done 5V sets I don't recall 30V+

author: james_s
author_rank: Super Contributor
timestamp: December 07, 2021, 12:11:56 am
content: Quote from: Tracker on December 06, 2021, 04:48:09 pmThe wire isn't a delay line. It's possible these are some sort of addressable LEDs with an IC in each LED.

author: BradC
author_rank: Super Contributor
timestamp: December 07, 2021, 02:11:08 am
content: I have a couple of other sets which use an even stranger technique. Each led has a completely stand alone pattern programmed in. The pattern runs for about 2 minutes and when it’s completed the led turns off. On power up they all start up and when a majority of them have completed the patterns, there is a tiny brick in the power line that detects the drop in current consumption and power cycles the string, repeating the process.Some leds run marginally faster than others giving a variation across the string. Very clever really.

author: Tracker
author_rank: Regular Contributor
timestamp: December 07, 2021, 10:56:24 am
content: Quote from: BradC on December 07, 2021, 02:11:08 amIndeed you are correct, no delay lineI did some more digging, which consisted of stripping a set down and removing the shrink wrap covering the LED's.There are no resistors or chips, just two LED's groups in series and parallel, taking advantage of the positive and negative PWM.The effects achievable are quite impressive given the simplicity of the circuit.If A is the Positive wired Led and B is the Negative wired led, the construction is as follows:The vertical icicle strings are parallel groups of A B A B A B A B etcThese icicles strings are in series groups of 10, thus applying 3 v to each parallel string.Then the pattern repeats these groups of 10 icicles along the main string.No delay line, just clever interleaving of two led groups.