David writes in –

Here’s page & description of how LED Christmas lights work. I found this site while searching for a way to repair a string of LED lights. This answered a lot of questions I had, like why the LED lights flicker, and why there is no obvious transformer or voltage converter in the string.
They also have a page on incandescent Christmas lights and how to troubleshoot/repair them.

LED Christmas Lights and how to fix them – Link.