The other day my son, age four, noticed that a string of lights were out on our Christmas tree. Last year, we switched to the new LED lights so I wasn't sure how long they should last or how to fix them.
I assumed that, like incandescent tree lights, a single failing LED would prevent the others from lighting. Unfortunately, we discarded the original box and therefore did not have any replacement bulbs (assuming that they came with some).
I picked up a new set of lights but apparently all "white" LEDs are not exactly the same - the new set had a bluer tint that didn't match the remaining sets on the tree. Oh bother, suddenly this became a "project". With the beauty of our tree at stake failure was not an option.
A Google search led me to this extremely informative web page. It turns out these LED lights use a much simpler circuit than I first imagined. No rectifiers, no smoothing capacitors - just 30 LEDs in series with a current limiting resistor. That explains the blinking that I sometimes find irritating.
My plan was to replace the faulting LED with one from the new set. First, however, I had to find it. I used a 9V battery and resistor to build a test rig, removed each LED, and tested it. Eventually, I found the broken one and of course it was the first one in the string that had the current limiting resistor soldered to it. So much for a quick replacement.
After a quick trip to the workshop to solder the resistor onto the new LED, we once again had a working set of really-white lights - except for one bluish one. Oh well, good enough is good enough.
An unexpected bonus from having one bluish LED is that my son now considers it a game to "find" the different one - it doesn't move but that doesn't appear to matter to a four year old :-).
