TapChristmas Lights

Yes, a sudden return to writing. I don’t know if I will keep this up.

I have often wondered how Christmas tree lights manage to display flashing patterns with only a few wires. In the old days of filament bulbs the cables were thick and heavy bundles of wires which weighed the tree branches down. Some bulbs were attached to to one string and others to different ones and the stings woven together to form the bundle. It addition, for fixed lights, the low voltage bulbs were connected in series usually 40 x 6v connected to a 240v mains supply. I haven;t looked at chaser bulbs to see if those were the same.

Now with LED bulbs there are many fewer wires—usually three or sometimes only two. So checking out a short set of two wire lights from our door wreath before putting them away for the year I took some measurements. This runs on 3 x AA cells so between about 3v and 4.5v supply. Getting a steady display with all lights on revealed an AC signal of about 2v. Looking at this on my little toy oscilloscope I could see that this is a 500Hz square wave. To get alternate lights on used a DC signal and the oposing alternate lights were obtained by the opposite polarity.

So that is how they work. The LEDs are all wired in parallel across the wires but alternate LEDs are wired the opposite way around so a +ve voltage lights up one set and a -ve voltage lights the other set. An alternating voltage of sufficiently high frequency appears to light both sets at once. By using 500Hz it avoids any apparent flicker. LEDs are not very tolerant of reverse current but presumably they are sufficiently under-run so as not to damage them.

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