Tim’s blog is somewhat drawn to clever and minimalistic implementations of consumer electronics.
The festive season is a boon to that, as we are bestowed with the latest innovation in animated RGB Christmas lights. I was obviously intrigued, when I learned from a comment on GitHub about a new type of RGB light chain that was controlled using only the power lines. I managed to score a similar product to analyze it.
All 100 (up to 300 in larger versions) LEDs are connected to the same two lines. These types of strings are known as “copper string lights”. It’s obvious that it is easier to change the LED than the string manufacturing process, so any improvement that does not require additional wires (or even a daisy chain connection like WS2812) is much easier to introduce.
Luckily, with only two wires involved, analyzing the protocol is not that complex.
The string is controlled by pulling the entire string voltage to ground for short durations of time (“pulses”). This is very simple to implement, but requires the LED controller to retain information without external power for a short time.
We can directly read from the string voltage whether LEDs are turned on or off.
The first half of the sequence obviously turns all LEDs off (indeed, the string flickers when changing color settings), while the second half of the sequence turns all LEDs on with the desired color setting.
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