Control 2 sets of lights using the same 2 wires?!

[star-art]

If you’re the type of person who likes to add “blinky” lights to your projects, here’s a clever little brain-teasing puzzle:

How can you make alternating flashing lights using only 2 wires?

Conventional thinking would suggest you need 3 wires to make this work. But, recently I purchased some LED lights for a project that proved otherwise.

I was making a custom little desktop Christmas tree and I purchased what are called LED “fairy lights.” They are made from tiny little SMD LEDs that are all wired in parallel. And yet, somehow, the controller for these lights was able to make them light up in many different patterns -- including alternating flashing!

For the longest time I just could not figure out how they controlled two different circuits with only two wires. Then, it finally dawned on me.
Each LED is a diode meaning current will flow in only one direction. If you reverse the current the LED won’t light. So, what if you were to wire every other LED backwards?

This way, if you were to apply direct current in one direction, only half of them would light up. But, if you were to apply current in the *opposite* direction, the rest would light up. To get all the LEDs to light up at the same time, you would need AC current.

So, it turns out it is indeed possible to control each set of lights independently. You can also turn them all on at the same time -- all using just one pair of wires. Clever!

[tetsujin]

There were (still are, I guess) red/green dual-color LEDs that worked on this kind of principle.

At one point I was exploring ways to computer-control individual LEDs in a model, which led to exploring the signaling systems used by model railroad setups for instance. (i.e. you have two "wires" - the rails, and you can individually control speed of multiple locomotives, turn on/off special effects and sounds, etc.) I don't remember the particulars of those systems but basically you'd alter the state of the lines in a way that would deliver data but still provide power (for instance, flip the polarity of the lines to produce a data signal, something like that) - then the individual devices would need to be able to process the data coming in and simultaneously get the power they need