1-wire fairy lights

Question

I have here some fairly lights. It's a string of coloured LEDs with a transformer and control box at one end. The control box lets you select a bazillion different lighting patterns. (Presumably the box contains a small microcontroller. I suspect it uses PWM to dim the LEDs.)

There are 4 colours of LED, but they always light in pairs. This leads me to believe they're wired as two independent circuits. Presumably wiring each colour individually would use twice as much wire, which would eat into the manufacturer's precious profit margins.

This got me thinking... Is there some way you could connect all the LEDs to a single circuit yet still be able to control them independently? In particular, I'm wondering if you would connect each LED across the power rails in series with some sort of trivial passive band-pass filter. That way, the microcontroller can generate some complex multi-band waveform, but each LED only receives a narrow(ish) frequency band to light it.

Basically, I'm wondering if something similar to this could be made to work:

^{simulate this circuit – Schematic created using CircuitLab}

Where each colour of LED has a band-pass filter tuned to a different frequency. (I gather capacity on its own isn't really a functioning filter...)

To be clear, I doubt this would end up actually being any cheaper to manufacture. But I'm wondering if it's technically possible. (I'm also not intending to try to make it, just figure out what the schematic would look like.)

Answer 1

One wire, are you sure? "Addressable" LEDs are becoming rather common and this might be what you are describing. This is where each "LED" includes a red, green, and blue LED inside, along with a serial interface. This uses four wires typically. The serial data piped into one LED is "daisy-chained" to the next, all the way to the end of the array. This data transfer typically happens very fast - hundreds of times per second, so isn't noticeable except for very long runs. The end result is "each LED can be a different color and they all can change in any way imaginable," depending on how "smart" the controller is.

In terms of datasheets, here is a 5mm WS2812 addressable LED module:

Answer 2

For two LEDs, on two wires

^{simulate this circuit – Schematic created using CircuitLab}

With LEDs attached as above, you can illuminate the red ones by applying a voltage in one direction, and the green ones by applying a voltage the other way. Just make sure you choose LEDs with which can safely withstand a reverse voltage sufficient to light the other type. If the LEDs have different forward voltages (and they probably will) you'll need series resistors on each one, or you'll need a current drive rather than a voltage.

If you want both on, switch back and forwards too fast for the eye to see.

You can even get a single IC containing the transistors necessary to switch the voltage on and off, backwards and forwards. It's called a H-Bridge, and they are typically used to control DC motors.

Answer 3

This is an old question, but as I had the same idea a few years ago I'll answer.

The basic idea is that each LED has a filter tuned to a different frequency range. To light any combination of LEDs you send the matching frequencies.

When I had the idea, I drew a simple LC filter and graphed the frequency response for a selection of component values. What I found was that the frequency response for each filter was not very sharp, and there was significant overlap even with the capacitor values spaced one decade apart. I concluded that for more a handful of channels some of the capacitors would be awkwardly large. Perhaps the non-linear response of the LEDs would mitigate this somewhat. I'd need to build it!

I can't find the files I made, but I think my proposed circuit would've looked something like this:

^{simulate this circuit – Schematic created using CircuitLab}

The idea is that each capacitor and inductor pair form a band-block filter that shorts out the LEDs at frequencies outside the relevant band. The resistor would be replaced with a constant current sink.