Thinking about sensory rooms, I was imagining that it could look pretty cool if there was a trail of light that zipped from the switch to the device or devices being controlled once the child activates the switch. Then I thought about even bringing that outside of the sensory room, a switch cable that you could use with any switch-adapted device and any switch. Here’s a quick demonstration of the idea:

This setup, a switch cable running in parallel to an LED strip, worked but was a little clunky. The increased thickness made the resulting cable very stiff and needing a way to keep the two cables together throughout the whole length made it a little less robust.

I wanted to work out a way to use the LED strip itself as the switch cable also. So instead of a solid-state relay (SSR) at the switch end, I moved it to the plug/toy end. Then I just needed a way to trigger it through the LED strip. I wanted to treat it just like another addressable LED in the strip that I could trigger when desired. I could have setup an MCU to sniff the signals coming out of the last LED and triggered off of that. But it turns out that the LED driver chips (WS2811) used in these strips are readily available so I started testing with those to drive the SSR as a last LED.

It turns out however that even when set to full “bright”, this driver still has a short off time at the 400 Hz PWM frequency. So the full circuit would need to be the WS2811, the SSR (VO14642), and an RC circuit to smooth the output. Note that the WS2811 outputs can only sink current.

Here was the first breadboarded prototype to test the idea:

After getting that working I laid out and milled the small PCB I wanted to use. Then I populated the components and soldered the tiny board in its place after the last LED and before the switch plug end. Here I show it before applying heat shrink for strain relief and electrical insulation.

Here’s another demonstration video of the cable and electronics before placing inside an enclosure: