Sunday, January 3, 2010

Raindrop Christmas lights

During a recent visit to the malls I saw what I would describe as raindrop type Christmas lights. Instead of bulbs there were these 5-inch transparent glass or plastic tubes which housed white LEDs. A cable ran from tube to tube. The LEDs started lighting from the top and simulated a falling raindrop. One LED would light, then two, then three, ... By the time the fifth LED was on, the first was off. The sixth would light and the second go out. So there was the illusion of a segment of four LEDs racing down. This went pretty fast--some two seconds.

I didn't bring my camera and I completely forgot that my phone has a camera and so I don't have a picture to show. I googled "raindrop christmas lights" and fortunately found this.

As far as I can see from this photo this tube has around twice more LEDs than the one I saw which had around 15 to 20 of what looked like 5mm white LEDs.

Each tube has its own controller chip since the tubes weren't lighting up at the same time. More evidently, there was no big cable running from tube to tube which you'd expect if these tubes were centrally controlled--what a cabling nightmare!

What intrigues me is the smallness of the unit and what must be a limited number of parts on that long but very narrow board. I can't imagine not having some cap for filtering/bypassing so it must have high value ceramics and/or tiny electrolytics. I presume that the cable is merely for 3.3 to 5V power. And the resistors for the LED? Chip resistors? Could these LEDs have internal resistors?

As for the controller chip, I'm wondering if they're using an MCU or some other programmable IC. Could each LED be switched by a dedicated pin? Or are they charlieplexed/multiplexed?

In the lights that I saw the last LED at the bottom remains lit for about a second and a half, fading out till it's off. Beautiful effect. PWM is being used of course.

This is a most interesting product and I'd love to take one apart and see its design. It can easily be effected using an MCU. Firmware is simple. Engineering a board this compact is the bigger challenge.


  1. if you discover how to make rain drop light,pls. send me email how to make rain drop xmas light ,very beatiful

  2. I want them! I see them in Sasebo, Japan and can't find them anywhere