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I'm planning on building some dimmable lighting controller for Christmas lights, but with LEDs being all the rage nowadays, I was thinking that instead of using TRIACs and dimming at 60/120 Hz, I would rectify the AC to DC and use MOSFETs to reduce the strobing that is more evident in LEDs. This might allow the controller to be smaller and/or cheaper.

This would depend on how the LED strings are actually wired up. I don't currently have any to look at, and I could imagine a few different ways they might be connected, such as diodes that only conduct for half the cycle, which would axe the DC idea if true.

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    \$\begingroup\$ I think you need to dissect one, in order to find out what it is doing... \$\endgroup\$
    – Johan
    Commented Nov 11, 2010 at 19:47
  • \$\begingroup\$ In the past a lot of the technology behind different Christmas light displays has had patents to how they translate different independence from failure and lifetime/safety features. I know this may not be the case with most simple designs, but just dropping a warning from what I have seen. \$\endgroup\$
    – Kortuk
    Commented Nov 11, 2010 at 21:53

4 Answers 4

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LED Christmas lights are wired in series to "gobble" up the 110V applied to them. A great site to read up on this is here.

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  • \$\begingroup\$ Nice read. Was very surprised about the breakdown rate though! \$\endgroup\$ Commented Nov 17, 2010 at 13:35
  • \$\begingroup\$ So an antiparallel pair of LED strings--guess powering them off rectified AC won't work that well; only half will ever be on. \$\endgroup\$
    – Nick T
    Commented Nov 17, 2010 at 18:25
  • \$\begingroup\$ @Nick T: Over the last couple years, I've seen "color-changing" LED lights which have anti-parallel LED strings in different colors. The supply selects one or other polarity to light a particular string. I've been thinking it would be fun to have some processor-controlled TRIAC outputs so I could nicely sequence the lights in different colors, but I don't know of any off-the-shelf controllers that would allow one to dim the hot-positive and hot-negative phases separately. \$\endgroup\$
    – supercat
    Commented Mar 11, 2011 at 18:15
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These days the way to do this is to string together WS2812 neopixels. They require a 5v DC supply capable of providing 60mA per light. They need a third wire from the output of each neopixel to the input of the next. You feed a digital signal into the first one and that gets transmitted along the chain. You can program each neopixel independently with 256 levels of brightness for each of its three LEDs (Red, Green, Blue.)

You can buy pre-wired flexible strips of neopixels. Some versions are waterproof.

The control signal is complicated to program. Use a microcontroller like an Arduino or Raspberry Pi Pico

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  • \$\begingroup\$ This is a product recommendation and could have been turned into a comment. It has no wiring information or supporting schematics or a helpful link. Please wait until a few comments are received and then modify your answer accordingly. \$\endgroup\$
    – Syed
    Commented Dec 11, 2021 at 18:26
  • \$\begingroup\$ @Syed I think this is the only reply pointing in the right direction and the product mentioned ( or compatible, I guess ) seems to be the one used everywhere, so better than a comment this should be the accepted answer. \$\endgroup\$ Commented Dec 11, 2023 at 10:31
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There are far less complicated ways to achieve what you want to do at a reasonable cost. There was a recent article on Planet Christmas about pixel lights where each individual LED is controllable. The WS2812 light string mentioned above is one example. I recently came across two 5 meter WS2811 ribbon strings at a local second hand store. They operate on 12V DC. I bought a controller on eBay for $12 that works with WS2811 and WS282B. It has 160 built in sequences on a remote control plus it has the option for four DIY programs. It also works with voice control by connecting to Alexa or your smart phone using something called the FairyNest app. It's pretty amazing for something with so many options. I tested it with the canned applications and was very happy with just those. It will control up to 900 pixels and has two separate outputs. You can search "WiFi Music LED Controller For WS2812B WS2811 Addressable RGB Pixel Lights" for similar controllers.

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I have been doing the following:

  1. Most standard LED strings work normally if I load the controller with one incandescent load. I used two 7 watt nightlight bulbs wired in series for a load (total load 3.5 watts). It works on most controllers. Increase the wattage if it doesn't work. I have one controller with two 15 watt bulbs in series. I put the lights in series so they don't burn out.

  2. A few controllers I have produce a rectified DC output. When used with my incandescent load, This lights half of the lamps in some strings and all of the lamps in some other strings. The strings that light only half the lamps have two series of lights, one connected in each direction.

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