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I'm curious about the Motorized Lighthouse (21335):

Set photo

As the title suggests, this set is motorized to allow the light on top to rotate continuously:

Continuous rotation animation

From the looking at the inventory, it is clear that it uses the Powered Up lights:

Power Up Lights

How does the wire not get tangled/twisted while the top of the lighthouse rotates?

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This required a clever design and one novel element.

The power for the light comes from the very bottom of the model where a battery box is stowed inside the mountain:

Step 60 (Book 1, page 64)

The wire leaves the mountain via the top and heads upward.

Step 210 (Book 1, page 192)

It then snakes its way around and up into the tower. Interestingly, the Powered Up lights include dual LEDs, as they are often used for headlights. Only one is needed for main light, so the other is used to add a glow to the woodstove in the cabin.

Step 309 (Book 2, page 38)

Up at the top, the light is attached facing upward underneath a construction that will be powered by the motor in the bottom of the lighthouse via a long vertical axle.

Step 488 (Book 2, page 141

We need the light traveling horizontally, so we add a stickered mirror at roughly 45 degrees to vertical:

Step 494 (Book 2, page 144)

This would sort of work, but the light from the Powered Up LEDs disperses significantly and is not able to travel very far. In real lighthouses, a Fresnel lens if often used to appropriately direct the beam from a diffuse light source. The basic operation can be seen here:

Fresnel lens diagram

TLG saw fit to create a Fresnel lens element to enhance the performance of this set. Here are some of the prototypes that folks iterated through:

Prototype lenses from set instructions

The final version is a unique element with the footprint of a 2x4 plate. It attaches to our mirror assembly providing light that is now traveling in the correct direction and is more focused than it would have otherwise been.

Step 496 (Book 2, page 144

This assembly is then attached to the top of the lighthouse and driven by gear so that it is able to rotate:

Step 497 (Book 2, page 145

In this way, the mirror and lens assembly is able to rotate without the wired lights rotating and creating twists and tangles.

Lightroom animation

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    Possibly of note, they theoretically could have created a new slip ring part to allow the LED itself to be rotated, but such a part would have been far more expensive to produce, as well as being far less reliable than the approach used, and even then they would probably still need the Fresnel lens piece to make it project the light well.
    – Austin Hemmelgarn
    Dec 13, 2022 at 15:45
  • +1 Only tangentially related but a nice image: What is a 3rd-order Fresnel lens?
    – uhoh
    Dec 14, 2022 at 10:59
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    @AustinHemmelgarn perhaps much easier would have been to rotate the battery box along with the light!
    – user253751
    Dec 14, 2022 at 16:59

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