I assume they’re very similar to wind up ones, but how does the cord work. Where is it stored? How does it avoid getting twisted or knotted up? Also as they’re often used in children’s toys how are they waterproof?
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With a wind up music box, you put energy in by turning the key, which pulls the spring which then works the mechanism. With a pull cord one, you put in energy in by pulling the cord, which pulls the spring…
The cord is wound around a bobbin inside it, so it doesn’t knot.
Both types of mechanism wind up a spring. The spring is a coil of metal that is fixed in place at one end (e.g. to the outer casing of the toy) while the other is attached to a rotatable axle. In the case of a wind-up mechanism, there’s a wind-key that you can slot into an opening at one end of the axle, or which is permanently affixed to it, and you wind up the spring by rotating this key. In the case of a pull cord, the cord winds around a spool attached to the axel. When you pull on the cord, the cord unspools off the spool and rotates the axel, thus again winding up the spring.
The cord comes out of a small hole in the outside of the toy that’s placed directly in front of the spool, and the spool has sufficiently large flanges on either side, so that the cord cannot slip off – it doesn’t have sufficient range of motion for that. It can only be spooled or unspooled. So that’s how it avoids getting knotted or twisted.
As for their being waterproof, since these music boxes are purely mechanical, there aren’t typically any components that are very sensitive to water, like you would have in an electronic device (which can short out and fry its circuits).
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