Electrical engineer here. I'm looking to build an incredibly simple mechanism that in my mind involves kind of a pulley. Below is a diagram.

enter image description here

So it's essentially like a mechanical servo. I would like to be able to pull something, and cause another body (attached to an arm) to rotate. However, I don't know what kind of mechanism or part I need.

I want to build this. And I'm talking real simple here - like household items (popsicle sticks!) or hardware store (though I do have access to a 3d printer).

Any ideas would be thoroughly appreciated!

  • $\begingroup$ Can we rely on gravity to return the arm back down when you let go of the string? $\endgroup$ Oct 24, 2019 at 14:48
  • $\begingroup$ @JonathanRSwift yes $\endgroup$
    – Vanush Vee
    Oct 24, 2019 at 22:17

1 Answer 1


The simplest way to achieve this in my mind, is with two gears - one should have a drum attached with the rope wound around, and the other has the arm attached.

One benefit of this method, is that you can have the gears sized such that pulling the string a long way causes the output arm to rotate more slowly than the input gear - this will give you more control than a simple lever. You can add more gears to the chain, or attach a drum that is much larger than the initial driving gear to give you even more advantage - let me know if this is something that you would like to explore and I'll add more details - I'm in a hurry at the moment, sorry!

gear diagram

You can make gears out of popsicle sticks, but give you have access to a 3D printer, you'd be pretty crazy not to use it in my opinion!

Popsicle stick gears

  • $\begingroup$ Cool. But will the arm go back down after I release the tension? $\endgroup$
    – Vanush Vee
    Oct 24, 2019 at 22:53
  • 1
    $\begingroup$ If you only lift it as high as in your original diagram, gravity will do it. If you want to go further, add a torsion spring, or a constant force spring - you can steal one from inside a steel tape measure $\endgroup$ Oct 25, 2019 at 5:41

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