I want to build a small car that is about 200 grams. I want it to run the following course and stop at exactly or about the finish line. I can't use any controlling devices or electronics.

enter image description here

I've an idea to count the number of revolution the wheels make to pass the finish line and the next time I stop it when it hit's it last wheel revolution.

Is there a device that can stop a wheel after a certain amount of turns? It must be mechanical and it doesn't have to be supplied by Lego either.

  • 1
    Ah, September, when the USA kiddies go back to school and start posting their homework questions.
    – Móż
    Commented Oct 5, 2013 at 22:27
  • 2
    Well, I'm not from the US and this is not a homework question. Even though I'm good with maths I'm extremely bad at construction so that's why I'm using legos.
    – B22
    Commented Oct 5, 2013 at 23:00

2 Answers 2


Here's the simplest way that I can think of to stop the vehicle at the finish line. You'll need basic LEGO Technic parts and a good length of lightweight string or thread.

Create two reels for the string. One should be connected to the wheels and the other may turn freely. Wind all the string onto the reel which can spin freely and attach the other end securely to the reel on the wheels so that it will wind up the string as the vehicle moves.

This mechanism should prevent the axle from turning after the vehicle has traveled the desired distance as long as you measure the string appropriately.

Here's an example:

example vehicle

Here's a similar model built in LDD showing exactly how this is put together:

LDD Model

The reel on the back axle will pull all of the string off of the center axle and then stop when the string runs out. If you wanted to be fancy, you could use some math to calculate how much string you need, or you could just test it to find out.

  • how is the string going to stop the wheels?
    – B22
    Commented Oct 5, 2013 at 15:31
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    I added an extra image to my answer to try to make this a little clearer for you. I'll be honest though, since this seems to be for some sort of educational assignment or competition, I don't want to give you a complete perfect answer. This is the basic concept. You'll need to adapt it and improve it to work for you.
    – jncraton
    Commented Oct 5, 2013 at 15:47
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    The general concept is that the reel on the wheels is consuming string from the other reel as the wheels turn. Once there is no additional string left, the wheels are forced to stop turning (or break the string).
    – jncraton
    Commented Oct 5, 2013 at 15:55
  • Thank you. How long did it take you to come up with this?
    – B22
    Commented Oct 5, 2013 at 16:20
  • 1
    You're welcome! It didn't take long. I've seen this sort of thing before.
    – jncraton
    Commented Oct 5, 2013 at 18:39

You might consider this cheating, but here we go:

There exist Lego wind-up motors:

enter image description here

With the number of turns of the key you should be able to control the distance rather precisely – unless the device gets too much momentum from the slope.

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