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Ball Roller Coaster
Building: Advanced
Program: Easy

Building Instructions




















Use the shortest (20 cm) wire to connect the motor to port A on the NXT, and one of the medium (35 cm) wires to connect the ultrasonic sensor to port 4 on the NXT. 

Important: Route both wires through the frame as shown, to keep them out of the way.

The track connects to the NXT/lift assembly using the two pegs already on the NXT side.


Ball Roller Coaster Programming

Use the Ball_Coaster program for the Ball Roller Coaster.  This is a simple program that uses the ultrasonic sensor to tell the lift motor when to tilt up.

How to Use the Ball Roller Coaster

If you start the Ball Roller correctly, it should be able to keep both of the balls on the track for a few minutes or so before something goes wrong.

  1. I used the Ball Roller Coaster on carpet, which I think allows it to work better than on a hard floor, because the position of the various angles is more forgiving and stays more stable.

  2. Start with the lift arm of the Ball Roller Coaster in the down position (ready for a ball to roll in).

  3. Run the Ball_Coaster program.

  4. Place a ball into the lift.

  5. When the first ball is about halfway down the track, place a second ball into the lift.

  • If you have trouble with the sensor thinking it sees a ball when one is not there, try removing the center rubber stopper piece in front of it.  Your sensor might be more sensitive than mine. 

  • If you have trouble with the ball falling off the track somewhere, try tweaking the angle of the track (or experiment with different stopping pieces).


  • As designed above, the Ball Roller Coaster can usually manage to keep both balls rolling on the track for a few minutes, but eventually something will go wrong.  Usually a ball will get stuck under the lift.  A more sophisticated design might have some kind of ball stopper at the end of the track that keeps a ball from rolling past the end of the track if the lift is not there to catch it.  But this ball stopper must be released somehow when the lift comes down into position.  Can you modify the design to add something like this, or come up with another solution?

  • You can imagine lots of additions or improvements you could try to make to this project, especially if you have a good supply of extra beams for the track.  Bigger?  Longer?  Higher?  Steeper?  Faster?  Angled turns instead of sharp corners?  Fun things for the balls to do on the way down?  If you come up with something good and post a photo or a video on YouTube, let me know and I will post a link to your photo or video.

Here is an effective ball stopper modification to keep the balls spaced correctly by YouTube user hiddenglass.

Here is a YouTube video of a design by Aaron Cocker with several modifications, including a ball stopper and release mechanism, steeper tracks, and conversion to use marbles instead of NXT balls.

Here is another video of an improved version by Ricardo and Ricky Arosemena.

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