Make a Marble Speed Trap with LEGO Mindstorms

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The next post in the LEGO® Mindstorms series is the first of the challenges I’m going to share. The valuable thing about a Mindstorms set is not only the open ended play it provides kids of all ages, but it’s easy to give challenges to your student using these kits. Today’s assignment is How to Make a Marble Speed Trap with LEGO® Mindstorms.

The Speed Trap Challenge

This is the second time we assigned the Mindstorms Speed Trap. When our 17yo was 12 he built his own speed trap which was actually the follow up activity to one in which we had our kids find the velocity of a marble manually using what we had on hand to find the distance the marble traveled and the time it took. All of this was from a question one morning while playing with the marble coaster, “I wonder how fast the marble is going.” My answer? Let’s find out! Then I had them design the test. I know…but it’s what science teachers do!

• The assignment in short is to build a speed trap which will tell your student how fast the marble is going as it travels down the track.
• Velocity (speed) is distance/time– If you forget this formula, just remember that cars travel in miles/hour.
• Your trap needs to measure the distance between the sensors and how long it takes to go between them– so your student will need to code that into the EV3 using the drag and drop programming that comes with the EV3.
• Have the EV3 calculate the velocity– Using the numbers it gets from the light sensors.

Of course, the real magic here is in your student figuring at least some of this stuff out by himself! I’ve included the basic information for you and you can use this to give the challenge to your student without giving away all of the things he’ll need along the way.

Equipment Needed to Complete the Speed Trap

You’ll need to gather together some basic tools for the job:

• EV3 or NXT LEGO Mindstorms Kit– We used a combination of the kits. The EV3 provided the brick (aka- Mindstorms computer) and most of the technic pieces for the trap itself.
• Technic Pieces– These are the studless LEGO pieces which go together with pegs.
• Light Sensors– You’ll need two sensors which we grabbed from our NXT kit. The EV3 home kit does not come with light sensors, but you can order them separately.
• Track for Rolling Marbles– We used a Skyrail set so that the marble could roll on top of the track and not inside a tube like it does with a typical marble run.
• White Background– To attach on the opposite side of the light sensors. Joshua used some large white LEGO pieces from a set we have.

Skills Involved in Completing the Speed Trap

There are a variety of skills used in putting the speed trap together. Younger engineers might need more guided help, but I like to encourage our students to come up with a solution on their own. When they get stuck, offering another clue can be helpful but I want them to stick with it as long as they can. I don’t worry about time limits when it’s just an exploration exercise. So, they have time to think.

• Designing the contraption which will hold the sensors– This includes how to hold the sensors so they can measure a change in light as the marble goes by. It’ll have to hold the track steady and it has to accommodate something which will reflect the light so there isn’t interference of ambient light.
• Writing the computer code– Which will have the light sensors detect a change in light and the time between each sensor seeing that change. This is what will give you the distance and time values for your formula for velocity. The software uses command blocks and it’s a very visual process for students. See the image below for how Joshua programmed his marble speed trap. You can see he added wait blocks (the blocks with little clocks) to the program to tell to wait until the marble disrupts the bream of light from the light sensors.
• Unit Conversion– Your student is going to measure the distance in mm and put that value into the program on the EV3 and the time is measured by the EV3 in seconds. It’s the student’s job to convert these numbers to mph so he can tell the EV3 to do the same. That way you’ll see the answer in miles/hour.

Challenge Results

Once your student thinks he has his program ready, it’s time to do some testing. Does the number make sense? Joshua hit a snag when he knew his numbers were way too large and it showed him he’d made a mistake in his unit conversion.

You can vary the track to see if you can increase the marble’s speed. Last time, they started the marble super high and let it go. This time it was not all that high. See how the track orientation changes the velocity. Have your kids make a prediction ahead of time. Remember they need to tell you what they think will happen and WHY. Those are the parts to a great hypothesis.

So, how did the marble do? The highest speed recorded was 8.5 mph!

Now it’s your turn! Give the marble speed trap challenge a try!