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# How to use the Ultrasonic Sensor with the Catapult built from EV3/NXT (Part 5) ProPreview

This video lesson shows a very interesting specific example on how to use the LEGO Mindstorms EV3 Ultrasonic Sensor. The sensor detects if the catapult is loaded or if we have just fired an element. At the end of the lesson I also offer an interesting challenge that you could use in your classroom or at home.

• #69
• 01 Feb 2015
• 7:52

### Ultrasonic Sensor

The sensor detects distance in centimeters or in inches. When the catapult is loaded the distance between the sensor and the base of the robot is about 7-9 centimeters (~3 inches).

It is more interesting however that after adding the sensor we should balance the nest holding the brick. Many novice robot builders make the mistake of adding parts, sensors and motors only on one side of the robot without thinking about how to balance it.

### The challenge

The challenge from this video is.

How to calculate the speed at which the nest falls? (Using those calculations in some of the next video lessons we would calculate the speed of the parts fired from the catapult). Use the knowledge from:

### Catapult build from LEGO Mindstorms EV3/NXT (Part 1)

Building instructions for EV3 Mindstorms Catapult Robot with Ultrasonic Sensor are given below

### Catapult Frame - LEGO Mindstorms construction for a frame of a catapult

This frame holds the motor and ev3 brick used in the catapult. The frame is placed on top of the rotating base and thus could be easily rotated. The brick could be both NXT and EV3

### Catapult Rotating Base - LEGO Mindstorms construction for a base for a catapult

This is the initial construction of a catapult. The base on which we will place the rest of the robot. The goal of this construction is to give the catapult the ability to rotate.

### Catapult Full - Full catapult from LEGO Mindstorms with a Motor and a Touch Sensor

These are building instruction for a working automatic catapult with a motor and a touch sensor.

### Catapult Full - Full catapult from LEGO Mindstorms with a Motor and ultrasonic sensor

The catapult robot could use the ultrasonic sensor to stop loading at a certain height

### Courses with this episode

The episode is used in the following courses where you can find additional tasks for it to use in class

#### A robot a "day" keeps the questions awake

Use the ultrasonic sensor to measure the speed of falling objects. Use the task in your classroom, for your team or at home to engage students in learning.

There is 1 task from a middle complexity giving 2 points.

#### Tasks description, submission and evaluation are available to subscribed users.

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