Introduction: Automated Newton's Cradle

By Jim Demello
About: I am an American teaching English at Shangluo University, Shaanxi. I like making machines that do interesting but fairly useless things - I call them Quixotic Machines.

This is my third effort to automate a Newton's Cradle, the first using a motor and custom built Newton's Cradle, the second using a custom built robot to pull the ball back and finally this one that uses an Arduino Nano, two servos and a magnet to pull the ball(s) back. This one is definitely the most interesting because it can pull one ball back, or two or three. Pulling multiple balls back also introduces a random element in how the balls will be released and therefore how they will "bounce" off one another. Most definitely another useless toy but love to automate these kinds of toys.

Assuming someone else would like to build one, the required parts are:

Arduino Uno or Nano

6 volt battery pack to power servos

9 volt to power Arduino

two micro servos

one magnet

some wood or fishing pole sections for servo arms

one piece of material to separate magnet from balls

hot glue and zip ties

Step 1: Make Servo Arms and Mount to Cradle Frame

For the main arm I bolted a section of fishing pole to the servo arm. At the end of the arm I hotglued a magnet and a piece of foam onto the magnet. The foam keeps the magnet away from the steel ball enough that it can let loose if pulled back far enough. Without the padding of the foam, the ball will never let loose of the magnet. You just have to experiment with the padding to see how thick it should be. I then cut a piece of plywood to hold the servo, drilled a couple of holes, mounted it to the frame with zip ties. Then zip tile the main arm servo to the plywood mount and use hot glue to keep it all from moving around.

The small arm's purpose is to stop the balls from swinging. It is not really necessary but without it the balls will swing and affect how they bounce or you have to wait until they stop swinging before releasing the ball. I just zip tied and hot glued the servo to the frame. I made a ball stopper arm with a couple of pieces of fishing pole glued together.

That is all the fabrication that needs to be done. The rest is just wiring up the Nano, servos and power.

Step 2: Wiring Up the Nano

I use a separate power supply for the Nano and the servos because the Nano cannot reliably power the servos with internal power. The only thing you have to do is wire the ground of the 6 volt power supply and the 9 volt power supply for the Nano together. The schematic shows how I wired it all up.

Step 3: Arduino Program

Here is the simple sketch to run the Nano. myServo is used to be able to slow the servo speed down. This is a very useful subroutine anytime you need to control the speed of a servo.

#include <Servo.h>

Servo myservoLeft,myservoRight, myservoCenter,myservoWeight; // create servo object to control a servo

void myServo(int newAngle,int angleInc,int incDelay,int servoNum) {

int curAngle;

if (servoNum== 1) { curAngle = myservoLeft.read(); }

if (servoNum== 2) { curAngle = myservoRight.read(); }

if (servoNum== 3) { curAngle = myservoCenter.read(); }

if (servoNum== 4) { curAngle = myservoWeight.read(); }

if (curAngle < newAngle) {

for(int angle=curAngle;angle < newAngle;angle += angleInc) {

if (servoNum == 1) myservoLeft.write(angle);

if (servoNum == 2) myservoRight.write(angle);

if (servoNum == 3) myservoCenter.write(angle);

if (servoNum == 4) myservoWeight.write(angle);

delay(incDelay); }

}

else if (curAngle > newAngle) {

for(int angle=curAngle;angle > newAngle;angle -= angleInc) {

if (servoNum == 1) myservoLeft.write(angle);

if (servoNum == 2) myservoRight.write(angle);

if (servoNum == 3) myservoCenter.write(angle);

if (servoNum == 4) myservoWeight.write(angle);

delay(incDelay); }

}

}

void singleBall() {

myServo(30,1,5,1); //open ball stopper

delay(100);

myServo(150,1,25,3); // pull ball back

delay(8000);

myServo(95,1,5,3); // down to get ball

delay(100);

myServo(134,1,10,1); // stop balls swinging

delay(3000);

}

void doubleBall() {

myServo(30,1,5,1); //open ball stopper

delay(100);

myServo(150,1,30,3);

delay(9000);

myServo(85,1,5,3);

delay(1000);

myServo(129,1,10,1); // stop balls swinging

delay(3000);

}

void setup()

{

Serial.begin(9600);

delay(100);

myservoCenter.attach(3); // attaches the servo on pin 9 to the servo object

myservoCenter.write(90);

myservoLeft.attach(4); // attaches the servo on pin 9 to the servo object

myservoLeft.write(90);

delay(50); // waits for the servo to get there

}

void loop() {

//doubleBall();

singleBall();

//}

// delay(1000);

// exit(0); //pause program - hit reset to continue

}