Magnetic Stirrer

/*
 Magnetic Stirrer
 
 Layman summary: 
 We will use a potiometer to control the speed of a DC motor.
 When the potiometer is turned to full Ohms then the motor should stop.
 When the potiometer is turned to 0 Ohms then the motor should be running full speed.
 And if the potiometer is somewhere in the middle then the motor speed will follow. 
 
 
 InDepth summary;
 This code will read an analog voltage of the potiometer using pin(A0). 
  We will take the analog to digital conversion (ADC) value and 
 produce a corresponding PWM signal on pin(9). The PWM signal will turn the 
 NPN transistor on/off very quickly to the point where the motor will spin like
 it is getting less than full power. 
 
 Notes:
 1. Please note the ADC on the Arduino mega is 10 bits. The PWM is 8 bits. 
 So we need to divide the ADC value from pin(A0) by 4 and then send that value to 
 the PWM on pin(9).  
 
 2. Intersting to note that if you leave out the division then 
 you will notice that a 1/4 turn of the potiometer will give you a 0 rpm to full speed 
 on your motor. This is because 8 bits will get you 255 steps for the PWM and the 
 ADC 10 bits will give you 1023 steps. 1023/255 = 4 (roughly) 
 
 3. I added a delay at end of loop to help stablize motor speed. I noticed
 with PC fans I was using that if you rotated the potiometere quickly or back and fourth
 that the load of the magnets and the drastic speed up/down would burn out the 
 cheap PC fan motors. So a word of caution is to turn the pot slowly to avoid motor burn out :)
 
 
 Brief Function description: 
 analogWrite() allows us to send a 0 to 255 value to a PWM pin. 
 ReadAnalogVoltage() allows us to read a 0 to 5Vdc voltage and convert
 voltage into a value between 0 to 1023. 
 Serial.Began starts serial communication to "Serial Monitor" 
 Serial.PrintLn writes one line to "Serial Monitor"


 Credits:
 Giving credit to the contributors of the example (basic) "Fade" program 
 and "ReadingAnalogVoltage" program since this code is really just a cut/paste of 
 both example programs into one working code.
 
 
 leaving the original ReadAnalogVoltage notes since I found them useful as well :) 
   ReadAnalogVoltage
  Reads an analog input on pin 0, converts it to voltage, and prints the result to the serial monitor.
  Graphical representation is available using serial plotter (Tools > Serial Plotter menu)
  Attach the center pin of a potentiometer to pin A0, and the outside pins to +5V and ground.

  This example code is in the public domain.
 */


int motor1 = 9;           // the PWM pin 
int motor1speed = 0;    // how fast motor1 is

// the setup routine runs once when you press reset:
void setup() {
  // declare pin 9 to be an output:
  pinMode(motor1, OUTPUT);
    // initialize serial communication at 9600 bits per second:
  Serial.begin(9600);
}

// the loop routine runs over and over again forever:
void loop() {
  
      // read the input on analog pin 0:
  int motor1speed = analogRead(A0);
  
    // Convert the analog reading (which goes from 0 - 1023) to a voltage (0 - 12V):
  float voltage = motor1speed * (12.0 / 1023.0);
  
  // print out the value you read:
  Serial.println(motor1speed);
  // print out the voltage 
  Serial.println(voltage);
  
  // set the motor1speed of pin 9:
  analogWrite(motor1, motor1speed/4);
  
  // delay 30 miliseconds. this pre
  delay(30);
}