How to Develop an Arduino-Based Optical Tachometer

/**
 * In this program, we develop an Arduino-Based Optical Tachometer 
 * that measures the speed of a brushless propeller.
 * 
 * The Infrared LED is connected to pin 13.
 * The Infrared Phototransistor is connected to pin 2 (interrupts).
 * 
 * @version 1.0 2019-02-18
 * @author Addison Sears-Collins
 */
 
// Assign a name to the DC motor pin on the Arduino Uno
const unsigned int MOTOR_PIN = 3;

// Assign a name to the infrared LED pin on the Arduino Uno
const unsigned int IR_LED = 13;

// The number of blades on the propeller. Adjust accordingly.
const unsigned int BLADE_COUNT = 3;

// Volatile keyword is used with interrupts
// This variable is subject to change inside an interrupt
// service routine
volatile unsigned int break_number = 0;

// Flag used to stop the program
bool done = false;

// Used for capturing the time
unsigned long time;

// Used for capturing the rpm (revolutions per minute)
unsigned int rpm;

/**
 *  Function runs only once, after each powerup or reset of the Arduino Uno
 */
void setup() {

  // Open the serial port and set the data transmission rate to 9600 bits 
  // per second. 9600 is the default baud rate for Arduino Uno.
  Serial.begin(9600);

  // Show a welcome message as human-readable ASCII text
  Serial.println("PROPELLER RPM PROGRAM");
  Serial.println("This program transmits the time and RPM of a propeller.");
  Serial.println("Created by Addison Sears-Collins");
  Serial.println("");
  Serial.println("Press ! to end the program");
  Serial.println("");
  Serial.println("Please enter the desired speed of the motor.");
  Serial.println("Must be a value between 100 and 255.");
  Serial.println("");
  Serial.println("TIME, RPM");

  // The Infrared phototransistor is connected to pin 2.
  // Interrupt triggers when signal goes from HIGH to LOW
  attachInterrupt(digitalPinToInterrupt(2), isr_break_count, FALLING); 

  // Turn on the IR Led
  pinMode(IR_LED, OUTPUT);
  digitalWrite(IR_LED, HIGH);

  // Enable output for the motor
  pinMode(MOTOR_PIN, OUTPUT);

  break_number = 0;
  rpm = 0;  
 
}

/**
 *  Main function
 */
void loop() {

  display_time_and_rpm();
  
  start_motor();

  while(!done) {    

    // Update time and rpm every second
    delay(1000);

    // Don't process interrupts during this calculation
    noInterrupts();

    // Calculate the RPM. If a 3-blade propeller, 3 breaks
    // per second would yield 1 rpm, which is 60 rpm. 
    rpm = (60 * break_number) / BLADE_COUNT;

    // Display the time and rpm
    display_time_and_rpm();

    // End program if sentinel is entered 
    end_program();    
    
    break_number = 0;
    rpm = 0;

    // Restart interrupts
    interrupts();
  }
  
  // Do nothing
  while (true) {}
}

/**
  * This function starts the motor
  */
void start_motor() {

  // Wait for the user to enter the speed of the motor
  while (Serial.available() == 0){};

  // Activate the motor
  int speed = Serial.parseInt();
  if (speed >= 100 && speed <= 255) {
    analogWrite(MOTOR_PIN, speed);
  }
}

/**
  * Interrupt service routine.
  * This function counts the number of interrupts
  */
void isr_break_count() {

  break_number++;
  
}

/**
  * Function displays the time and rpm
  */
void display_time_and_rpm() {
  
  // Capture the time and covert to seconds
  time = millis() / 1000;

  // Display the time
  Serial.print(time); 
  Serial.print(" , ");
  // Println so the next line begins on a new line
  // Display the rpm
  Serial.println(rpm);   
}

/**
  * This function ends the program
  */
void end_program() {
  
  // Used for reading data from the serial monitor
  char ch;

  // Check to see if ! is available to be read
  if (Serial.available()) {     
  
    // Read the character
    // Serial.read() returns the first (oldest) character in the buffer 
    // and removes that byte of data from the buffer
    ch = Serial.read();    

    // End the program if an exclamation point is entered in the
    // serial monitor
    if (ch == '!') {
      done = true;  

      // Turn off the motor
      analogWrite(MOTOR_PIN, 0);

      // Turn off the IR LED
      digitalWrite(IR_LED, LOW);
      
      Serial.println("Finished recording RPM. Goodbye.");
    }
  }    
}