5. Serial Monitor Debugging

This lesson uses: Arduino UNO R3 board, TinkerBlock TK04 Push Button module, TinkerBlock TK01 XL LED module, jumper wires (or breadboard). Learn to use the serial monitor and Serial to print. Outcome: open the serial monitor (9600 baud) to see “Program started” and button state (pressed/released); the button still controls the LED, which helps debugging.

1. What is serial communication?

Serial communication is one way the Arduino and the computer exchange information. Like two people on a call:

• The Arduino “talks” over the USB cable (sends data)
• The computer “listens” with the serial monitor (receives and displays data)

Why use serial?

• While the program runs we cannot see what is happening inside the Arduino
• Serial lets us “see” what the program is doing, which helps debugging (finding problems)
• We can show sensor readings, program state, and so on

2. Wiring

Same as Lesson 03:

• TK04 button: VCC → 5V, GND → GND, SIGNAL → D2, NC leave unconnected
• TK01 LED: GND → GND, SIGNAL → D13

3. New sketch

1. Arduino IDE: File → New (or Ctrl/Cmd + N) to create a blank sketch
2. Confirm board and port are selected (same as Lesson 01)
3. Type the code below into the default setup() and loop() yourself

4. Program structure

Same as last lesson: two fixed blocks setup() and loop().

• setup():
  • Tell the board “pin 2 for button, pin 13 for LED”
  • Start serial (so the Arduino can talk to the computer)
  • Print one startup message
  • Runs once
• loop(): Keep repeating “read button → print button state to serial → control LED from button → short delay”.

Open the serial monitor (choose 9600 baud), type the code, upload, and you will see on the computer whether the button is pressed or released; the LED will follow.

5. Code to write

Type the code below line by line into setup() and loop() (do not copy-paste; typing it yourself helps you remember):

// Serial debug: read button and print state to serial, and control LED; baud 9600
#define BUTTON_PIN 2   // button to D2
#define LED_PIN 13     // LED to D13

void setup() {
  pinMode(BUTTON_PIN, INPUT);   // D2 read button
  pinMode(LED_PIN, OUTPUT);     // D13 control LED
  Serial.begin(9600);           // serial baud 9600, must match monitor
  Serial.println("Program started");  // printed once after power-up
}

void loop() {
  int state = digitalRead(BUTTON_PIN);

  Serial.print("Button: ");   // print prefix first, no newline
  if (state == HIGH) {
    Serial.println("pressed");   // then print state and newline
    digitalWrite(LED_PIN, HIGH);
  } else {
    Serial.println("released");
    digitalWrite(LED_PIN, LOW);
  }
  delay(200);   // lower refresh rate so you can read it
}

Program notes

Overall idea: In setup() use Serial.begin(9600) to start serial; then in loop() each time read the button and print “Button: pressed/released”, and control the LED from that state. On the computer, open the serial monitor at 9600 baud to see the text and check that the program behaves as expected.

print vs println: Serial.print() only sends the content, no newline, so the next output continues on the same line; Serial.println() sends and then adds a newline, so each message can be one line. Baud rate must match the monitor or you get garbage.

Difference between print and println:

• Serial.print(): After printing, no newline; next print continues on the same line.
• Serial.println(): After printing, newline; next print starts on a new line.

Example:

Serial.print("Button: ");   // print "Button: ", no newline
Serial.println("pressed");  // print "pressed", then newline

Serial monitor shows:

Button: pressed

What you can print:

• Text: Serial.print("Temperature: ");
• Numbers: Serial.print(123);
• Variables: Serial.print(state); (prints the variable’s value)

7. Hands-on

7.1 Basic steps

1. After entering the code, click Verify (✓) to compile
2. Click Upload (→) to upload to the board; make sure the board is connected and upload succeeds
3. Open the serial monitor: Click the serial monitor icon in the red box at the top right of the Arduino IDE (or menu Tools → Serial Monitor, shortcut Ctrl/Cmd + Shift + M) to open the serial window; the program output will appear there as in the figure.
   • Important: Set baud at the bottom right of the window to 9600 (must match Serial.begin(9600) in the code)

4. Watch the serial monitor:

• You should see “Program started” once (printed in setup())
• Then it keeps printing “Button: pressed” or “Button: released” (printed in loop())

5. Test the button:
   • Press TK04: serial shows “Button: pressed” and TK01 LED turns on
   • Release: serial shows “Button: released” and TK01 LED turns off

Expected result: As in the figure above.

7.2 Debug tips

Tip 1: Use serial to see where the program has reached

void loop() {
  Serial.println("Start reading button");   // marker: we reached here
  int state = digitalRead(BUTTON_PIN);
  Serial.println("Read done");             // marker: read finished
  // ...
}

Tip 2: Print the variable’s value to check it

int state = digitalRead(BUTTON_PIN);
Serial.print("Button raw value: ");
Serial.println(state);   // print 0 or 1 to see if read is correct

Tip 3: Use different labels for different states

if (state == HIGH) {
  Serial.println("[pressed] LED on");
} else {
  Serial.println("[released] LED off");
}

7.3 Try it yourself

1. Change baud rate:

   • Change Serial.begin(9600) to Serial.begin(115200)
   • After upload, set the serial monitor to 115200 as well
   • Observe: mismatched baud gives garbage

2. Combine print and println:

   • Try combining Serial.print() and Serial.println()
   • Example: Serial.print("Value: "); Serial.println(state);

3. Print numbers:

   • Try Serial.println(123); to print a number
   • Try Serial.println(state); to print the variable’s value

Proceed to Lesson 06.

8. Common issues

9. Summary

In this lesson you learned:

1. Serial communication: How the Arduino and computer exchange information
2. Serial.begin(9600): Start serial; baud must match the serial monitor
3. Serial.print(): Print text/numbers without newline
4. Serial.println(): Print text/numbers with newline
5. Debug tips: Use serial to see program state and find problems

Reminders:

• Serial monitor baud must match Serial.begin() in the code
• Serial.begin() must be called once in setup()
• The serial monitor can show text, numbers, and variable values; it’s a good tool for debugging

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