16. PWM Fading LED

This lesson uses: Arduino UNO R3 board, TinkerBlock TK01 XL LED module, jumper wires (or breadboard). Learn PWM and analogWrite(); use a for loop for outcome: LED fades from dim to bright and back to dim—a breathing light.

1. Wiring

Connect TK01 XL LED to Arduino (must use a PWM pin, e.g. D3, D5, D6, D9, D10, D11):

• GND → Arduino GND
• SIGNAL → Arduino D3 (PWM pin)
• NC leave unconnected

Note: analogWrite() only works on PWM pins (Arduino Uno: D3, D5, D6, D9, D10, D11, marked with ~).

2. What is PWM?

PWM (Pulse Width Modulation) is a way to “simulate” different brightness or voltage with a digital pin.

• Principle: The pin switches quickly between high (on) and low (off). The ratio of high time to the total period is the duty cycle: higher duty cycle → higher average voltage → LED looks brighter; lower → dimmer.
• On Arduino: In analogWrite(pin, value), value is 0–255, corresponding to 0%–100% duty cycle. You don’t compute the waveform; write 0 (off), 255 (full), or something in between (e.g. 128 ≈ half), and the board outputs the right duty cycle.
• Why it “fades”: Step through 0–255 in small steps; each step analogWrite a slightly larger (or smaller) value and wait a few ms—to the eye it looks like smooth brightness change, a breathing light.

With PWM in mind, wire up and type the code so the LED really “breathes.”

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 3 for LED (PWM output)”; runs once.
• loop(): Keep repeating “for loop dim→bright → for loop bright→dim”.

Type the code below and the LED will slowly brighten then dim, like breathing.

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):

#define LED_PIN 3   // LED on D3 (PWM pin)

void setup() {
  pinMode(LED_PIN, OUTPUT);   // D3 output for LED
}

void loop() {
  // Dim to bright (0 → 255)
  for (int brightness = 0; brightness <= 255; brightness++) {
    analogWrite(LED_PIN, brightness);   // Set brightness (0=dim, 255=bright)
    delay(10);   // 10 ms for smoother fade
  }
  
  // Bright to dim (255 → 0)
  for (int brightness = 255; brightness >= 0; brightness--) {
    analogWrite(LED_PIN, brightness);   // Output current brightness
    delay(10);   // 10 ms per step for smoother fade
  }
}

Program notes

Overall idea: Two for loops: brightness 0→255 then 255→0; each step analogWrite and delay(10) for a smooth fade. PWM pin 0–255 is duty cycle; visually that’s brightness change.

About the for loop

• Three parts in parentheses: In for, two semicolons separate: init (e.g. int brightness = 0), condition (e.g. brightness <= 255), update (e.g. brightness++). Init runs once; then each iteration: check condition → if true run body → run update → check again, until condition is false.
• brightness++: “Add 1 to brightness”, same as brightness = brightness + 1.
• brightness--: “Subtract 1 from brightness”, same as brightness = brightness - 1.

6. Hands-on

1. After entering the code, click Verify (✓) to compile
2. Click Upload (→) to upload to the board
3. Watch TK01 XL LED: it should gradually brighten then dim—breathing light
4. Try yourself:
   • Change delay(10) to delay(5) or delay(20) and see speed change
   • Change brightness++ to brightness = brightness + 5 for a faster fade

Expected result: As in the figure.

Proceed to Lesson 17.

7. Common issues

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