What is PWM

PWM (Pulse Width Modulation) is a way to control power delivery by rapidly turning a signal on and off. Instead of reducing voltage, you change the duty cycle — the percentage of time the signal is HIGH.

Why PWM

• LED dimming — PWM controls brightness without changing voltage
• Motor speed — PWM controls RPM without losing torque
• Servo position — PWM pulse width sets the servo angle
• Audio generation — PWM creates tones and melodies

How It Works

Duty Cycle = T(on) / T(total) × 100%

0% duty:    ────────────────────────────   (always OFF)
25% duty:   ████────────────────────────   (1/4 time ON)  
50% duty:   ████████────────────────────   (half time ON)
75% duty:   ████████████────────────────   (3/4 time ON)
100% duty:  ████████████████████████████   (always ON)

The frequency (how many cycles per second) stays constant. Only the ON/OFF ratio changes.

Arduino Example: LED Fade

int led = 9; // PWM-capable pin (~ on Uno)
void setup() { pinMode(led, OUTPUT); }
void loop() {
  for (int brightness = 0; brightness < 255; brightness++) {
    analogWrite(led, brightness); // 0-255 = 0-100% duty
    delay(10);
  }
  for (int brightness = 255; brightness >= 0; brightness--) {
    analogWrite(led, brightness);
    delay(10);
  }
}

Common Pitfalls

Issue	Cause	Fix
LED flickers at low duty	Frequency too low	Default ~490 Hz is fine for LEDs; for motors use higher
Motor buzzes	PWM frequency in audible range	Use >20 kHz (above hearing)
Pin does not work	Not a PWM-capable pin	Check board pinout for ~ or PWM mark
Servo jitters	PWM frequency wrong for servo	Servos need exactly 50 Hz (20ms period)

PWM Pins by Board

Board	PWM Pins	Resolution	Frequency
Arduino Uno	3,5,6,9,10,11 (~)	8-bit (0-255)	~490/980 Hz
ESP32	All GPIOs	16-bit (0-65535)	Adjustable
Raspberry Pi	GPIO12,13,18,19	Hardware PWM	Adjustable
STM32	Most timers	16-bit	Up to MHz

Related

• Arduino Getting Started
• [What is GPIO](coming soon)