analogWrite & PWM

Learning Goals 5 min

1. Explain in your own words how a digital pin can pretend to be analog — by switching on and off very fast.
2. Use analogWrite(pin, value) with values from 0 to 255 to set LED brightness on any PWM-capable pin (~3, 5, 6, 9, 10, 11 on the UNO).
3. Identify which UNO pins support PWM at a glance — the ones with the squiggly ~ symbol next to their number.

Warm-Up 10 min

So far every output pin you've driven has been fully on or fully off. An LED is bright or dark; a motor is spinning or stopped. There's no half-bright, no slow-spin.

But you know they're possible — a TV brightness slider clearly works. So how do we get "in-between" from a chip that can only output 0 V or 5 V?

Predict-the-output puzzle

Wei Jie thinks he can dim an LED by switching it on and off really fast. He writes this:

void loop() {
  digitalWrite(9, HIGH);
  delay(5);
  digitalWrite(9, LOW);
  delay(5);
}

What does the LED look like?

New Concept · PWM, the "fake analog" trick 20 min

The big idea — duty cycle

The chip can only output 0 V or 5 V. So it cheats: it switches between them very fast. The fraction of time the pin spends HIGH is the duty cycle.

• 0% duty → always LOW → LED off.
• 25% duty → on for 1 unit of time, off for 3 → LED at about a quarter brightness.
• 50% duty → on/off equal → LED at about half brightness.
• 100% duty → always HIGH → LED full bright.

Your eye averages the on/off flicker into a single perceived brightness. Same trick used in TV displays, dimmer switches, motor controllers, even server fans.

The Arduino built-in: analogWrite

analogWrite(pin, value);
// value is an integer 0..255
// 0   = always LOW   (0% duty)
// 64  = ~25% duty
// 128 = ~50% duty
// 192 = ~75% duty
// 255 = always HIGH  (100% duty)

The number 255 isn't magic — it's 2⁸ − 1, the biggest value that fits in one byte. The Arduino uses an 8-bit register for PWM, so you get 256 brightness steps from 0 to 255.

Only PWM-capable pins

Not every UNO pin can do PWM. Look at your board: pins with a squiggly ~ mark are the PWM pins. On a standard UNO they are ~3, ~5, ~6, ~9, ~10, ~11 — six pins out of fourteen.

If you call analogWrite on a non-PWM pin (say, pin 7), the IDE doesn't crash. The pin just acts like a regular digitalWrite: 0 = LOW, anything else = HIGH. No dimming. The lesson: always wire LEDs you want to fade onto a ~ pin.

The PWM frequency

On most UNO pins, PWM runs at ~490 Hz. On D5 and D6 it's ~980 Hz. Either way, much faster than the human eye can see (we top out at roughly 60 Hz of flicker perception). For LEDs you can ignore the frequency entirely; for motors and audio it matters more — but that's a Level 3 conversation.

No pinMode required

One small surprise — you don't need pinMode(pin, OUTPUT) before analogWrite. The function sets the pin to output for you. Adding it does no harm, and most authors include it for symmetry with their other pins. Up to you.

Worked Example · Three brightness levels 20 min

Step 1 — wire one LED on a PWM pin

Same wiring as the L01-07 single-LED circuit, but use pin ~9 (which supports PWM) instead of pin 13. Anode of the LED → 220 Ω resistor → Arduino pin ~9. Cathode → GND rail.

Step 2 — set three brightnesses

Save as three-brightnesses.ino:

// L02-02: three brightness steps with analogWrite
const int LED = 9;

void setup() {
  pinMode(LED, OUTPUT);
}

void loop() {
  analogWrite(LED, 64);   // ~25% — dim
  delay(1000);
  analogWrite(LED, 128);  // ~50% — medium
  delay(1000);
  analogWrite(LED, 255);  // 100% — full
  delay(1000);
}

Step 3 — upload and watch

The LED cycles between dim, medium, full, dim, medium, full... at one-second steps. No on/off flicker visible — the PWM is far too fast for your eye.

What you should see

(dim ─ 1s) → (medium ─ 1s) → (full ─ 1s) → (dim ─ 1s) → …

Step 4 — predict before you tweak

Before you change any number, ask yourself: what would analogWrite(LED, 0) look like? What about analogWrite(LED, 1)? Or analogWrite(LED, 254)?

• 0 → dead off, indistinguishable from a broken wire.
• 1 → just barely glowing — in a dark room you'll see it, in daylight you might not.
• 254 → looks identical to 255 to your eye. The eye can't tell the difference of one PWM step at the top of the range.

Try It Yourself 20 min

const int LED_A = 9;
const int LED_B = 10;

void setup() {
  pinMode(LED_A, OUTPUT);
  pinMode(LED_B, OUTPUT);
}

void loop() {
  analogWrite(LED_A, 64);
  analogWrite(LED_B, 192);
}

for (int v = 0; v <= 255; v += 51) {
  analogWrite(LED, v);
  delay(300);
}

Mini-Challenge · Three-level mood lamp 15 min

Build a small sketch where pressing the button (on pin 2 with INPUT_PULLUP) cycles the LED through three brightness levels: low, medium, high, low, medium, high...

It works if:

• Power-up state is low brightness.
• Each press of the button advances to the next level (low → medium → high → low → ...).
• The button works reliably — no skipping levels because of debounce noise (recall L01-18). A 200 ms blocking delay after each press is fine here.
• The LED is on a PWM pin (~9 / ~10 / ~11) so the brightness is visibly different at each level.

const int LED = 9;
const int BTN = 2;
int level = 0;
const int LEVELS[3] = {32, 128, 255};

void setup() {
  pinMode(LED, OUTPUT);
  pinMode(BTN, INPUT_PULLUP);
  analogWrite(LED, LEVELS[0]);
}

void loop() {
  if (digitalRead(BTN) == LOW) {
    level = (level + 1) % 3;
    analogWrite(LED, LEVELS[level]);
    delay(200);   // simple debounce
  }
}

Recap 5 min

PWM gets analog-style output from a digital pin by switching very fast. analogWrite(pin, value) takes a value from 0 to 255 and sets the duty cycle accordingly. Only the six ~-marked UNO pins support it; the others fall back to plain on/off. Tomorrow we'll do the maths on duty cycle properly — frequency, period and the difference between "average voltage" and "perceived brightness".

PWM (Pulse-Width Modulation)

Switching a digital pin on and off so quickly that the average voltage acts analog. Your eye / a motor / a speaker doesn't see the flicker — only the average.

Duty cycle

The fraction of time the pin spends HIGH within one PWM period. 0% = always off, 100% = always on. analogWrite's value maps to duty as value / 255 × 100%.

analogWrite(pin, value)

Arduino built-in. value is 0..255 (8-bit). Only works on PWM-capable pins. No pinMode required.

PWM pins (UNO)

The six pins marked with ~ on the board silkscreen: D3, D5, D6, D9, D10, D11.

Homework 5 min

Predict-then-test. In your notebook, write down what each of these analogWrite values will look like on the LED:

• analogWrite(LED, 0) → ____
• analogWrite(LED, 32) → ____
• analogWrite(LED, 96) → ____
• analogWrite(LED, 160) → ____
• analogWrite(LED, 224) → ____
• analogWrite(LED, 255) → ____

Then write a sketch that sets the LED to each value for 800 ms in turn, upload it, and check your guesses. Note in your notebook anywhere your guess was off — that's the gap between the duty-cycle maths (linear) and your eye's response (a curve we'll meet tomorrow).

Bring back next class:

• Your predicted vs actual table.
• The .ino file (rename to hw-l02-02.ino).