analogWrite Not Fading? Check the Pin

You copied a fade sketch, changed the pin number to match your wiring, and pressed Run. The LED does not fade. It stays dark for a while, then snaps fully on. The code is right and the wiring is right. The problem is the pin. On an Arduino Uno, analogWrite only fades some pins. In this post you will see the problem happen on a breadboard, read what Mokxi's Circuit Doctor says about it, and fix it.

What PWM is, in one paragraph

An Arduino pin is either 5 volts or 0 volts. It cannot output 2.5 volts. To dim an LED anyway, the chip switches the pin on and off hundreds of times a second and changes how long each "on" lasts. That is pulse width modulation, or PWM. Your eye averages the flicker into a dimmer light. A long "on" looks bright and a short "on" looks dim. analogWrite(pin, value) takes a value from 0 to 255, where 0 is always off and 255 is always on.

The catch is that the switching is done by timer hardware inside the chip, and only some pins are connected to it.

Which Uno pins have PWM

On the Arduino Uno, six pins can do PWM: 3, 5, 6, 9, 10 and 11. On the real board each one is marked with a tilde (~) next to its number. Every other digital pin, such as 2, 4, 7, 8, 12 and 13, cannot. Mokxi's Uno follows the same list.

So what does analogWrite do on a pin without PWM? In Mokxi's Uno runtime, it falls back to the nearest thing it can do: a value under 128 sets the pin low, and 128 or more sets it high. No dimming at all, just on or off. That is why your LED stayed dark and then snapped on.

What you need

An Arduino Uno
Two LEDs
Two 220 ohm resistors
Jumper wires

Wire it up

We will put one LED on a PWM pin and one on a pin without PWM, so you can watch them side by side.

From	To	What it does
Uno pin 9	Resistor 1	Pin 9 has PWM.
Resistor 1, other end	LED 1 anode (long leg)	Positive side.
LED 1 cathode (short leg)	Uno GND	Completes the loop.
Uno pin 7	Resistor 2	Pin 7 has no PWM.
Resistor 2, other end	LED 2 anode (long leg)	Positive side.
LED 2 cathode (short leg)	Uno GND	Completes the loop.

The complete sketch

Both LEDs run exactly the same fade code. Only the pin numbers differ.

const int LED_PWM = 9;    // has PWM (marked ~ on the board)
const int LED_PLAIN = 7;  // no PWM

void setup() {
  pinMode(LED_PWM, OUTPUT);
  pinMode(LED_PLAIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Fading pin 9 and pin 7 together");
}

void loop() {
  for (int level = 0; level <= 255; level += 5) {
    analogWrite(LED_PWM, level);
    analogWrite(LED_PLAIN, level);
    delay(30);
  }
  for (int level = 255; level >= 0; level -= 5) {
    analogWrite(LED_PWM, level);
    analogWrite(LED_PLAIN, level);
    delay(30);
  }
}

I compiled this sketch for the Uno with Mokxi's own build path and ran it in the simulator before writing this post.

What you should see

Press Run. The LED on pin 9 rises smoothly from dark to bright and back again. The LED on pin 7 does something else. It stays dark for the first half of each rise, and then jumps to full brightness and stays there until the level falls back under 128. In my run, the pin 9 brightness climbed in small steps while pin 7 only ever read off or fully on.

Same code, same loop, different result. The only difference is the pin.

What the Circuit Doctor says

Open the Doctor tab in the editor. The Circuit Doctor reads your wiring and your code together, in your browser, and lists what looks wrong. For this circuit it shows one warning, in these words:

analogWrite on pin 7 will not fade: that pin has no PWM on the Arduino Uno.

Under it, the detail line says: "It just switches fully on or off. Use a PWM pin (marked ~): 3, 5, 6, 9, 10 and 11." It points at pin 7, the one that is wired and used with analogWrite. I checked, and it does not offer a one-tap fix for this one. The fix is a small change that you make yourself, and it is below.

The fix

Move the wire from pin 7 to a pin with a tilde, such as pin 10.
In the sketch, change one line: const int LED_PLAIN = 7; becomes const int LED_PLAIN = 10;.

Press Run again. Both LEDs fade now. The warning disappears from the Doctor.

Line by line

LED_PWM = 9 and LED_PLAIN = 7 name the pins. A change in one place changes every line that uses it.
pinMode(..., OUTPUT) makes both pins outputs. It is good practice even though analogWrite also sets a pin up.
Serial.println(...) prints one line so you know the sketch started.
The first for loop counts level from 0 up to 255 in steps of 5. On each step it writes the same number to both pins and waits 30 milliseconds.
The second for counts back down, so the LEDs fade out again.
analogWrite(pin, level) is the line that behaves differently depending on the pin.

Other gotchas worth knowing

0 and 255 are not really PWM. On the Uno, analogWrite(pin, 0) just turns the pin off and analogWrite(pin, 255) just turns it on. That is the same as the Arduino core.
tone() takes timer 2. On an Uno, playing a note with tone() uses the timer behind pins 3 and 11, so PWM on those two pins pauses while a note plays.
Other boards differ. On an ESP32-C3, analogWrite works on any GPIO, because the chip routes its PWM controller to whichever pin you pick. The Uno does not have that freedom, so always check the board.

Try next

Swap pin 9 for pin 3, 5, 6, 10 and 11 in turn. All of them fade.
Make the fade smoother by changing the step from 5 to 1 and the delay from 30 to 6.
Use a potentiometer on A0 and write analogRead(A0) / 4 to analogWrite, so a knob sets the brightness.
Read the Arduino PWM fade guide for a longer walkthrough and the brightness curve, and the PWM guide for what the duty cycle means.

Try it on Mokxi

Open the Fade an LED project to see a fade on a PWM pin, then move its wire and the pin number in the sketch to one without a tilde and watch the Doctor. The Arduino simulator is where to build the two-LED circuit above.

Ready to build it yourself? Create your free Mokxi account and open the editor in your browser. No installs, no parts to buy.