Unit B - Inputs & Outputs

11. PWM, millis() & State Machines

Explain PWM duty cycle with waveforms, drive brightness using analogWrite, and keep several tasks alive with millis and a simple state machine.

Estimated time 4 hours

Learning outcomes

Explain PWM as rapid digital switching with a duty cycle (not true analogue voltage)
Use analogWrite on Uno PWM pins (~) with values 0-255
Replace long delays with millis timing so loop can do more than one job
Implement a simple state machine

Parts and preparation

Uno, LED, 330 ohm resistor and potentiometer.

Before power: inspect wiring, confirm supply voltage and ensure all connected circuits share GND.

PWM, millis() & State Machines instructional connection diagram

What PWM is

PWM means Pulse-Width Modulation. The pin still only drives LOW or HIGH (digital levels), but it switches many times per second. Duty cycle is the percentage of each period spent HIGH.

A larger duty cycle means more average power to a load such as an LED, so it looks brighter. A motor driver or MOSFET can use the same idea for average speed or power. PWM is not a true analogue mid-voltage from a DAC.

Three PWM waveform traces at about 25 percent, 50 percent and 75 percent duty cycle showing longer HIGH time as duty cycle rises — Same switching period; longer HIGH fraction = higher duty cycle = more average power.

Duty cycle	HIGH time share	Typical LED look
0%	Always LOW	Off
25%	Short HIGH	Dim
50%	Half HIGH	Medium
75%	Long HIGH	Bright
100%	Always HIGH	Full on

PWM is not true analogue

Lesson 10 used the ADC to measure many voltages. PWM goes the other way for control: it fakes a mid level by pulsing. An LED and your eye average the pulses. A slow multimeter may show a mid reading, but the pin is still switching HIGH/LOW.

Do not treat analogWrite as producing a clean DC voltage for precision analogue circuits. Use it for brightness, simple motor enable/PWM speed, and similar average-power jobs.

Comparison of a PWM pulse train with a dashed average line versus a true steady mid-level DC voltage — Left: PWM pulses and their average. Right: true steady DC (what Uno PWM pins do not output).

analogWrite on the Uno

analogWrite(pin, value) sets the PWM duty on pins marked ~ : 3, 5, 6, 9, 10 and 11. value is 0-255 (8-bit). 0 is off, 255 is full on, mid values set the duty cycle.

Call pinMode(pin, OUTPUT) first. Heavy loads still need a driver (lesson 12) - PWM only controls the pin; it does not raise the pin current limit.

Scale from analogWrite 0 to 255 with cards for 0 always LOW, 127 about half duty, and 255 always HIGH — Map a pot (0-1023) to 0-255 when linking analogRead to LED brightness.

const byte ledPin = 9;  // must be a ~ PWM pin

pinMode(ledPin, OUTPUT);
analogWrite(ledPin, 0);    // off
analogWrite(ledPin, 127);  // about half
analogWrite(ledPin, 255);  // full

Non-blocking time with millis

delay() stops the whole sketch until the wait finishes, so other work cannot run. millis() returns milliseconds since the board started (unsigned long). Save a timestamp and compare: if enough time has passed, do the task and update the timestamp. loop() can then keep reading sensors and updating PWM while reports run on a slower schedule.

unsigned long lastReport = 0;

if (millis() - lastReport >= 500) {
  lastReport = millis();
  // do the slow task here (e.g. Serial.println)
}

State machine idea

A state variable records the current phase of a behaviour (for example IDLE, ON, WAIT). Events and timers decide when to move to another state. Combined with millis, you can run timed sequences without long delay() calls blocking the rest of the sketch.

Wiring and safe build sequence

Breadboard wiring for lesson 11: PWM, millis() & State Machines — Breadboard layout for this lesson. Match colours and pins before powering the circuit. Click the image for a larger view.

D9 -> 330 ohm -> LED anode
LED cathode -> GND
Potentiometer wiper -> A0; outer legs -> 5 V and GND

Power rule: switch off before moving wires. Arduino I/O pins are control signals; high-current loads require a driver and suitable external supply.

Worked sketch

Download .ino sketch

const byte ledPin = 9;
unsigned long lastReport = 0;

void setup() {
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  int raw = analogRead(A0);
  byte brightness = map(raw, 0, 1023, 0, 255);
  analogWrite(ledPin, brightness);

  if (millis() - lastReport >= 500) {
    lastReport = millis();
    Serial.println(brightness);
  }
}

How the code works

D9 is a PWM (~) pin so analogWrite can set duty cycle.
map converts the ADC reading (0-1023) into a PWM value (0-255).
LED response stays immediate because no delay blocks loop.
The Serial report task runs only every 500 ms via millis.

Test and record evidence

Expected result: The potentiometer smoothly controls brightness while Serial reports twice per second.

Practical evidence checklist

Labelled wiring or block diagram Uploaded sketch and observed result At least one measured/tested value Fault or improvement recorded in logbook

Common faults and checks

Use a pin marked ~ for PWM (3, 5, 6, 9, 10, 11 on Uno).
Do not connect a motor in place of the LED without a driver and flyback protection.
If brightness never changes, check pot wiring to A0 and that ledPin is a PWM pin.

Extension challenge: Create traffic-light states with separate durations using one state variable and millis.

Check your understanding

Q1. Is PWM a true analogue output?

Show answer

No; it is a digital pulse train with variable duty cycle.

Q2. What does duty cycle mean?

Show answer

The percentage of each PWM period that the pin is HIGH.

Q3. What range does analogWrite use on the Uno?

Show answer

0 to 255.

Q4. Why prefer millis for multi-task programs?

Show answer

It schedules work without stopping the rest of loop.