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PWM and Analog Output on Arduino – A Technical Overview

What is PWM (Pulse Width Modulation)?

PWM (Pulse Width Modulation) is a technique used to simulate analog voltage levels using a digital output.
Instead of varying the actual voltage, PWM rapidly switches the signal ON and OFF at a fixed frequency.

The duty cycle of this signal determines the effective output:

  • 0% duty cycle → Always OFF → 0V average
  • 50% duty cycle → ON half the time → ~2.5V average
  • 100% duty cycle → Always ON → 5V average (on typical Arduino)

Although the voltage is either HIGH (5V) or LOW (0V), the rapid toggling tricks components into perceiving a smoothed average voltage.

What Happens Electrically?

Let's take an LED as the load on a PWM pin:

  • If the pin is HIGH for 25% of the time and LOW for 75%, the LED is dimly lit
  • If the pin is HIGH for 75%, the LED is much brighter

This is because the average power delivered is higher at a higher duty cycle.

PWM signals are essentially square waves, but for slow-reacting loads (like LEDs), the human eye integrates the flashes and sees a consistent brightness.

Visual Representation (Duty Cycles)

Duty Cycle Description Approx. Avg Voltage (5V)
0% Always OFF 0V
25% Dim ~1.25V
50% Medium brightness ~2.5V
75% Bright ~3.75V
100% Fully ON 5V

Filtering the PWM – Smooth Analog Output

In some cases, especially for sensitive analog electronics (like audio or DAC simulation), PWM can be passed through a low-pass filter (capacitor + resistor) to smooth the signal.

This gives a nearly constant DC voltage corresponding to the duty cycle.

PWM on Arduino – Behind the Scenes

Arduino's analogWrite(pin, value) function sets the PWM duty cycle:

  • value ranges from 0 to 255
  • The default frequency for most PWM pins is ~490 Hz
  • Pins 5 and 6 often use ~980 Hz (on Uno)

Example:

analogWrite(9, 128);  // ~50% duty cycle on pin 9

Note: Even though it's called analogWrite(), it does not produce a true analog voltage – just a digital PWM signal.

Which Arduino Pins Support PWM?

On the Arduino Uno, the following pins support PWM:

  • 3, 5, 6, 9, 10, 11

You can recognize them by the ~ symbol next to their number on the board silkscreen.

How Does It Work Internally?

PWM is usually implemented using hardware timers on the microcontroller. These generate consistent square wave signals with high timing accuracy, independent of your code’s execution.

Summary

  • PWM is a digital technique to simulate analog voltages
  • Used in LED dimming, motor control, audio, and communication
  • Arduino makes PWM easy with analogWrite()
  • Filtering can convert PWM to real analog voltage

PWM is an essential technique for combining digital control with analog behavior – a fundamental tool in embedded systems.

Documentation written by: Jose Luis Ocana (GitHub: 0xZorro)
Last updated: April 2025