PWM (Pulse Width Modulation)

PWM stands for Pulse Width Modulation. It's a trick that lets a digital chip — which can only turn things fully on or fully off — behave as if it's dimming a light, fading a sound, or running a motor at half-speed.

The fan analogy

Imagine a desk fan with only two settings: full blast or completely off. You want to feel a gentle breeze.

You could flip the switch on for 1 second, off for 1 second, on for 1 second — and you'd feel pulses of cold air. Not great.

Now flip it 100 times per second: on for 1/200 of a second, off for 1/200 of a second. Your skin and eyes can't possibly track that fast. What you'd feel is a steady, half-strength breeze.

That's PWM. The fan is still only ever fully on or fully off — but the fraction of the time it spends on (the "duty cycle") decides the average power.

Duty cycle, the only thing that matters

A 0% duty cycle means it's always off. A 100% duty cycle means it's always on. 50% means it's on half the time and off half the time. 25% means a quarter of the time.

When you write analogWrite(pin, 128) in Arduino, you're telling the chip: "send PWM at 50% duty cycle to this pin" (128 is half of 255, which is the max).

Why robots can't live without it

Every motor in a robot is controlled with PWM. So is every LED that dims. So is every servo that holds a precise angle. PWM is the universal language between a tiny digital brain and a physical, analog world.

LEDs dim by being on-off fast enough that your eyes blur it into "half-bright."

DC motors spin at variable speed because their inertia smooths out the on-off pulses into a steady rotation.

Servos (the small motors with a horn that can hold a specific angle) read the length of the PWM pulse — usually between 1 and 2 milliseconds — and rotate to a position based on it.

Sound — PWM at audio frequencies (a few kilohertz) can make a piezo buzzer play a melody.

What can go wrong

If the PWM frequency is too slow (say, 50 Hz on an LED), you'll see it flickering. That's why nicer LED bulbs and dashboards use 1 kHz or higher.

If a motor is run at very low duty cycle, it might just hum without spinning, because the small bursts of power aren't enough to overcome friction. You need a minimum duty cycle to get motion. This is called the stall threshold.

If the chip's pin can't supply enough current, the motor or LED will dim only weakly, or the chip itself might overheat. That's why we use a motor driver (like an L298N or a TB6612FNG) — a beefy intermediate chip that the microcontroller commands via PWM, but which actually carries the current.

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Curious how a servo actually decodes a PWM signal into an angle? Read Servo motor.