# How does the PWM signal really work?

I am confused exactly how the PWM signal works in DC motors, LEDs, and resistors.

• How do our eyes perceive the brightness of the LED as the average voltage?
• Does a DC motor work as a low pass filter?
• Do the eye's perception and DC motor voltage result in the same graphics?
• What about a resistor working as a low pass filter or is it something else?
• Motors coils have inductance (electrical inertia) and motor rotors have mass (mechanical inertia). Cones and rods have persistence. Commented Sep 18, 2021 at 20:23
• Too many questions. You want to know: (1) How a motor acts like a low-pass filter?; (2) How do humans perceive blinking/flickering light sources?; (3) How do humans perceive brightness?; (4) How can a resistor participate as a low pass filter element?; (5) Does any discussion about human perception relate to any discussion about motors as low-pass filters? --- These questions bury in them false assumptions, which have to be unwound before any answers could be then added. This is way, way too much to handle in a single answer here.
– jonk
Commented Sep 18, 2021 at 22:18

There shouldn't be any great confusion here.

How our eyes perceive the brightness of the LED as the average voltage?

Our eyes don't detect variations in light intensity above about 30 to 50 Hz very well. If they did we would have been driven mad by AC powered lighting and TV/monitor flicker.

It should make sense that if you blink a light at full power and high frequency that the average light level will be proportional to the PWM pulse width.

Is a dc motor working as a low pass filter?

Yes. The mechanical inertia of a motor means it smooths out the bursts of acceleration and deceleration to provide a smooth motion. If you drop the PWM frequency low enough you will start to get vibration.

Do the eye's perception and dc motor voltage result in the same graphics?

DC motor and graphics? You'll need to explain more.

What about a resistor working as a low pass filter or is it something else?

No, the eyes and the motor provide the low-pass filtering.

Similarly with the motor, the average motor current will vary with the PWM pulse width.

• electronics.stackexchange.com/questions/242293/… grapisc that i mentioned Commented Sep 18, 2021 at 20:50
• I think you mean "graphs", not graphics. Higher pulse width = more light or faster motor. Commented Sep 18, 2021 at 21:01
• how does this resistor do the smoothing? Commented Sep 18, 2021 at 21:09
• imgur.com/a/JkHwtmv is this same result ? Commented Sep 18, 2021 at 21:10
• Yes. You can think of the motor and the eye integrating the waveform. Commented Sep 18, 2021 at 21:12