Keyestudio Smart Little Turtle Robot V3 Manual page 73

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In the above figure, the green line represents a period, and value of
analogWrite() corresponds to a percentage which is called Duty Cycle as
well. Duty cycle implies that high-level duration is divided by low-level
duration in a cycle. From top to bottom, the duty cycle of first square wave
is 0% and its corresponding value is 0. The LED brightness is lowest, that is,
turn off. The more time high level lasts, the brighter the LED. Therefore, the
last duty cycle is 100%, which correspond to 255, LED is brightest. 25%
means darker.
PWM mostly is used for adjusting the LED brightness or rotation speed of
motor.
It plays vital role in controlling smart robot car. I believe that you can' t wait
to enter next project.
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