What is PWM

PWM stands for Pulse Width Modulation and it is a technique used in controlling the brightness of LED, speed control of DC motor, controlling a servo motor or where you have to get analog output with digital means.

Terminologies in PWM

TON (On Time): It is the time when the signal is high.

TOFF (Off Time): It is the time when the signal is low.

Period: It is the sum of on time and off time.

Duty Cycle: It is the percentage of time when the signal was high during the time of period.

PWM in Arduino

Pulse Width Modulation, or PWM, is a technique for getting analog results with digital means. Digital control is used to create a square wave, a signal switched between on and off. This on-off pattern can simulate voltages in between the full Vcc of the board (e.g., 5 V on Uno, 3.3 V on a MKR board) and off (0 Volts) by changing the portion of the time the signal spends on versus the time that the signal spends off. The duration of “on time” is called the pulse width. To get varying analog values, you change, or modulate, that pulse width. If you repeat this on-off pattern fast enough with an LED for example, the result is as if the signal is a steady voltage between 0 and Vcc controlling the brightness of the LED.

In the graphic below, the green lines represent a regular time period. This duration or period is the inverse of the PWM frequency. In other words, with Arduino’s PWM frequency at about 500Hz, the green lines would measure 2 milliseconds each. A call to analogWrite() on a scale of 0 – 255, such that analogWrite(255) requests a 100% duty cycle (always on), and analogWrite(127) is a 50% duty cycle (on half of the time)

So at 50% duty cycle and 1Hz frequency, the led will be high for half a second and will be low for the other half second. If we increase the frequency to 50Hz (50 times ON and OFF per second), then the led will be seen glowing at half brightness by the human eye.

Basics of PWM Inclusion in Arduino Code

The Arduino IDE has a built-in function “analogWrite()” which can be used to generate a PWM signal. The frequency of this generated signal for most pins will be about 490Hz and we can give the value from 0-255 using this function.

analogWrite(0) means a signal of 0% duty cycle.

analogWrite(127) means a signal of 50% duty cycle.

analogWrite(255) means a signal of 100% duty cycle.

On Arduino Uno, the PWM pins are 3, 5, 6, 9, 10 and 11. The frequency of PWM signal on pins 5 and 6 will be about 980Hz and on other pins will be 490Hz. The PWM pins are labeled with ~ sign.

Get Started with PWM using IDE examples .

The Fading example demonstrates the use of analog output (PWM) to fade an LED.

It is available in the File->Sketchbook->Examples->Analog menu of the Arduino software.

Connect an LED to Arduino by connecting the anode of the LED to pin number “9” of the arduino and cathode to the ground pin of the arduino uno or refer the below the given circuit.

The code inside “fading” of “analog” found inside examples will look like the below given figure,

To change the intensity of light of the LED ,change the value inside the analogWrite() function to any value between 0(lowest intensity/off) and 255(highest intensity/on)


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