Generate variable voltages using PWM in MicroPython

PWM, a trick to generate variable voltages on digital pins

PWM is a technique that generates a voltage between 0 and 3.3V using only digital outputs . PWM stands for Pulse With Modulation . It’s based on the period ratio of a logic signal in its high state (3.3V) and in its low state (0V): the PWM consists in varying the duty of an electrical pulse.

PWM operation

The succession of pulses with a given duty is seen on average as a constant voltage between 0V and 3.3V, whose value is determined by:

with α the duty cycle

A PWM signal is configured via the pulse duty and its frequency. We can tune these 2 parameters in MicroPython.

In practice, PWM is used for:

• Controlling a motor speed

• Control LED brightness

• Generate square signals (with α = 0.5)

• Generate music notes (like the sound in retro consoles)

PWM performance on ESP32

The PWM signals are not permanently generated by the microprocessor but by dedicated hardware blocks. Therefore, it is sufficient to configure the PWM blocks only once in the script: the signal will be generated continuously in the background. We associate an output of a PWM block with a pin on your ESP32 board. Processor resources will be free to perform other tasks.

Each PWM block can have an independent frequency.

PWM on MicroPython

Using PWM with MicroPython is very easy. MicroPython takes care of automatically selecting an available PWM block: it is not necessary to indicate which one you intend to use. The hardware stuff described above is completely hidden.

The configuration involves associating a PWM object with a Pin object and choosing the PWM frequency.

from machine import Pin, PWM

pin = Pin(25, mode=Pin.OUT)
pin_with_pwm = PWM(pin) # Attach PWM object on a pin

from machine import Pin
from machine import PWM

from machine import Pin, PWM

Once we have linked an ESP32 pin to our PWM object, all the functions are done directly on the PWM object (Unlike the Arduino code where we specify the pin number with the analogWrite(pin_number) .

We use the .duty() function to choose the duty cycle with a value between 0 and 2 ¹⁰ (0-1023). The voltage is set instantly at the output of the pin selected previously.

To see the voltage variation, you can use the built-in LED of your ESP32 board with the following script:

from machine import Pin, PWM

LED_BUITLTIN = 2 # For ESP32

pwm_led = PWM(Pin(LED_BUITLTIN, mode=Pin.OUT)) # Attach PWM object on the LED pin

# Settings
pwm_led.freq(1_000)

while True:
    for duty in range(0,1024):
        pwm_led.duty(duty) # For ESP32

To be easier, we could specify the duty cycle percentage and then apply a formula.

from machine import Pin, PWM

LED_BUITLTIN = 2 # For ESP32

pwm_led = PWM(Pin(LED_BUITLTIN, mode=Pin.OUT)) # Attach PWM object on the LED pin
pwm_led.freq(1_000)

duty_cycle = 50 # Between 0 - 100 %
pwm_led.duty(int((duty_cycle/100)*1024))

The LED brightness can be changed by ranging the duty cycle between 0 and 100%.

from machine import Pin, PWM
import time

LED_BUITLTIN = 2 # For ESP32

pwm_led = PWM(Pin(LED_BUITLTIN, mode=Pin.OUT)) # Attach PWM object on the LED pin
pwm_led.freq(1_000)

while True:
    for duty in range(100): # Duty from 0 to 100 %
        pwm_led.duty(int((duty/100)*1024))
        time.sleep_ms(10)

Project: Diming the ESP32 built-in LED

With everything we have seen, the script is straightforward to achieve:

from machine import Pin, PWM

LED_BUITLTIN = 2 # For ESP32
pwm_led = PWM(Pin(LED_BUITLTIN, mode=Pin.OUT)) # Attach PWM object on the LED pin

# Settings
pwm_led.freq(1_000)

while True:
    for duty in range(0,1024, 5):
        pwm_led.duty(duty)
    for duty in range(1024,0, -5):
        pwm_led.duty(duty)