Read MH_Z19 PWM with RPi gpio 12.

pwm.py

# coding: utf8
import RPi.GPIO as GPIO
import time

class GPIO_Edge_Timeout(Exception):
  pass

CYCLE_START_HIGHT_TIME = 2 

GPIO.setmode(GPIO.BCM)
GPIO.setup(12,GPIO.IN)

channel = GPIO.wait_for_edge(12, GPIO.FALLING, timeout=2000)
if channel is None:
  raise GPIO_Edge_Timeout("gpio 12 edge timeout")

channel = GPIO.wait_for_edge(12,GPIO.RISING, timeout=2000)
if channel is None:
  raise GPIO_Edge_Timeout("gpio 12 edge timeout")
else:
  rising = int(time.time() * 1000)

channel = GPIO.wait_for_edge(12, GPIO.FALLING, timeout=2000)
if channel is None:
  raise GPIO_Edge_Timeout("gpio 12 edge timeout")
else:
  falling = int(time.time() * 1000)
  print(rising)

co2 = (falling -rising - CYCLE_START_HIGHT_TIME) / 2
print ("co2", co2)
print ("range2000", co2*(2000/500))
print ("range5000", co2*(5000/500))
print ("range10000", co2*(10000/500))