"""CircuitPython library for MQ-135 CO2 gas censor.
Based on Micropython library by rubfi, which is based
on Arduino Library developed by G.Krocker (Mad Frog Labs)
and the corrections from balk77 and ViliusKraujutis
More info:
https://hackaday.io/project/3475-sniffing-trinket/log/12363-mq135-arduino-library
https://github.com/ViliusKraujutis/MQ135
https://github.com/balk77/MQ135
https://github.com/rubfi/MQ135
"""
import math
import time
import analogio
class MQ135:
""" Class for dealing with MQ13 Gas Sensors """
# The load resistance on the board
RLOAD = 10.0
# Calibration resistance at atmospheric CO2 level
RZERO = 76.63
# Parameters for calculating ppm of CO2 from sensor resistance
PARA = 116.6020682
PARB = 2.769034857
# Parameters to model temperature and humidity dependence
CORA = 0.00035
CORB = 0.02718
CORC = 1.39538
CORD = 0.0018
CORE = -0.003333333
CORF = -0.001923077
CORG = 1.130128205
# Atmospheric CO2 level for calibration purposes
ATMOCO2 = 397.13
def __init__(self, pin):
self.pin = analogio.AnalogIn(pin)
def get_correction_factor(self, temperature, humidity):
"""Calculates the correction factor for ambient air temperature and relative humidity
Based on the linearization of the temperature dependency curve
under and above 20 degrees Celsius, asuming a linear dependency on humidity,
provided by Balk77 https://github.com/GeorgK/MQ135/pull/6/files
"""
if temperature < 20:
return self.CORA * temperature * temperature - self.CORB * temperature + self.CORC - (humidity - 33.) * self.CORD
return self.CORE * temperature + self.CORF * humidity + self.CORG
def get_resistance(self):
"""Returns the resistance of the sensor in kOhms // -1 if not value got in pin"""
if self.pin.value == 0:
return -1
return (61722./self.pin.value - 1.) * self.RLOAD
def get_corrected_resistance(self, temperature, humidity):
"""Gets the resistance of the sensor corrected for temperature/humidity"""
return self.get_resistance()/ self.get_correction_factor(temperature, humidity)
def get_ppm(self):
"""Returns the ppm of CO2 sensed (assuming only CO2 in the air)"""
return self.PARA * math.pow((self.get_resistance()/ self.RZERO), -self.PARB)
def get_corrected_ppm(self, temperature, humidity):
"""Returns the ppm of CO2 sensed (assuming only CO2 in the air)
corrected for temperature/humidity"""
return self.PARA * math.pow((self.get_corrected_resistance(temperature, humidity)/ self.RZERO), -self.PARB)
def get_rzero(self):
"""Returns the resistance RZero of the sensor (in kOhms) for calibratioin purposes"""
return self.get_resistance() * math.pow((self.ATMOCO2/self.PARA), (1./self.PARB))
def get_corrected_rzero(self, temperature, humidity):
"""Returns the resistance RZero of the sensor (in kOhms) for calibration purposes
corrected for temperature/humidity"""
return self.get_corrected_resistance(temperature, humidity) * math.pow((self.ATMOCO2/self.PARA), (1./self.PARB))
