8000 drivers: Add driver for MH-Z19 (CO2 sensor) by frestr · Pull Request #3778 · micropython/micropython · GitHub
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drivers: Add driver for MH-Z19 (CO2 sensor) #3778

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drivers: Add driver for MH-Z19 (CO2 sensor) #3778

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drivers: Add driver for MH-Z19 (CO2 sensor)
frestr May 10, 2018
689196c
drivers/mhz19: Replace IRQ with polling & use time_pulse_us.
frestr May 11, 2018
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49 changes: 49 additions & 0 deletions drivers/mhz19/mhz19.py
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# MH-Z19 CO2 sensor driver for MicroPython.
# MIT license; Copyright (c) 2018 Fredrik Strupe

import machine
import utime


class TimeoutError(Exception):
pass


class MHZ19:
'''MH-Z19 CO2 sensor driver'''

def __init__(self, pin, max_value=5000):
'''
Args:
pin: the pin that the PWM pin on the MH-Z19 is connected to.
max_value: upper bound of measuring range. usually 2000 or 5000.
'''
self.pin = pin
self.max_value = max_value

def _wait_on_condition(self, cond, timeout=5000):
start = utime.ticks_ms()
while not cond():
if utime.ticks_diff(utime.ticks_ms(), start) > timeout:
raise TimeoutError

def pwm_read(self):
'''Read CO2 value via PWM pin.

Reading usually takes 1-2 seconds.

Returns:
CO2 value in ppm (parts per million), with an accuracy of
±(50 + result * 0.05) ppm.
Raises:
TimeoutError: if the reading takes more than 5 seconds.
'''
# Wait until a new cycle starts
self._wait_on_condition(lambda: self.pin.value() == 0)
self._wait_on_condition(lambda: self.pin.value() == 1)
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This second wait shouldn't be needed because the time_pulse_us that follows will wait until the pin goes high.

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Oh, I must've overlooked that in the docs.


# Measure high and low duration during cycle
t_h = machine.time_pulse_us(self.pin, 1, 1500000) / 1000
t_l = machine.time_pulse_us(self.pin, 0, 1500000) / 1000
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Did you test that this works and gives accurate readings?

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@frestr frestr May 15, 2018 •
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I did and yes, it does. The results are slightly different than before because of the higher precision in the time readings, but this is negligible compared to the accuracy of the sensor itself. In the datasheet the pulses are given in 2 ms increments though, so it may be possible to round the reading to the nearest increment for more consistent readings.

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Ok, that's good to know that it works well. I think this is a good demonstration of how to use the time_pulse_us function.


return self.max_value * (t_h - 2) / (t_h + t_l - 4)
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