Gravity UART CO2 sensor with Raspberry Pi

This tutorial explains how to connect Gravity UART MH-Z16 CO₂ sensor from DFRobot to Raspberry Pi and get CO₂ readings. This is a nice NDIR-sensor with a wide range, up to 50000 ppm.

I have used Raspberry Pi 3 B+ for the tutorial below, but it can be applied for other models as well.

Restore UART0

There are two UARTs on Raspberry Pis: UART0 (PL011) and miniUART. miniUART is not suitable for this sensor. It could be used, but baud rate and communications would not be stable.

UART0 is connected to GPIO 14 and 15. The interface level is 3.3V which corresponds to interface level of MH-Z16 CO₂ sensor.

Raspberry Pi uses UART0 for Bluetooth module by default. Adding dtoverlay=pi3-disable-bt to /boot/config.txt disables the Bluetooth device and restores UART0 to GPIO 14 and 15 (ttyAMA0). Apparently the modem has to be disabled as well: sudo systemctl disable hciuart.

Note that you might need to update your Raspberry Pi if the kernel does not support device overlay (dtoverlay):

$ sudo apt-get update && sudo apt-get upgrade

If commands above did not bring required firmware, you can get the very latest firmware running sudo rpi-update.

See The Raspberry Pi UARTs for more information.

Testing UART0

After reboot, connect GPIO 14 and 15 pins together. Install minicom to test serial communications:

sudo apt-get install minicom

Run minicom -D /dev/ttyAMA0 and start typing to verify whether serial communictions work on UART0.

Connecting Gravity UART CO₂ sensor

Gravity UART has 4 pins: 5V, GND, TX, RX. Note that interface level of TX/RX is 3.3V, but power is 5V.

Connections:

Getting CO₂ measurements

Communication protocol is described in Datasheet MH-Z16.

I have used python3 script to get CO₂ measurements from Gravity UART MH-Z16 sensor. python-serial is required as a dependency.

#!/usr/bin/python3

import serial
import binascii
import sys

def verify_checksum(bytes):
  if len(bytes) != 9:
    return False
  sum = 0
  for i in range(1, 8):
    sum += bytes[i]
  sum = sum % 256
  checksum = 255 - sum + 1

  return bytes[8] == checksum

def get_co2(con):
  con.write(bytearray(b'\xff\x01\x86\x00\x00\x00\x00\x00\x79'))
  rcv = con.read(9)
  if not verify_checksum(rcv):
    print("Checksum error from received: `{}'".format(binascii.hexlify(rcv)), file=sys.stderr)
    return -1
  return rcv[2] * 256 + rcv[3]

def calibrate_span(con):
  con.write(bytearray(b'\xff\x01\x88\x07\xd0\x00\x00\x00\xa0'))

def calibrate_zero(con):
  con.write(bytearray(b'\xff\x01\x87\x00\x00\x00\x00\x00\x78'))

con = serial.Serial("/dev/ttyAMA0", 9600, timeout=5)
co2 = get_co2(con)
con.close()

if co2 < 100 or co2 > 6000:
  print("CO2 concentration {} is out of range".format(co2), file=sys.stderr)
  exit(1)

print(co2)

You can get very nice historical CO₂ measurements inside or outside: