See extras for I2C version using the SC16IS750 UART bridge (slightly outdated, can be updated on request).

A library for the MH-Z19 & MH-Z19B CO2 sensor on the Arduino platform which unlocks significant commands. Supports Hardware/Softeware serial and arduino based boads.

See examples for the recovery code. *Note, Only use if your sensor is not recoverable by other means as it is somewhat untested, however from feedback and personal experience, it can often work.

• Automatically sends "autocalibration off".
• Filter mode, to detect invalid readings when sensor is recovering from power loss / boot (see example)
• Option to print communcation between device and sensor (for debugging)
• Communication error checking
• Examples

^Transmittance is based upon the backwards projection of the raw value (which decreases with CO2) - see examples or My Notes (Ravings)

^* submitted by SFeli

The library can be found in the IDE/IO library manager. Alternatively, simply clone this library to your working library folder and include "MHZ19.h" in your main sketch.

If you are having issues with specific boards, please contact me (find my details below)

Advice: The MH-Z19 works best in the Range of 2000ppm, outside of this accuracy begins to fall away. This is supported by documentation by also by features such as the Analog Out. I would suggest keeping to this range if you need accuracy and a maximum of 5000ppm.

Relevant Datasheets

• The Englisih datasheet for the MH-Z19: MH-Z19

• The English datasheet for the non-JST MH-Z19B version: MH-Z19B NON-JST.

• The Chinese datasheet for the JST MH-Z19B version (more detailed): MH-Z19B JST.

Auto Calibration: The MH-Z19 is a sensor that is designed to be powered on and rarely turned off. The sensor calibrates over time (if autocalibration is on) tuning the Zero at the end of each "ABC" (auto calibration) period (0 - 24hrs currently). After 3 weeks, a value is stored to represent it's accuracy, this can be requested using getAccuracy(). ABC must be disabled by sending the appropriate command before the end of the ABC period to ensure it remains off - this is handled by the library.

Calibration: If you plan to manually calibrate sensor (in my experience this is often be better) then it's important to be aware that Zero calibration does not refer to 0ppm (often a nitrogen environment), instead it refers to 400ppm.

Background Calibration: It's currently unclear how to change this, if possible at all. The value stored on the MH-Z19 and is set to 400ppm. This is used as the zeroing point.

Span: This should be sent after calibrateZero() is used and if readings after reset have failed. I would advise to be set to 2000 as it represents the segment of radiation that interferes with the active element of the sensor.

Zero Calibration: This can be made in two ways: By pulling the zero HD low (0V) for 7 Secs, or be sending command 135 (0x87). As above, the Zero refers to the background CO2 value of 400ppm, not 0ppm. Currently testing is the ability to send an adjustment with command at byte 7, however it is unclear the affects this has.

Range: This is essentially your CO2 scale high/low. 2000ppm is the most accurate and advised. Changing the value will adjust all your readings slightly. However, more importantly this limits the received value from command 134 (0x86). Therefore, it can be useful to use a software alarm.

All parameters can be bypassed using command 132 (0x84), however this requires manual calculations from the Raw value.

Alarm: The analog output is located on the brown wire on the JST version. On the non-JST version it can be found on the far side, beside the Rx pin. It's unclear at the moment how to change the threshold and is not affected by Range. However, it is possible to attach an amplifier to the Analog Out pin and create an interrupt.

Analog Out: An additional feature of MH-Z19. The output in mV corresponds to ppm when using a range of 2000ppm. Alternatively, calculations can be made to adjust the value (See Examples).

• Include most useful and working recovered commands

• Confirm corret units of CO2 Raw

• Determine the functioning of sending byte 7 with Zero Calibration and it's relationship with Range

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Myself, you can find my contact details below.

This project is licensed under the Lesser General Public License v3.0 License - see the LICENSE.md file for details

This library was originaly inspired by Strange-V's work! https://github.com/strange-v/MHZ19;

This is one of my first long pieces of code after starting the journey 6 months ago, I'm still learning the ropes - so constructive feedback is more than welcome; [email protected]

Also, feel free to improve on the project and propose appropriate changes.

If this library was particularly helpful, and you feel like funding a replacement sensor (brutalised from testing!)