Add CO2 sensing with SCD4X

[Coloradohusky]

This PR adds support for CO2 sensing with the SCD40/SCD41 sensors. These sensors also support temperature and relative humidity.

Currently, it is using startLowPowerPeriodicMeasurement, which has an update interval of ~30 seconds. However, the datasheet mentions a measure_single_shot for lower power use, but says it is limited to SCD41 sensors only. Nonetheless, I tried it on my SCD40 sensor and it appeared to work fine. measure_single_shot also includes a delay of 5 seconds for each use.

Average Supply Current:

• High Performance Mode: 11 mA
• Low Power Mode: 2.8 mA
• Idle Single Shot Mode: 1.1 mA for 1 min and 0.36 mA for 5 min interval
• Power-Cycled Single Shot Mode: 0.18 mA for 10 min, 0.09 mA for 20 min, 0.03 mA for 1 hour interval

Other than that, the only differences between SCD40 and SCD41 are a wider CO2 interval and higher accuracy.

See SCD4x Low Power Operation and the Datasheet for more information.

Thanks

[fifieldt]

Since this one has temperature and humidity in addition to co2, it looks like it spans our EnvironmentMetrics and AirQualityMetrics. Do you have a sense for how easy it would be to put the co2 in AirQualityMetrics and keep the temp/humidity in EnvironmentMetrics?

[Coloradohusky]

I'll work on moving it over to AirQualityMetrics instead of EnvironmentMetrics - although I noticed that IAQ is also in EnvironmentMetrics, should that be moved as well?

[Coloradohusky]

Hm yeah, I'm looking closer at AirQualityModule and it's going to be more involved than I'd hoped - AirQualityModule explicitly wants a Adafruit_PM2.5 sensor (aqi = Adafruit_PM25AQI()) so I'm not entirely sure how to handle that with the addition of the CO2 metric

[fifieldt]

Thanks for the heads up! Will take a look in the next few days.

[fifieldt]

@Coloradohusky - I hacked together some things over at
https://github.com/fifieldt/meshtastic-firmware/tree/scd4x

It compiles ... but I haven't got it further than that :)

Do you think you'd be able to hack from there and test with your sensor?

[fifieldt]

@Coloradohusky , have hacked this a bit more. In theory you might also get a screen on your UI with CO2 readings too. Can you test & improve? =D

[Coloradohusky]

Was checking it out just now! Will keep testing 😄

[fifieldt]

OK, over to you -- I'll hold off making any more commits

[Coloradohusky]

Haven't had much luck yet - it properly detects the sensor, but doesn't grab any measurements. Not sure what's preventing the measurements but will keep testing