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CCS811 Gas sensor

CCS811 is an ultra-low power digital gas sensor solution for monitoring indoor air quality. CCS811 integrates a gas sensor solution for detecting low levels of Volatile Organic Compounds typically found indoors, with a microcontroller unit (MCU) and an Analog-to-Digital converter to monitor the local environment and provide an indication of the indoor air quality via an equivalent CO2 or Total Volatile Organic Compounds output over a standard I2C digital interface.

Documentation

Device information

Important:

  • CCS811 needs 20 minutes to warm up before giving any accurate measurement. Once, you'll select a mode and the internal resistor will start heating, keep in mind that accurate results will show up after 20 minutes approximately
  • When you'll receive it, the device needs to be put on reading mode every second for about 48h as it needs time to get a a stable internal resistor
  • The sensor autocalibrate over time. There is a notion of baseline. This baseline should be handle with care and is not the same for all the devices. Also it does evolve over time.

CCS811 exposes 3 pins, here is a short information on every one:

  • The Address pins allows you to select the first of second I2C address. Place it to the ground to select the first one (0x5A) or to VCC to select the second one (0x5B).
  • The Reset pin is sometime present. If present and you want to use it, this will perform a full hard reset.
  • The Wake pin is used to select the chip and wake it up. If you don't want to use it, just put it to the ground.
  • The Interrupt pin allows interruption, if used, the interrupt mode and events will be activated. This needs to be activated to be able to use the embedded Threshold feature.

Understanding the measurement:

  • CCS811 provides equivalent CO2 in part per millions as well as Total Volatile Organic Compounds in part per billion. Those equivalents are calculated based on the own internal mechanism
  • You have as well the raw data reading from the current gas sensor in micro Ampere and the raw voltage ADC. The ADC voltage is 1.65 V for a reading 1023 in a linear mode.

Important to understand:

In order to have this sensor working on a Raspberry Pi, you need to lower the bus speed. This sensor uses a mode called I2C stretching and it is not supported natively on Raspberry Pi. So you must lower the I2C clock to the minimum to make it working properly or use a software I2C with a low clock as well.

In order to do so, open a ssh session with your Raspberry and edit the /boot/firmware/config.txt file:

sudo nano /boot/firmware/config.txt

Note

Prior to Bookworm, Raspberry Pi OS stored the boot partition at /boot/. Since Bookworm, the boot partition is located at /boot/firmware/. Adjust the previous line to be sudo nano /boot/firmware/config.txt if you have an older OS version.

Lowering the hardware I2C clock

Locate the line where you have dtparam=i2c_arm=on, make sure you'll remove any # which can be in front and add ,i2c_arm_baudrate=10000 so the line will now bocome: dtparam=i2c_arm=on,i2c_arm_baudrate=10000

Reboot:

sudo reboot

Notes:

  • This has an impact on the all bus! So if you are using other sensors, this will decrease the speed of all other sensors.
  • Even with the bus speed reduced, you may have issues.

Activating the Sofware I2C

Add the following line to use GPIO 17 for SCA and GPIO 27 for SCL:

dtoverlay=i2c-gpio,i2c_gpio_sda=17,i2c_gpio_scl=27,bus=3,i2c_gpio_delay_us=20

You can of course adjust the GPIO you want to use. The delay os 20 micro seconds correspond to about 10000 Hz. You can change as well the bus number. In this case, bus 3 will be used.

Reboot:

sudo reboot

Notes:

  • This uses 2 extra GPIO
  • This is the best solution especially if you are using extra I2C devices

Usage

You'll find below how to use the sensor. A full example covering in details all the usage can be found in the samples directory.

Create the device

To create a device without any of the pins:

var ccs811 = new Css811Sensor(I2cDevice.Create(new I2cConnectionSettings(1, Ccs811Sensor.I2cFirstAddress)));

To create a device with a wake pin and interrupt pin:

var ccs811 = new Css811Sensor(I2cDevice.Create(new I2cConnectionSettings(1, Ccs811Sensor.I2cFirstAddress)), pinWake: 3, pinInterruption: 2);

Note:

  • If you are using the software I2C device instead of the hardware I2C, adjust the bus number. If like in the previous section, you've setup the software I2C, the bus number is 3. So instancing the device will be then:
var ccs811 = new Css811Sensor(I2cDevice.Create(new I2cConnectionSettings(3, Ccs811Sensor.I2cFirstAddress)));

To create a device using an external chipset like FT4222 to offer GPIO and I2C support including Wake and Interrupt pins:

var ftdiI2C = new Ft4222I2c(new I2cConnectionSettings(0, Ccs811Sensor.I2cFirstAddress));
var gpioController = new GpioController(PinNumberingScheme.Board, new Ft4222Gpio());
ccs811 = new Ccs811Sensor(ftdiI2C, gpioController, 3, 2, -1, false);

You can then display basic information of the device:

Console.WriteLine($"Hardware identification: 0x{ccs811.HardwareIdentification:X2}, must be 0x81");
Console.WriteLine($"Hardware version: 0x{ccs811.HardwareVersion:X2}, must be 0x1X where any X is valid");
Console.WriteLine($"Application version: {ccs811.ApplicationVersion}");
Console.WriteLine($"Boot loader version: {ccs811.BootloaderVersion}");

Select a measurement mode

This is needed to start the measurement in the constant power 1 measurement per second mode. Keep in mind the important notes regarding data accuracy.

ccs811.OperationMode = OperationMode.ConstantPower1Second;

Once the measurement is set to anything else than idle, you can see the next section how to read a measure

Getting measures

If you have selected an Interruption pin, an event mode is put in place. If not, you'll have to check if any measurement is available.

Case of not using the Interrupt pin

The basic example shows how to check if any data is ready and then ready the Gas sensor data.

while (!ccs811.IsDataReady)
{
    Thread.Sleep(10);
}

var error = ccs811.TryReadGasData(out VolumeConcentration eCO2, out VolumeConcentration eTVOC, out ElectricCurrent curr, out int adc);
Console.WriteLine($"Success: {error}, eCO2: {eCO2.PartsPerMillion} ppm, eTVOC: {eTVOC.PartsPerBillion} ppb, Current: {curr.Microamperes} µA, ADC: {adc} = {adc * 1.65 / 1023} V.");

Case of using the Interrupt pin

You can use the previous way or use the Event:

// In the code after initialization
ccs811.MeasurementReady += Ccs811MeasurementReady;

// And a function to be called when a measurement is ready
private static void Ccs811MeasurementReady(object sender, MeasurementThresholdArgs args)
{
    Console.WriteLine($"Measurement Event: Success: {args.MeasurementSuccess}, eCO2: {args.EquivalentCO2.PartsPerMillion} ppm, " +
        $"eTVOC: {args.EquivalentTotalVolatileOrganicCompound.PartsPerBillion} ppb, Current: {args.RawCurrentSelected.Microamperes} µA, " +
        $"ADC: {args.RawAdcReading} = {args.RawAdcReading * 1.65 / 1023} V.");
}

Setting a threshold

This feature is only available if the interruption pin is used. Events needs to be activated as well. This is an example of setting up a threshold between 400 and 600 ppm for the eCO2. Note that the threshold needs to have at least 50 of difference between the minimum and maximum values.

ccs811.MeasurementReady += Ccs811MeasurementReady;
ccs811.SetThreshold(VolumeConcentration.FromPartsPerMillion(400), VolumeConcentration.FromPartsPerMillion(600));

You will then receive an event with the first data point crossing up the threshold. No other data point will raise an event.

Adjusting temperature and humidity

The calculation is sensitive to temperature and humidity. It is recommended to adjust the default values with an accurate temperature and relative humidity source sensor. Default values are 25°C for the temperature and 50% for the relative humidity. The following example shows how to adjust for 21.3°C and 42.5%:

ccs811.SetEnvironmentData(Temperature.FromDegreesCelsius(21.3), Ratio.FromPercent(42.5));

Reading and loading the baseline

The baseline is used to calculate the eCO2 and eTVOC based on the raw data. It is not intended to be human readable. Refer to the documentation to understand more about the concept.

var baseline = ccs811.BaselineAlgorithmCalculation;
Console.WriteLine($"Baseline calculation value: {baseline}, changing baseline");
// Please refer to documentation, baseline is not a human readable number
ccs811.BaselineAlgorithmCalculation = 50300;
Console.WriteLine($"Baseline calculation value: {ccs811.BaselineAlgorithmCalculation}, changing baseline for the previous one");
ccs811.BaselineAlgorithmCalculation = baseline;
Console.WriteLine($"Baseline calculation value: {ccs811.BaselineAlgorithmCalculation}");

CCS811 Samples

This sample application contains flow and menus allowing you to test easily all the feature of the CSS811 and also show how to implement properly all readings.

You can test it thru:

  • A native platform like a Raspberry PI
  • A chip set providing GPIO and I2C like the FT4222

You can use the native GPIO support for the following pins or not:

  • The address pin is used to select primary (0x5A) or secondary (0x5B) I2C device address.
  • The Reset pin is sometime present or not. If present and you want to use it, this will perform a full hard reset.
  • The Wake pin is used to select the chip and wake it up. If you don't want to use it, just put it to the ground.
  • The Interrupt pin allows interruption, if used, the interrupt mode and events will be activated. This needs to be activated to be able to use the embedded Threshold feature.

You can select any of the mode.

A variety of tests and reading, including changing the temperature and humidity correction is proposed.

You can log the date an nicely import them later on in Excel. The following example shows a measurement over time. In blue, the equivalent CO2 in ppm and in orange the equivalent TVOC in ppb. Note that the measurement started to be accurate around 11:35 on this graph.

Graph

Sample wiring

Important to understand:

In order to have this sensor working on a Raspberry Pi, you need to lower the bus speed. This sensor uses a mode called I2C stretching and it is not supported natively on Raspberry Pi. So you must lower the I2C clock to the minimum to make it working properly or use a software I2C with a low clock as well. See the section above.

This example uses the software I2C with GPIO 17 and 27 as explained in the previous section on a Raspberry Pi, Wake pin on GPIO 23 and Interrupt on GPIO 22.

Wiring sample