|Playing around with a breakout board for the CSS-811 IC|
It appears what they accomplished is integration of a metal-oxide gas sensor into a tiny package so it uses very little power and has a fast response. For example, the well-known MQ-135 typically uses 750 mW just for heating the sensitive MOX layer.
The datasheet can be found here.
It measures volatile organic compounds, but it is a bit unclear what it measures exactly. It appears to be sensitive to temperature and humidity levels too, although the chip provides a way to let it know the humidity and temperature so it can compensate for it.
Sensitivity to organic solvents
Very roughly from experimentation (and memory):
- Alklanet (contains 2-butoxyethanol), very sensitive to this stuff, we use it everywhere in the space to clean working surfaces
- permanent marker: very sensitive to it
- toluene: very sensitive
- acetone: very sensitive
- butanol: sensitive
- butylacetate: sensitive
- butane, it is detected but not impressively so
- chloroform, almost no response!
- dichloro-methane: insensitive
- acetaldehyde: not very sensitive to it
Pins on the breakout board:
- VCC this is directly connected to the IC (there is no voltage regulator on board), connect this to 3.3V
- GND to ground obviously
- SCL signal from I2C
- SDA signal from I2C
- WAK this needs to be low (!) in order for the chip to be active
- INT can be left unconnected
- RST can be left unconnected
- ADD can be left unconnected. It has a 10k pull-down to GND, setting the I2C address to 0x5A.
I'm wiring it up to a Wemos D1 mini board with an ESP8266.
There is a firmware update available for the microcontroller inside the CCS811. My board shipped with the V1.1 version. I flashed it to the V2.0 version with this firmware updater by Maarten Pennings.
The newer 2.0 firmware fixes the upper range of eTVOC. In the older version, the maximum reported concentration was 1156 bpm, this limit is no longer present with the 2.0 firmware.
I found the following libraries (in order of preference, top one most likely to use):
Interesting projects using it together with a BME280:
Application to air quality monitoring at RevSpace
A sensor like this could be used to monitor indoor air quality at Revspace.
The plan is to build a TVOC sensor with the following features:
- compensation of influence of temperature and humidity
- keeping track of the baseline of the sensor
- display of current TVOC concentration on an RGB LED stick
- reporting of current TVOC concentration over MQTT
- pin D3 goes to WS2812 VDC
- pin D4 goes to WS2812 DIN
- pin GND goes to WS2812 GND
- pin D5 goes to CCS811 WAK
- pin D6 goes to CCS811 SDA
- pin D7 goes to CCS811 SCL
- pin D8 goes to CCS811 GND, we use a data pin as ground because the D1 mini has only one ground pin
- pin 3V3 goes to CCS811 VCC
temperature and humidity correction
A BME280 is used to read ambient temperature and humidity. The microcontroller inside the CCS811 can use this information to improve the reading.
It can be connected on the same I2C bus, but the software has to toggle the CCS811 WAK pin. The CCS811 doesn't play nice with other I2C devices on the same bus (perhaps this is caused by the clock stretching requirement).
WS2812 LED bar display
A RGB LED bargraph shows the current TVOC concentration, using a WS2812 RGB LED stick. On a logarithmic scale of 8 pixels, LEDs light for each of the following levels: 62, 125, 250, 500, 1000, 2000, 4000, 8000 ppb
Also, I don't know what a specific TVOC concentration means, e.g. what level is considered as clean air?
The air at RevSpace appears to default to a quite high value of approximately 500-1000, probably because of Alklanet vapours.
The microcontroller inside the CCS811 keeps track of the properties of the particular sensor and the environment it is in. The state of this is kept in a baseline register.
We periodically read this register and save it in flash/EEPROM (once an hour). Upon startup, the baseline register is restored from the saved value.
Saving and restoring the register is described in AMS Application Note AN000370.
Reading and writing the baseline register is supported by the Sparkfun library.
- Sara HÖgdahl, "Placing VOC Sensors for Assessing Air Quality - A CFD Study of Indoor VOC Distribution", MJ243X Degree project in Sustainable Buildings, second level, 2018-06-13, Stockhom.
- Ultrafine particles from 3d printing: https://www.3ders.org/images2016/new-study-shows-that-3d-printing-in-confined-spaces-can-be-bad-for-your-health-3.jpg
- Volatile organic compounds from 3d printing: https://www.3ders.org/images2016/new-study-shows-that-3d-printing-in-confined-spaces-can-be-bad-for-your-health-4.jpg