BME680 Element

The BME680Element implements reading the 4 sensor values from a BME680 environment air sensor chip. This sensor can capture temperature, humidity, air pressure and air resistance.

The Element implementation uses the Bosch bme680 sensor library that comes with the element implementation and need no further installation. It uses the I2C bus for connection.

To make it available for configuration it needs to be included into the compilation by activating it using the macro

The Element is included in the standard firmware but can be excluded to save memory by removing the definition:

#define HOMEDING_INCLUDE_BME680

in the element register section of the sketch.

Sensor Connectivity

The sensor chip uses a 3.3 V Supply voltage on VCC and GND and needs to be connected to the SDA and SCL line of the I2C bus. The I2C pins are configured using the properties of the device element and the I2C bus is normally shared by multiple elements e.g. sensors.

{
  "device": {
    "0": {
      "i2c-scl": "D2",
      "i2c-sda": "D1"
    }
  }
}

The I2C address of the sensor is 0x77 = 119 by default and is configured using the element configuration. The chip offers the possibility using an alternate address. Please refer to the documentation of your sensor board.

Using the default I2C bus pins:

Signal	ESP8266	ESP32	Sensor	Description
GND	GND	GND	GND	Ground for I2C and Power Supply
VCC	3.3v	3.3v	VCC/VDD	Power Supply
SDA	GPIO4(D2)	IO21	SDA	I2C Data Signal
SCL	GPIO5(D1)	IO22	SCL	I2C Clock Signal
	-	-	SDD	not used
	-	-	CS	not used in I2C Mode

BME680 wiring

Element Configuration

These properties can be configured:

address The I2C Address of the sensor. The default value is 0x77 the other possible value is 0x76.

readTime Time between 2 probes from the sensor are fetched. The default value is 1 minute.

Configuration Example

This example shows how to configure this element:

{
  "bme680": {
    "bd": {
      "address": "0x77",
      "readtime": "10s",
      "ontemperature": "device/0?log=temp=$v",
      "onhumidity": "device/0?log=humidity=$v",
      "onpressure": "device/0?log=pressure=$v",
      "ongas": "device/0?log=gas=$v"
    }
  }
}

Outbound Actions

The actions emitted by the BME680 Element has read some new data (with changed value) from the sensor.

onTemperature - These actions are send when a new temperature value has been read. The value given in the actions is actual temperature in Celcius.

onHumidity - These actions are send when a new temperature value has been read. The value given in the actions is actual Humidity in percentage.

onPressure - These actions are send when a new pressure value has been read. The value given in the actions is actual pressure in hPA.

onGas - These actions are send when a new gas resistance value has been read. The value given in the actions is actual gas resistance value in kOhm.

State

{
  "bme680/0": {
    "active":"true",
    "temperature":"21.21",
    "humidity":"43.48",
    "pressure":"99784",
    "gas":"72724"
  }
}

Interpreting the GAS-OHM values

There are some discussions about how to interpret the values from the gas sensor.

The number is not related directly to quality of the air and Bosch supports a (closed source) library to calculate a proprietary Air Quality Index (IAQ).

Here is a link on how to integrate the algorithm from BOSCH that can be found in the the BSEC SDK:

https://www.bosch-sensortec.com/bst/products/all_products/bsec Bosch Sensortec Software Downloads
https://wolfgangklenk.wordpress.com/2017/11/05/indoor-air-quality-iaq-measurement-with-bosch-bme680-and-stm32f103c8t6/ A example using the BOSCH SDK directly.

I avoid this huge library for now.

However there is a table driven and a formula driven approach for some mapping of the sensor values to a Quality Indicator available. The humidity of the air might be a good factor for the calculation of “good” air quality.

range	quality
521177 - 431331	good
297625 - 213212	average
148977 - 108042	little bad
75010 - 54586	bad
37395 - 27080	worse
18761 - 13591	very bad
9008 - 8371	totally bad

https://forums.pimoroni.com/t/bme680-observed-gas-ohms-readings/6608
https://github.com/G6EJD/BME680-Example Here you find an example for BME680 that calculates a quality index by combining a quality from gas and humidity.