Kitchen air on an Arduino Uno

// Kitchen air: a BME680 heats a tiny plate and reads its resistance, which
// falls when the air carries cooking smoke or solvents. It learns the clean
// air's figure first, then calls out bad air when the reading drops well
// below it.
//
//   BME680    SDA -> SDA   SCL -> SCL   VIN -> 5 V   GND -> GND
//
// The gas figure means nothing on its own: every sensor reads a different
// resistance in the same air. What means something is the fall from the
// sensor's own clean-air baseline, so the sketch takes that as the first
// reading and compares against it.
//
// Things to try. Drag the gas slider on the BME680 down to half: the sketch
// calls out bad air. Drag the temperature and humidity sliders and watch the weather
// line follow.

#include <Wire.h>
#include "Adafruit_BME680.h"

Adafruit_BME680 bme;
uint32_t baseline = 0;

void setup() {
  Serial.begin(9600);
  if (!bme.begin()) {
    Serial.println("No BME680");
    while (1);
  }
  // The plate at 320 C for 150 ms each reading.
  bme.setGasHeater(320, 150);
  bme.performReading();
  baseline = bme.gas_resistance;
  Serial.print("Clean air: ");
  Serial.print((baseline + 500) / 1000);
  Serial.println(" kohm");
}

void loop() {
  if (!bme.performReading()) {
    return;
  }
  // Below two thirds of the baseline is worth a look.
  bool bad = bme.gas_resistance < baseline / 3 * 2;
  Serial.print(bme.temperature, 1);
  Serial.print(" C  ");
  Serial.print(bme.humidity, 0);
  Serial.print(" %RH  ");
  Serial.print(bme.pressure / 100.0, 1);
  Serial.print(" hPa  gas ");
  Serial.print((bme.gas_resistance + 500) / 1000);
  Serial.println(bad ? " kohm  bad air" : " kohm");
  delay(1000);
}