CO2 monitor on an Arduino Uno

// CO2 monitor: a Sensirion SCD41 counts the CO2 in the room and an SHT40
// reads the temperature and humidity, both on one I2C bus.
//
//   SCD41, SHT40   SDA -> A4   SCL -> A5   VIN -> 5 V   GND -> GND
//
// The SCD41 measures real CO2 by the sound a light pulse makes in it, and
// gives a new reading every five seconds. Fresh outdoor air is about 420 ppm;
// a closed room full of people climbs past 1000 ppm, and that is the point
// where a window wants opening.
//
// Things to try. Drag the CO2 slider on the SCD41 past 1000 ppm and watch
// the sketch ask for a window.

#include <Wire.h>
#include <SensirionI2cScd4x.h>
#include "Adafruit_SHT4x.h"

const uint16_t STUFFY_PPM = 1000;

SensirionI2cScd4x scd41;
Adafruit_SHT4x sht40;

void setup() {
  Serial.begin(9600);
  Wire.begin();
  scd41.begin(Wire, SCD41_I2C_ADDR_62);
  delay(30);
  scd41.stopPeriodicMeasurement();
  if (scd41.startPeriodicMeasurement() != NO_ERROR) {
    Serial.println("No SCD41");
  }
  if (!sht40.begin()) {
    Serial.println("No SHT40");
  }
  Serial.println("CO2 monitor: the first reading comes in five seconds");
}

void loop() {
  bool ready = false;
  scd41.getDataReadyStatus(ready);
  if (!ready) {
    delay(500);
    return;
  }
  uint16_t co2 = 0;
  float t = 0, rh = 0;
  if (scd41.readMeasurement(co2, t, rh) != NO_ERROR) {
    return;
  }
  sensors_event_t humidity, temp;
  sht40.getEvent(&humidity, &temp);

  Serial.print("CO2 ");
  Serial.print(co2);
  Serial.print(" ppm  ");
  Serial.print(temp.temperature, 1);
  Serial.print(" C  ");
  Serial.print(humidity.relative_humidity, 0);
  Serial.println(" %RH");
  if (co2 >= STUFFY_PPM) {
    Serial.println("Stuffy: open a window");
  }
}