arduinocode

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

// Example testing sketch for various DHT humidity/temperature sensors
// Written by ladyada, public domain

#include "DHT.h"

#define DHTPIN 2     // what digital pin we're connected to


// Uncomment whatever type you're using!
//#define DHTTYPE DHT11   // DHT 11
#define DHTTYPE DHT22   // DHT 22  (AM2302), AM2321
//#define DHTTYPE DHT21   // DHT 21 (AM2301)

// Connect pin 1 (on the left) of the sensor to +5V
// NOTE: If using a board with 3.3V logic like an Arduino Due connect pin 1
// to 3.3V instead of 5V!
// Connect pin 2 of the sensor to whatever your DHTPIN is
// Connect pin 4 (on the right) of the sensor to GROUND
// Connect a 10K resistor from pin 2 (data) to pin 1 (power) of the sensor

// Initialize DHT sensor.
// Note that older versions of this library took an optional third parameter to
// tweak the timings for faster processors.  This parameter is no longer needed
// as the current DHT reading algorithm adjusts itself to work on faster procs.
DHT dht(DHTPIN, DHTTYPE);
int sensorIn = A0; 
LiquidCrystal_I2C lcd(0x27,20,4); 

void setup() {
  lcd.init();
  lcd.backlight();
  Serial.begin(9600);
  Serial.println("DHTxx test!");
  analogReference(DEFAULT);
  dht.begin();
}

float read_co2() {
  int sensorValue = analogRead(sensorIn);
  float voltage = sensorValue*(5000/1024.0); 
  //Serial.println(sensorValue);
  //Serial.println(voltage);
  if(voltage == 0) {
   //Serial.println("Fault");
   return -1;
  } else if(voltage < 400) { 
   //Serial.println("preheating");
   return 0;
  } else {
   int voltage_diference=voltage-400;
   float concentration=voltage_diference*50.0/16.0;
   // Print Voltage
   //Serial.print("voltage:");
   //Serial.print(voltage);
   //Serial.println("mv");
   //Print CO2 concentration
   //Serial.print(concentration);
   //Serial.println("ppm");
   return concentration;
 }

}

void loop() {
  delay(2000);
  float co2_ppm = read_co2();
  // Reading temperature or humidity takes about 250 milliseconds!
  // Sensor readings may also be up to 2 seconds 'old' (its a very slow sensor)
  float h = dht.readHumidity();
  // Read temperature as Celsius (the default)
  float t = dht.readTemperature();

  // Check if any reads failed and exit early (to try again).
  if (isnan(h) || isnan(t)) {
    Serial.println("Failed to read from DHT sensor!");
    return;
  }

  // Compute heat index in Celsius (isFahreheit = false)
  float hic = dht.computeHeatIndex(t, h, false);

  Serial.print("{ \"humidity\": \"");
  Serial.print(h);
  Serial.print("\", \"");
  Serial.print("temperature\": \"");
  Serial.print(t);
  Serial.print("\", \"");
  Serial.print("heatindex\": \"");
  Serial.print(hic);
  Serial.print("\", \"co2\": \"");
  Serial.print(co2_ppm);
  Serial.print("\" }\n");
  lcd.setCursor(0,0);
  lcd.print("Temperatur:");
  lcd.print(t);
  lcd.print("C");
  lcd.setCursor(0,1);
  lcd.print("Feuchtigkeit:");
  lcd.print(h);
  lcd.print("%");
  lcd.setCursor(0,2);
  lcd.print("CO2:");
  lcd.print(co2_ppm);
  lcd.print("ppm");  
  
}