Try to get hardware working. Proper regulation is missing.

firmware/weckkubator/weckkubator.ino

@@ -1,10 +1,24 @@
-#include <Wire.h> // Support for I2C --> LCD
-#include <LiquidCrystal_I2C.h> // Support for PCF8574 --> LCD
+#include <Wire.h>  // Support for I2C --> LCD
+#include <LiquidCrystal_I2C.h>  // Support for PCF8574 --> LCD
+#include <OneWire.h>  // 1-Wiresupport for DS18B20
+#include <DallasTemperature.h>  // Easy reading of DS18B20
 
-LiquidCrystal_I2C lcd(0x3F,20,4); // declare LCD
+#define ONE_WIRE_BUS 10  // Enable 1-Wirebus on digital 10 (PB2 - Pin16)
+
+LiquidCrystal_I2C lcd(0x3F,20,4);  // declare LCD
+OneWire oneWire(ONE_WIRE_BUS);  // declare 1-Wirebus
+DallasTemperature sensors(&oneWire);  // declare sensors 
+
+float tempset;
+float watertemp;
+int poti;
 
 void setup() {
-
+  // Setup GPIO
+  pinMode(9, OUTPUT);  // Output "Wecktopf"
+  pinMode(8, OUTPUT);  // Fan
+  // Setup sensors
+  sensors.begin();
   // Setup I2C-LCD
   lcd.init();
   lcd.backlight();
@@ -19,15 +33,53 @@ void setup() {
 
 void loop() {
   //
-  lcd.setCursor(6, 0);
-  lcd.print("Nothing");
-  lcd.setCursor(9, 1);
-  lcd.print("to");
-  lcd.setCursor(9, 2);
-  lcd.print("do");
-  lcd.setCursor(8, 3);
-  lcd.print("jet.");
+  settemp();
+  readtemp();
+  lcdoutput();
 
-  delay(1000000000);
+  delay(100);
+  
+  if (tempset >= watertemp) {
+    digitalWrite(9, HIGH);
 
+    
+  }
+  else {
+    digitalWrite(9, LOW);
+
+  }
+  
+}
+
+//------------------------------------------
+// Read temp from DS18B20
+void readtemp() {
+
+  /* Read temp of first sensor and take if for watertemp.
+   * In future we might add others.
+   */
+  sensors.requestTemperatures();
+  watertemp = sensors.getTempCByIndex(0);
+  
+}
+
+//------------------------------------------
+// LCD output
+void lcdoutput() {
+  // Output temperatureset
+  lcd.setCursor(0, 1);
+  lcd.print("s:");
+  lcd.print(tempset);
+
+  // Output watertemp
+  lcd.setCursor(9, 1);
+  lcd.print("t:");
+  lcd.print(watertemp);
+}
+
+//------------------------------------------
+// Read Poti
+void settemp() {
+  poti = analogRead(A3);
+  tempset = (poti*0.1);
 }