Ok, ich dachte es gibt vielleicht eine andere Ursache als den Code und wollte mal einfach so fragen 
Hab mal die Codeschnipsel zusammengefasst welche mein Problem betreffen.
Verwendet wird akuell ein Mega, hab das gleiche Problem auch mit einem UNO.
Schaltplan werde ich morgen nachreichen 
Hoffe soweit genügt das mal
Danke
#include <Wire.h> // I2C
#include <LiquidCrystal_I2C.h> //Display
#include <LCDMenuLib2.h> //Menü
#include <DS1302.h> //RealTimeClock
#include <EEPROM.h> //Eeprom
#include <Encoder.h> //Clickencoder Menüverwaltung
#include <TimerOne.h> //Interrupt
#include <ClickEncoder.h> //Clickencoder Werteverstellung
#include <OneWire.h> //1-Draht Sensoren
#include <DallasTemperature.h> //Temperatursensor DS18B20
#include <ErriezBH1750.h> //Luxsensor BH1750
#include <PID_v1.h> //PID Regler
#include <Adafruit_MCP4725.h> // DAC MCP4725
OneWire tempsens(4); // on pin 4 (a 4.7K resistor is necessary) Temperatursensor
// *********************************************************************
// Lux Sensor
// *********************************************************************
uint16_t luxInnen;
uint16_t luxAussen;
// *********************************************************************
// Temperatur Sensor
// *********************************************************************
int set_barnOffset = EEPROM.get(21, set_barnOffset); // den Wert aus EEPROM lesen
// *********************************************************************
// PID Regler
// *********************************************************************
const int PIN_OUTPUT = 5; //PWM LED Pin
double Setpoint, Input, Output;
double Kp=1, Ki=0.2, Kd=0.05; //Reglerfaktoren
double dblLuxInnen;
// *********************************************************************
// Objects
// *********************************************************************
LCDMenuLib2_menu LCDML_0 (255, 0, 0, NULL, NULL); // root menu element (do not change)
LCDMenuLib2 LCDML(LCDML_0, _LCDML_DISP_rows, _LCDML_DISP_cols, lcdml_menu_display, lcdml_menu_clear, lcdml_menu_control);
DS1302 rtc(8, 12, 10); // (RST,DAT,CLK) Creation of the Real Time Clock Object
ClickEncoder *encoder; // Dreh-Enkoder mit Klickbutton
BH1750 HelligkeitInnen(LOW); // ADDR line LOW/open: I2C address 0x23 (0x46 including R/W bit) [default]
BH1750 HelligkeitAussen(HIGH); // ADDR line HIGH: I2C address 0x5C (0xB8 including R/W bit)
PID myPID(&Input, &Output, &Setpoint, Kp, Ki, Kd, DIRECT); //PID Regler
Adafruit_MCP4725 dac; //DAC
void setup()
{
Serial.begin(9600);
Wire.begin(); // Initialize I2C bus
lcd.init(); //LCD initialisieren
lcd.backlight(); //LCD Licht EIN
// Lux Sensor * * * * *
HelligkeitInnen.begin(ModeContinuous, ResolutionHigh); // Initialize sensor in continues mode, high 0.5 lx resolution
HelligkeitAussen.begin(ModeContinuous, ResolutionHigh); // Initialize sensor in continues mode, high 0.5 lx resolution
HelligkeitInnen.startConversion(); // Start conversion
HelligkeitAussen.startConversion();// Start conversion
//***************************PID***************************
//initialize the variables we're linked to
Input = double(luxInnen/2);
Setpoint = 80;
//turn the PID on
myPID.SetMode(AUTOMATIC);
pinMode(PIN_OUTPUT, OUTPUT);
myPID.SetSampleTime(110); //Sample Time Regler
//DAC * * * * * * * * * * * * * * * * *
dac.begin(0x62); // For Adafruit MCP4725A1 the address is 0x62 (default) or 0x63 (ADDR pin tied to VCC)
void loop()
{
//PID
dblLuxInnen = double(luxInnen);
Input = dblLuxInnen/2;
double gap = abs(Setpoint-Input); //distance away from setpoint
myPID.SetTunings(Kp, Ki, Kd);
myPID.Compute();
analogWrite(PIN_OUTPUT, Output);
//PID ENDE
//DAC - Digital Analog Converter 0-10VDC ANM.: *25 Skalierungsfaktor für Ausgangsspannung, false: Wert nicht in EEPROM speichern
dac.setVoltage((uint16_t(Output)*25), false);
//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//Lux Sensor ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
if (HelligkeitInnen.isConversionCompleted()) // Wait for completion (blocking busy-wait delay)
{
luxInnen = HelligkeitInnen.read(); // Read light
}
if (HelligkeitAussen.isConversionCompleted()) // Wait for completion (blocking busy-wait delay)
{
luxAussen = HelligkeitAussen.read(); // Read light
}
}
// *********************************************************************
void mFunc_temp_barn(uint8_t param)
// *********************************************************************
{
byte i;
byte present = 0;
byte type_s;
byte data[12];
byte addr[8];
float celsius;
unsigned long barnmillis; //Hilfsvariable für Tempsens delay
unsigned long delaytime = 1200; //Zeit in Millisekunden Tempsens delay
bool flag_barndelay = LOW;
if(LCDML.FUNC_setup()) // ****** SETUP *********
{
LCDML.FUNC_disableScreensaver();
barnmillis = millis();
LCDML.FUNC_setLoopInterval(100); // starts a trigger event for the loop function every 100 milliseconds
lcd.setCursor(2, 1);
lcd.print("Temperatur Stall");
lcd.setCursor(6, 2);
lcd.print(celsius); // aktuelle Temperatur
lcd.setCursor(12, 2);
lcd.write(223); //223 °-Symbol ANM.:HD44780U Hitachi char library
lcd.setCursor(13, 2);
lcd.print("C");
encoder = new ClickEncoder(enc_PinA, enc_PinB, enc_PinC, enc_Step); // den Dreh-Enkoder initialisieren
Timer1.initialize(1000); // den Interrupt-Timer fuer den Dreh-Enkoder initialisieren
Timer1.attachInterrupt(timerIsr); // und die Interrupt-Funktion festlegen
}
if(LCDML.FUNC_loop()) // ****** LOOP *********
{
ClickEncoder::Button b = encoder->getButton(); // den Button-Status abfragen
if ( !tempsens.search(addr)) {
tempsens.reset_search();
return;
}
// the first ROM byte indicates which chip
switch (addr[0])
{
case 0x10:
type_s = 1;
break;
case 0x28:
type_s = 0;
break;
case 0x22:
type_s = 0;
break;
default:
return;
}
tempsens.reset();
tempsens.select(addr);
tempsens.write(0x44, 1); // start conversion, with parasite power on at the end
if(millis() > delaytime + barnmillis)
{
barnmillis = millis();
flag_barndelay = HIGH;
}
if(flag_barndelay == HIGH)
{
present = tempsens.reset();
tempsens.select(addr);
tempsens.write(0xBE); // Read Scratchpad
for ( i = 0; i < 9; i++) // we need 9 bytes
{
data[i] = tempsens.read();
}
flag_barndelay = LOW;
int16_t raw = (data[1] << 8) | data[0];
if (type_s)
{
raw = raw << 3; // 9 bit resolution default
if (data[7] == 0x10) {
// "count remain" gives full 12 bit resolution
raw = (raw & 0xFFF0) + 12 - data[6];
}
}
else
{
byte cfg = (data[4] & 0x60);
// at lower res, the low bits are undefined, so let's zero them
if (cfg == 0x00) raw = raw & ~7; // 9 bit resolution, 93.75 ms
else if (cfg == 0x20) raw = raw & ~3; // 10 bit res, 187.5 ms
else if (cfg == 0x40) raw = raw & ~1; // 11 bit res, 375 ms
//// default is 12 bit resolution, 750 ms conversion time
}
celsius = ((float)raw / 16.0)+float(set_barnOffset);
}
Serial.println(celsius);
lcd.setCursor(6, 2);
lcd.print(celsius); // aktuelle Temperatur
if (b != ClickEncoder::Open)
{
switch (b)
{ // und entsprechend darauf reagieren
case ClickEncoder::Clicked: // Button wurde einmal angeklickt
LCDML.FUNC_goBackToMenu();
break;
}
}
}
}