Het is gelukt de oplossing was simpel 
de (if else) funtie is de oplossing! Ik was veelste moeilijk aan het denken. het void verhaal maakte het verwarrend.
Wat ik niet begrijp wat is nou de redenen dat sommige mensen de void functie meerdere malen gebruiken?
Ik kan hier over ook niks vinden in mijn boek.
nu alleen nog een vertraging bijvoegen zodat wanneer er beweging gedetecteerd word. dat de code "if (Sensor == HIGH) {}" voor een half uur word uitgevoerd.
Bedankt mannen!
#include <GravityTDS.h>
#include <EEPROM.h>
#include <Bounce2.h>
#include <DHT.h>
#include <LiquidCrystal_I2C.h>
#define setBUTTON 12
#define upBUTTON 8
#define downBUTTON 13
#define DHTPIN 7
#define DHTTYPE DHT11
#define TdsSensorPin A1
Bounce set;
Bounce up;
Bounce down;
GravityTDS gravityTds;
DHT dht(DHTPIN, DHTTYPE);
LiquidCrystal_I2C lcd(0x27, 20, 4); // set the LCD address to 0x27 for a 16 chars and 2 line display
byte HsetpointHIGH = 55;
byte HsetpointLOW = 50;
byte FANspeed = 70;
float temperature = 25, tdsValue = 0;
int Relaystate;
int outputValue;
int sensorValue = 0;
int ledState;
int Sensor;
unsigned long previousMillis1;
unsigned long previousMillis2;
unsigned long previousMillis3;
static byte prevdownButton;
static byte prevupButton;
static byte prevsetButton;
static byte prevSetMode;
byte setMode;
const int Hrelay = 5;
const int Mosfet = 3;
const int sensorWater = A0;
const int waterklep = 6;
const int led = 9;
const int PIR = 2;
const int Display = 10;
long OnTime = 0;
long OffTime = 10000;
void setup() {
 digitalWrite (Display, LOW);
 lcd.init();           // initialize the lcd
 lcd.backlight();
 lcd.begin(16, 2);
 dht.begin();
 digitalWrite (Hrelay, LOW);
 digitalWrite (waterklep, HIGH);
 analogWrite(led, 255);
 set.attach (setBUTTON);
 set.interval(30);
 up.attach (upBUTTON);
 up.interval(30);
 down.attach (downBUTTON);
 down.interval(30);
 pinMode (setBUTTON, INPUT_PULLUP);
 pinMode (upBUTTON, INPUT_PULLUP);
 pinMode (downBUTTON, INPUT_PULLUP);
 pinMode (waterklep, OUTPUT);
 pinMode (Hrelay, OUTPUT);
 pinMode (Display, OUTPUT);
 pinMode (PIR, INPUT);
 HsetpointHIGH = EEPROM.read(0);
 HsetpointLOW = EEPROM.read(1);
 FANspeed = EEPROM.read(2);
 gravityTds.setPin(TdsSensorPin);
 gravityTds.setAref(5.0); //reference voltage on ADC, default 5.0V on Arduino UNO
 gravityTds.setAdcRange(1024); //1024 for 10bit ADC;4096 for 12bit ADC
 gravityTds.begin(); //initialization
}
void loop() {
 Sensor = digitalRead(PIR);
 if (Sensor == HIGH) {
 digitalWrite (Display, LOW);
 unsigned long currentMillis = millis();
 set.update();
 up.update();
 down.update();
 sensorValue = analogRead(sensorWater);
 unsigned long h = dht.readHumidity();
 float    t = dht.readTemperature();
 analogWrite(Mosfet, outputValue);
 //temperature = readTemperature(); //add your temperature sensor and read it
 gravityTds.setTemperature(t); // set the temperature and execute temperature compensation
 gravityTds.update(); //sample and calculate
 tdsValue = gravityTds.getTdsValue(); // then get the value
 if (isnan(h) || isnan(t)) {
  lcd.clear();
  lcd.setCursor(1, 0);
  lcd.print("Failed to read ");
  lcd.setCursor(0, 1);
  lcd.print("from DHT sensor!");
  delay(2000);
  lcd.clear();
  return;
 }
 if (h < HsetpointHIGH) {
  Relaystate = LOW;
  outputValue = map(FANspeed, 0, 100, 126, 140);
 }
 if (h > HsetpointLOW) {
  Relaystate = HIGH;
  analogWrite(Mosfet, 0);
 }
 digitalWrite (Hrelay, Relaystate);
 sensorValue = analogRead(sensorWater);
 if (sensorValue <= 20) {
  ledState = LOW;
 }
 if (sensorValue >= 21) {
  ledState = HIGH;
 }
 if ((ledState == HIGH) && (currentMillis - previousMillis1 >= OnTime))
 {
  ledState = LOW; // Turn it off
  analogWrite(led, 0);
  previousMillis1 = currentMillis; // Remember the time
  digitalWrite(waterklep, ledState); // Update the actual LED
 }
 else if ((ledState == LOW) && (currentMillis - previousMillis1 >= OffTime))
 {
  ledState = HIGH; // turn it on
  analogWrite(led, 255);
  previousMillis1 = currentMillis; // Remember the time
  digitalWrite(waterklep, ledState);  // Update the actual LED
 }
 if ( setMode != 0) {
  if (currentMillis - previousMillis2 >= 5000) {
   setMode = 0;
   previousMillis2 = currentMillis;
  }
 }
 if (set.fell()) {
  setMode++;
 }
 prevsetButton = setBUTTON;
 if (setMode >= 4) {
  setMode = 0;
 }
 if (setMode != prevSetMode) {
  lcd.clear();
  prevSetMode = setMode;
 }
 switch (setMode) {
  case 0://normal
   lcd.setCursor(0, 0);
   lcd.print("Hum:");
   lcd.setCursor(5, 0);
   lcd.print(h);
   lcd.setCursor(7, 0);
   lcd.print("%");
   lcd.setCursor(0, 1);
   lcd.print("Temp:");
   lcd.setCursor(7, 1);
   lcd.print(t);
   lcd.setCursor(12, 1);
   lcd.print((char)223);
   lcd.print("C");
   lcd.setCursor(9, 0);
   lcd.print(tdsValue, 0);
   lcd.setCursor(12, 0);
   lcd.print("ppm");
   previousMillis2 = currentMillis;
   break;
  case 1://edit temp
   HsetpointLOW = constrain(HsetpointLOW, 00, 100);
   if ('upButton' != prevupButton) {
    if (up.fell()) {
     HsetpointLOW = HsetpointLOW + 1;
    }
    prevupButton = 'upButton';
   }
   if ('downButton' != prevdownButton) {
    if (down.fell()) {
     HsetpointLOW = HsetpointLOW - 1;
    }
    prevdownButton = 'downButton';
   }
   lcd.setCursor(0, 0);
   lcd.print("Set Humidity ON:");
   lcd.setCursor(7, 1);
   lcd.print(HsetpointLOW);
   lcd.setCursor(10, 1);
   lcd.print("%");
   EEPROM.write(1, HsetpointLOW);
   break;
  case 2://edit humidity
   HsetpointHIGH = constrain(HsetpointHIGH, 00, 100);
   if ('upButton' != prevupButton) {
    if (up.fell()) {
     HsetpointHIGH = HsetpointHIGH + 1;
    }
    prevupButton = 'upButton';
   }
   if ('downButton' != prevdownButton) {
    if (down.fell()) {
     HsetpointHIGH = HsetpointHIGH - 1;
    }
    prevdownButton = 'downButton';
   }
   lcd.setCursor(0, 0);
   lcd.print("Set Humidity OFF:");
   lcd.setCursor(7, 1);
   lcd.print(HsetpointHIGH);
   lcd.setCursor(10, 1);
   lcd.print("%");
   EEPROM.write(0, HsetpointHIGH);
   break;
  case 3://edit FANspeed
   FANspeed = constrain(FANspeed, 0, 100);
   if ('upButton' != prevupButton) {
    if (up.fell()) {
     FANspeed = FANspeed + 10;
    }
    prevupButton = 'upButton';
   }
   if ('downButton' != prevdownButton) {
    if (down.fell()) {
     FANspeed = FANspeed - 10;
    }
    prevdownButton = 'downButton';
   }
   lcd.setCursor(0, 0);
   lcd.print("Set FANspeed:");
   lcd.setCursor(7, 1);
   lcd.print(FANspeed);
   lcd.setCursor(9, 1);
   lcd.print("%");
   EEPROM.write(2, FANspeed);
   break;
 }
 }
else{
 digitalWrite (Display, HIGH);
 digitalWrite (Hrelay, HIGH);
 digitalWrite (waterklep, HIGH);
 analogWrite(led, 0);
 analogWrite(Mosfet, 0);
}
}