@BartE Adding the extra code done the job, now its working great. Thank you for your help!

The final working code if someone wonders is:

#include <SPI.h>
#include <MySensor.h>
#include <DHT.h>
#include <Bounce2.h>

#include <MySigningNone.h>
#include <MyTransportNRF24.h>
#include <MyTransportRFM69.h>
#include <MyHwATMega328.h>

#define NODE_ID 1
#define CHILD_ID_HUM 0
#define CHILD_ID_TEMP 1
#define CHILD_ID_RELAY 2
#define RELAY_PIN 3
#define BUTTON_PIN 4
#define HUMIDITY_SENSOR_DIGITAL_PIN 2
//unsigned long SLEEP_TIME = 30000; // Sleep time between reads (in milliseconds)

#define RELAY_ON 1  // GPIO value to write to turn on attached relay
#define RELAY_OFF 0 // GPIO value to write to turn off attached relay

MyTransportNRF24 radio(RF24_CE_PIN, RF24_CS_PIN, RF24_PA_LEVEL_GW);

MyHwATMega328 hw;

MySensor gw(radio, hw);

Bounce debouncer = Bounce(); 
int oldValue=0;
bool state;
MyMessage msg(CHILD_ID_RELAY,V_LIGHT);

DHT dht;
float lastTemp;
float lastHum;
boolean metric = true;
MyMessage msgHum(CHILD_ID_HUM, V_HUM);
MyMessage msgTemp(CHILD_ID_TEMP, V_TEMP);

void setup()
{
  gw.begin(incomingMessage, AUTO, true);
  dht.setup(HUMIDITY_SENSOR_DIGITAL_PIN);
  gw.sendSketchInfo("Combo", "1.0");
  gw.present(CHILD_ID_HUM, S_HUM);
  gw.present(CHILD_ID_TEMP, S_TEMP);
  pinMode(BUTTON_PIN,INPUT);
  digitalWrite(BUTTON_PIN,HIGH);
  debouncer.attach(BUTTON_PIN);
  debouncer.interval(5);
  gw.present(CHILD_ID_RELAY, S_LIGHT);
  metric = gw.getConfig().isMetric;
  digitalWrite(RELAY_PIN, RELAY_OFF);
  pinMode(RELAY_PIN, OUTPUT);
  state = gw.loadState(CHILD_ID_RELAY);
  digitalWrite(RELAY_PIN, state?RELAY_ON:RELAY_OFF);
  
}


unsigned long lastCheckTimestamp = 0;

// setup stuff

void incomingMessage(const MyMessage &message) 
{
  if (message.type == V_LIGHT) 
  {
    digitalWrite(RELAY_PIN, message.getBool() ? RELAY_ON : RELAY_OFF);
    gw.saveState(1, message.getBool());
    Serial.print("Incoming change for sensor:");
    Serial.print(message.sensor);
    Serial.print(", New status: ");
    Serial.println(message.getBool());
  }
}

void loop()
{
    gw.process();
    if (debouncer.update() && debouncer.read() == 0) {
       gw.send(msg.set(gw.loadState(1)?false:true), true); // Send new state and request ack back
    }

    // Check if the minimal sample period has been passed 
    if ((lastCheckTimestamp < (millis() - dht.getMinimumSamplingPeriod()) ) || lastCheckTimestamp > millis())
    {
      lastCheckTimestamp  = millis();

      // do the normal stuff after the delay
      float temperature = dht.getTemperature();
      if (isnan(temperature))
      {
        Serial.println("Failed reading temperature from DHT");
      } 
      else if (temperature != lastTemp) 
      {
        lastTemp = temperature;
        if (!metric) 
        {
          temperature = dht.toFahrenheit(temperature);
        }
        gw.send(msgTemp.set(temperature, 1));
        Serial.print("T: ");
        Serial.println(temperature);
      }
      float humidity = dht.getHumidity();
      if (isnan(humidity))
      {
        Serial.println("Failed reading humidity from DHT");
      } else if (humidity != lastHum)
      {
        lastHum = humidity;
        gw.send(msgHum.set(humidity, 1));
        Serial.print("H: ");
        Serial.println(humidity);
      }
   }
}

When i try your sketch in this format:

#include <SPI.h>
#include <MySensor.h>
#include <DHT.h>
#include <Bounce2.h>

#include <MySigningNone.h>
#include <MyTransportNRF24.h>
#include <MyTransportRFM69.h>
#include <MyHwATMega328.h>

#define NODE_ID 1
#define CHILD_ID_HUM 0
#define CHILD_ID_TEMP 1
#define CHILD_ID_RELAY 2
#define RELAY_PIN 3
#define BUTTON_PIN 4
#define HUMIDITY_SENSOR_DIGITAL_PIN 2
//unsigned long SLEEP_TIME = 30000; // Sleep time between reads (in milliseconds)

#define RELAY_ON 1  // GPIO value to write to turn on attached relay
#define RELAY_OFF 0 // GPIO value to write to turn off attached relay

MyTransportNRF24 radio(RF24_CE_PIN, RF24_CS_PIN, RF24_PA_LEVEL_GW);

MyHwATMega328 hw;

MySensor gw(radio, hw);

Bounce debouncer = Bounce(); 
int oldValue=0;
bool state;
MyMessage msg(CHILD_ID_RELAY,V_LIGHT);

DHT dht;
float lastTemp;
float lastHum;
boolean metric = true;
MyMessage msgHum(CHILD_ID_HUM, V_HUM);
MyMessage msgTemp(CHILD_ID_TEMP, V_TEMP);

void setup()
{
  gw.begin(incomingMessage, AUTO, true);
  dht.setup(HUMIDITY_SENSOR_DIGITAL_PIN);
  gw.sendSketchInfo("Combo", "1.0");
  gw.present(CHILD_ID_HUM, S_HUM);
  gw.present(CHILD_ID_TEMP, S_TEMP);
  pinMode(BUTTON_PIN,INPUT);
  digitalWrite(BUTTON_PIN,HIGH);
  debouncer.attach(BUTTON_PIN);
  debouncer.interval(5);
  gw.present(CHILD_ID_RELAY, S_LIGHT);
  metric = gw.getConfig().isMetric;
  digitalWrite(RELAY_PIN, RELAY_OFF);
  pinMode(RELAY_PIN, OUTPUT);
  state = gw.loadState(CHILD_ID_RELAY);
  digitalWrite(RELAY_PIN, state?RELAY_ON:RELAY_OFF);
  
}


unsigned long lastCheckTimestamp = 0;

// setup stuff

void loop()
{
    gw.process();
    if (debouncer.update() && debouncer.read() == 0) {
       gw.send(msg.set(gw.loadState(1)?false:true), true); // Send new state and request ack back
    }

    // Check if the minimal sample period has been passed 
    if ((lastCheckTimestamp < (millis() - dht.getMinimumSamplingPeriod()) ) || lastCheckTimestamp > millis())
    {
      lastCheckTimestamp  = millis();

      // do the normal stuff after the delay
      float temperature = dht.getTemperature();
      if (isnan(temperature))
      {
        Serial.println("Failed reading temperature from DHT");
      } 
      else if (temperature != lastTemp) 
      {
        lastTemp = temperature;
        if (!metric) 
        {
          temperature = dht.toFahrenheit(temperature);
        }
        gw.send(msgTemp.set(temperature, 1));
        Serial.print("T: ");
        Serial.println(temperature);
      }
      float humidity = dht.getHumidity();
      if (isnan(humidity))
      {
        Serial.println("Failed reading humidity from DHT");
      } else if (humidity != lastHum)
      {
        lastHum = humidity;
        gw.send(msgHum.set(humidity, 1));
        Serial.print("H: ");
        Serial.println(humidity);
      }
   }
}

Forumhlp.ino: In function 'void setup()':
Forumhlp:44: error: 'incomingMessage' was not declared in this scope
'incomingMessage' was not declared in this scope

Seems something is not right, tried deleting incomingMessage line, but no dice.

@BartE Is there any way to decrease the amount of time the button needs to be pushed ? Maybe my choice of the button is not good, will try another button when i come home?

Hi, thank you for the suggestions, its working now!
The final code looks like this:

#include <SPI.h>
#include <MySensor.h>
#include <DHT.h>
#include <Bounce2.h>

#include <MySigningNone.h>
#include <MyTransportNRF24.h>
#include <MyTransportRFM69.h>
#include <MyHwATMega328.h>

#define NODE_ID 1
#define CHILD_ID_HUM 0
#define CHILD_ID_TEMP 1
#define CHILD_ID_RELAY 2
#define RELAY_PIN 3
#define BUTTON_PIN 4
#define HUMIDITY_SENSOR_DIGITAL_PIN 2
//unsigned long SLEEP_TIME = 30000; // Sleep time between reads (in milliseconds)

#define RELAY_ON 1  // GPIO value to write to turn on attached relay
#define RELAY_OFF 0 // GPIO value to write to turn off attached relay

MyTransportNRF24 radio(RF24_CE_PIN, RF24_CS_PIN, RF24_PA_LEVEL_GW);

MyHwATMega328 hw;

MySensor gw(radio, hw);

Bounce debouncer = Bounce(); 
int oldValue=0;
bool state;
MyMessage msg(CHILD_ID_RELAY,V_LIGHT);

DHT dht;
float lastTemp;
float lastHum;
boolean metric = true;
MyMessage msgHum(CHILD_ID_HUM, V_HUM);
MyMessage msgTemp(CHILD_ID_TEMP, V_TEMP);

void setup()
{
  gw.begin(incomingMessage, AUTO, true);
  dht.setup(HUMIDITY_SENSOR_DIGITAL_PIN);
  gw.sendSketchInfo("Combo", "1.0");
  gw.present(CHILD_ID_HUM, S_HUM);
  gw.present(CHILD_ID_TEMP, S_TEMP);
  pinMode(BUTTON_PIN,INPUT);
  digitalWrite(BUTTON_PIN,HIGH);
  debouncer.attach(BUTTON_PIN);
  debouncer.interval(5);
  gw.present(CHILD_ID_RELAY, S_LIGHT);
  metric = gw.getConfig().isMetric;
  digitalWrite(RELAY_PIN, RELAY_OFF);
  pinMode(RELAY_PIN, OUTPUT);
  state = gw.loadState(CHILD_ID_RELAY);
  digitalWrite(RELAY_PIN, state?RELAY_ON:RELAY_OFF);
  
}



void loop()
{
  gw.process();
   debouncer.update();
   int value = debouncer.read();
   if (value != oldValue && value==0) {
       gw.send(msg.set(gw.loadState(1)?false:true), true); // Send new state and request ack back
   }
   oldValue = value;
  delay(dht.getMinimumSamplingPeriod());
  float temperature = dht.getTemperature();
  if (isnan(temperature))
  {
    Serial.println("Failed reading temperature from DHT");
  } 
  else if (temperature != lastTemp) 
  {
    lastTemp = temperature;
    if (!metric) 
    {
      temperature = dht.toFahrenheit(temperature);
    }
    gw.send(msgTemp.set(temperature, 1));
    Serial.print("T: ");
    Serial.println(temperature);
  }
  float humidity = dht.getHumidity();
  if (isnan(humidity))
  {
    Serial.println("Failed reading humidity from DHT");
  } else if (humidity != lastHum)
  {
    lastHum = humidity;
    gw.send(msgHum.set(humidity, 1));
    Serial.print("H: ");
    Serial.println(humidity);
  }
}

void incomingMessage(const MyMessage &message) 
{
  if (message.type == V_LIGHT) 
  {
    digitalWrite(RELAY_PIN, message.getBool() ? RELAY_ON : RELAY_OFF);
    gw.saveState(1, message.getBool());
    Serial.print("Incoming change for sensor:");
    Serial.print(message.sensor);
    Serial.print(", New status: ");
    Serial.println(message.getBool());
  }
}

The only thing that remains is that the button needs to be pressed for about 2 secs to actuate. When measured with a multimeter, it takes about 2 secs to get from 1 to 0. Tried inserting a 4.7k resistor between PIN4 and the button, but still 2 sec for the button to actuate.

Thee sketch i used is:

#include <SPI.h>
#include <MySensor.h>
#include <DHT.h>
#include <Bounce2.h>

#include <MySigningNone.h>
#include <MyTransportNRF24.h>
#include <MyTransportRFM69.h>
#include <MyHwATMega328.h>

#define NODE_ID 1
#define CHILD_ID_HUM 0
#define CHILD_ID_TEMP 1
#define CHILD_ID_RELAY 2
#define RELAY_PIN 3
#define BUTTON_PIN 4
#define HUMIDITY_SENSOR_DIGITAL_PIN 2
//unsigned long SLEEP_TIME = 30000; // Sleep time between reads (in milliseconds)

#define RELAY_ON 1  // GPIO value to write to turn on attached relay
#define RELAY_OFF 0 // GPIO value to write to turn off attached relay

MyTransportNRF24 radio(RF24_CE_PIN, RF24_CS_PIN, RF24_PA_LEVEL_GW);

MyHwATMega328 hw;

MySensor gw(radio, hw);

Bounce debouncer = Bounce(); 
int oldValue=0;
bool state;
MyMessage msg(CHILD_ID_RELAY,V_LIGHT);

DHT dht;
float lastTemp;
float lastHum;
boolean metric = true;
MyMessage msgHum(CHILD_ID_HUM, V_HUM);
MyMessage msgTemp(CHILD_ID_TEMP, V_TEMP);

void setup()
{
  gw.begin(incomingMessage, AUTO, true);
  dht.setup(HUMIDITY_SENSOR_DIGITAL_PIN);
  gw.sendSketchInfo("Combo", "1.0");
  gw.present(CHILD_ID_HUM, S_HUM);
  gw.present(CHILD_ID_TEMP, S_TEMP);
  pinMode(BUTTON_PIN,INPUT);
  digitalWrite(BUTTON_PIN,HIGH);
  debouncer.attach(BUTTON_PIN);
  debouncer.interval(5);
  gw.present(CHILD_ID_RELAY, S_LIGHT);
  metric = gw.getConfig().isMetric;
  digitalWrite(RELAY_PIN, RELAY_OFF);
  pinMode(RELAY_PIN, OUTPUT);
  state = gw.loadState(CHILD_ID_RELAY);
  digitalWrite(RELAY_PIN, state?RELAY_ON:RELAY_OFF);
  
}



void loop()
{
  gw.process();
  debouncer.update();
  int value = debouncer.read();
  if (value != oldValue && value==0) {
      gw.send(msg.set(state?false:true), true); // Send new state and request ack back
  }
  oldValue = value;
  delay(dht.getMinimumSamplingPeriod());
  float temperature = dht.getTemperature();
  if (isnan(temperature))
  {
    Serial.println("Failed reading temperature from DHT");
  } 
  else if (temperature != lastTemp) 
  {
    lastTemp = temperature;
    if (!metric) 
    {
      temperature = dht.toFahrenheit(temperature);
    }
    gw.send(msgTemp.set(temperature, 1));
    Serial.print("T: ");
    Serial.println(temperature);
  }
  float humidity = dht.getHumidity();
  if (isnan(humidity))
  {
    Serial.println("Failed reading humidity from DHT");
  } else if (humidity != lastHum)
  {
    lastHum = humidity;
    gw.send(msgHum.set(humidity, 1));
    Serial.print("H: ");
    Serial.println(humidity);
  }
}

void incomingMessage(const MyMessage &message) 
{
  if (message.type == V_LIGHT) 
  {
    digitalWrite(RELAY_PIN, message.getBool() ? RELAY_ON : RELAY_OFF);
    gw.saveState(1, message.getBool());
    Serial.print("Incoming change for sensor:");
    Serial.print(message.sensor);
    Serial.print(", New status: ");
    Serial.println(message.getBool());
  }
}

With this sketch i can control the relay with domoticz, but i can only activate the relay, not deactivate.
I connected the button to GND pin and PIN4 on the Arduino.

Hi,
i started playing with Mysensors and Arduino and managed to make a Gateway and the relay/humidity node with the following code:

#include <SPI.h>
#include <MySensor.h>
#include <DHT.h>

#include <MySigningNone.h>
#include <MyTransportNRF24.h>
#include <MyTransportRFM69.h>
#include <MyHwATMega328.h>

#define NODE_ID 1
#define CHILD_ID_HUM 0
#define CHILD_ID_TEMP 1
#define CHILD_ID_RELAY 2
#define RELAY_PIN 3
#define HUMIDITY_SENSOR_DIGITAL_PIN 2
//unsigned long SLEEP_TIME = 30000; // Sleep time between reads (in milliseconds)

#define RELAY_ON 1  // GPIO value to write to turn on attached relay
#define RELAY_OFF 0 // GPIO value to write to turn off attached relay

MyTransportNRF24 radio(RF24_CE_PIN, RF24_CS_PIN, RF24_PA_LEVEL_GW);

MyHwATMega328 hw;

MySensor gw(radio, hw);

DHT dht;
float lastTemp;
float lastHum;
boolean metric = true;
MyMessage msgHum(CHILD_ID_HUM, V_HUM);
MyMessage msgTemp(CHILD_ID_TEMP, V_TEMP);

void setup()
{
  gw.begin(incomingMessage, AUTO, true);
  dht.setup(HUMIDITY_SENSOR_DIGITAL_PIN);
  gw.sendSketchInfo("Combo", "1.0");
  gw.present(CHILD_ID_HUM, S_HUM);
  gw.present(CHILD_ID_TEMP, S_TEMP);
  gw.present(CHILD_ID_RELAY, S_LIGHT);
  metric = gw.getConfig().isMetric;
  pinMode(RELAY_PIN, OUTPUT);
  digitalWrite(RELAY_PIN, gw.loadState(1) ? RELAY_ON : RELAY_OFF);
}

void loop()
{
  gw.process();
  delay(dht.getMinimumSamplingPeriod());
  float temperature = dht.getTemperature();
  if (isnan(temperature))
  {
    Serial.println("Failed reading temperature from DHT");
  } 
  else if (temperature != lastTemp) 
  {
    lastTemp = temperature;
    if (!metric) 
    {
      temperature = dht.toFahrenheit(temperature);
    }
    gw.send(msgTemp.set(temperature, 1));
    Serial.print("T: ");
    Serial.println(temperature);
  }
  float humidity = dht.getHumidity();
  if (isnan(humidity))
  {
    Serial.println("Failed reading humidity from DHT");
  } else if (humidity != lastHum)
  {
    lastHum = humidity;
    gw.send(msgHum.set(humidity, 1));
    Serial.print("H: ");
    Serial.println(humidity);
  }
}

void incomingMessage(const MyMessage &message) 
{
  if (message.type == V_LIGHT) 
  {
    digitalWrite(RELAY_PIN, message.getBool() ? RELAY_ON : RELAY_OFF);
    gw.saveState(1, message.getBool());
    Serial.print("Incoming change for sensor:");
    Serial.print(message.sensor);
    Serial.print(", New status: ");
    Serial.println(message.getBool());
  }
}

Since this is copy/pasted from the forum, i am wondering if it is possible to incorporate a hardware button to control the relay. I tried adding code from the Relay with actuator example, but i failed to make it work, i can only turn the relay on with the button. I am using Domoticz as a controller. Any help would be welcome.