Hi!

Parts that I used to make this happen.

EtherTen Ethernet Arduino Compatibele Microcontroller

12 Volt Power Supply - 5.0 Amp Single Output

AIWAN LEZHI 10 Meters GT2 Timing Belt Width 6mm

GT2 Pulley 20 Teeth 5mm bore 6mm Width

Stepper Motor - Stepper Motor 17HS24-2104S

Optical Photosensitive LDR light sensor Module

GeeekPi I2C 20x4 LCD Module

5Volt Solid State Relay Module

Various hooks, screws, bolts and other mounting materials

I wrote my own arduino code, I am sure it can be done better and cleaner, but this works for me. I am still thinking about adding more Homekit compatibility, but I am not an programmer/coder. I would like to have some kind of status report back to Homekit/Homebridge so I can tell if the curtains are open or closed but that might never happen as longs as this code does what it needs to do.
The curtains close when it gets dark outside and they open again when it gets light outside, no manual actions required there. I added some kind of Homekit support by means of a webserver setup with two embedded links, one to open and one to close the curtains by manual override the light sensor.

                                    
#include 
#include 
#include  
#include 

AccelStepper stepper(1, 5, 6); //setup the steppermotor
LiquidCrystal_I2C lcd(0x27, 20, 4); //setup LCD screen

boolean CURTAIN_STATE = false; //state of the curtains, open or closed
boolean CURTAIN_MAN = false; //state of control, automatic or manually

int DRIVER_ENABLE = 9; //pin to enable or disable the stepper driver
int RELAY1_PIN = 8; //pin for the relay to activate the sensor for the back curtains
//int RELAY2_PIN = 7; //pin for the relay to activate the sensor for the back curtains
int SENSOR_PIN = A3; //pin for the LDR sensor
int LIGHT_THRESHOLD = 125; //threshold for opening the curtains
int DARK_THRESHOLD = 25; //threshold for closing the curtains

unsigned int SENSOR_VALUE = 0;
unsigned int MAPPED_VALUE = 0;
unsigned int RELAY1_VALUE = 0;
//unsigned int RELAY2_VALUE = 0;

unsigned long CURRENT_TIME = 0;
long PREVIOUS_MILLIS = 0;
long INTERVAL = 5000; //time between LDR readings

byte mac[] = {  0x86, 0xB4, 0xEF, 0x8E, 0xA3, 0x17 }; //network MAC address
byte myip[] = { 192, 168, 1, 2 };//network IP address
byte gwip[] = { 192, 168, 1, 1 };//network gateway IP address
byte dnsip[] = { 192, 168, 1, 1 }; //network DNS server address

EthernetServer server(80); //setup webserver TCP Port

void setup() {
  pinMode(RELAY1_PIN, OUTPUT); //setup the relay-pin as output
  //pinMode(RELAY2_PIN, OUTPUT); //setup the relay-pin as output
  digitalWrite(RELAY1_PIN, LOW); //make sure the relay-pin is LOW
  //digitalWrite(RELAY2_PIN, LOW); //make sure the relay-pin is LOW
  pinMode(DRIVER_ENABLE, OUTPUT); //setup the driver-pin as output
  digitalWrite(DRIVER_ENABLE, LOW); //make sure the driver is set to disabled, the curtains can run free now
  Serial.begin(9600); //setup the serial connection for troubleshooting
  lcd.init(); // initialiseer het LCD scherm
  lcd.backlight(); // zet de backlight aan
  lcd.clear(); // wis het scherm
  lcd.setCursor(3, 0); // zet de cursor op positie 1, regel 1
  lcd.print("Curtain status"); // schrijf op scherm
  lcd.setCursor(1, 1); // zet de cursor op positie 1, regel 2
  lcd.print("LDR value:"); // schrijf op scherm
  lcd.setCursor(1, 2); // zet de cursor op positie 1, regel 3
  lcd.print("Curtains mode "); // schrijf op scherm
  lcd.setCursor(1, 3); // zet de cursor op positie 1, regel 4
  lcd.print("Curtains are "); // schrijf op scherm
  Ethernet.begin(mac, myip, gwip, dnsip); //start the ethernet connection
  server.begin(); //start the webserver for remote control by Homebridge
  stepper.setMaxSpeed(5500); //set the speed of the steppermotor
  stepper.setAcceleration(4000); //set the acceleration for the steppermotor
}

void loop() {
  read_ldr();
  if (MAPPED_VALUE > LIGHT_THRESHOLD && CURTAIN_STATE == false) {
    auto_open(); //if it is light outside and the cutains are closed, open the curtains automatically
  }
  if (MAPPED_VALUE < DARK_THRESHOLD && CURTAIN_STATE == true) {
    auto_close(); //if it is dark outside and the curtains are open, close the curtains automatically
  }
  EthernetClient client = server.available();
  if (client) {
    Serial.println("new client");
    boolean currentLineIsBlank = true;
    String postText = "";
    while (client.connected()) {
      if (client.available()) {
        char c = client.read();
        if (postText.length() < 10) {
          postText += c;
        }
        if (c == '\n' && currentLineIsBlank) {
          client.println("HTTP/1.1 200 OK");
          client.println("Content-Type: text/html");
          client.println("Connection: close");
          client.println();
          client.println("");
          client.println("");
          client.println("");
          client.println("  ");
          client.println("");
          client.println("");
          break;
        }
        if (c == '\n') {
          currentLineIsBlank = true;
        }
        else if (c != '\r') {
          currentLineIsBlank = false;
        }
      }
    }
    if (postText.indexOf("?on1") > 0) {
      if (MAPPED_VALUE < DARK_THRESHOLD && CURTAIN_STATE == false && CURTAIN_MAN == false) {
        manual_open();
      }
      if (MAPPED_VALUE > LIGHT_THRESHOLD && CURTAIN_STATE == true && CURTAIN_MAN == false) {
        manual_close();
      }
    }
    if (postText.indexOf("?off1") > 0 ) {
      if (MAPPED_VALUE > LIGHT_THRESHOLD && CURTAIN_STATE == true && CURTAIN_MAN == true) {
        auto_open();
      }
      if (MAPPED_VALUE < DARK_THRESHOLD && CURTAIN_STATE == false && CURTAIN_MAN == true) {
        auto_close();
      }
    }
    delay(1);
    client.stop();
    Serial.println("client disconnected");
  }
}

//gordijn open Homekit
void manual_open() {
  digitalWrite(DRIVER_ENABLE, LOW);
  Serial.println("Manually Opening");
  delay(200);
  stepper.moveTo(000);
  while (stepper.distanceToGo() != 0)
  {
    stepper.run();
  }
  CURTAIN_MAN = true;
  CURTAIN_STATE = false;
  digitalWrite(DRIVER_ENABLE, HIGH);
}

//gordijn dicht Homekit
void manual_close() {
  digitalWrite(DRIVER_ENABLE, LOW);
  Serial.println("Manually Closing");
  delay(200);
  stepper.moveTo(48500);
  while (stepper.distanceToGo() != 0)
  {
    stepper.run();
  }
  CURTAIN_MAN = true;
  CURTAIN_STATE = true;
  digitalWrite(DRIVER_ENABLE, HIGH);
}

//gordijn open LDR
void auto_open() {
  digitalWrite(DRIVER_ENABLE, LOW);
  digitalWrite(RELAY1_PIN, LOW);
  Serial.println("Opening by LDR");
  delay(200);
  stepper.moveTo(000);
  while (stepper.distanceToGo() != 0)
  {
    stepper.run();
  }
  CURTAIN_MAN = false;
  CURTAIN_STATE = true;
  digitalWrite(DRIVER_ENABLE, HIGH);
}

//gordijn dicht LDR
void auto_close() {
  digitalWrite(DRIVER_ENABLE, LOW);
  digitalWrite(RELAY1_PIN, HIGH);
  Serial.println("Closing by LDR");
  delay(200);
  stepper.moveTo(48500);
  while (stepper.distanceToGo() != 0)
  {
    stepper.run();
  }
  CURTAIN_MAN = false;
  CURTAIN_STATE = false;
  digitalWrite(DRIVER_ENABLE, HIGH);
}

//read LDR value
void read_ldr() {
  unsigned long currentMillis = millis();
  if (currentMillis - PREVIOUS_MILLIS > INTERVAL) {
    PREVIOUS_MILLIS = currentMillis;
    SENSOR_VALUE = analogRead(SENSOR_PIN);
    MAPPED_VALUE = map(SENSOR_VALUE, 0, 1023, 1023, 0);
    RELAY1_VALUE = digitalRead(RELAY1_PIN);
    //RELAY2_VALUE = digitalRead(RELAY2_PIN);
    lcd.setCursor(16, 1); // zet de cursor op positie 17, regel 2
    lcd.print(MAPPED_VALUE); // schrijf op scherm
    lcd.print(" ");
    Serial.println("");
    Serial.print("SENSOR_VALUE: ");
    Serial.println(MAPPED_VALUE);
    Serial.print("RELAY1_VALUE: ");
    Serial.println(RELAY1_VALUE);
    //Serial.print("RELAY2_VALUE: ");
    //Serial.println(RELAY2_VALUE);
    if (CURTAIN_MAN == true) {
      Serial.println("curtains in manual mode");
      lcd.setCursor(16, 2); // zet de cursor op positie 16, regel 3
      lcd.print("MAN"); // schrijf op scherm
      lcd.print(" ");
    }
    else {
      Serial.println("curtains in automatic mode");
      lcd.setCursor(16, 2); // zet de cursor op positie 16, regel 3
      lcd.print("AUTO"); // schrijf op scherm
      lcd.print(" ");
    }
    if (CURTAIN_STATE == true) {
      Serial.println("curtains are open");
      lcd.setCursor(15, 3); // zet de cursor op positie 15, regel 4
      lcd.print("OPEN"); // schrijf op scherm
      lcd.print(" ");
    }
    else {
      Serial.println("curtains are closed");
      lcd.setCursor(15, 3); // zet de cursor op positie 15, regel 4
      lcd.print("CLOSE"); // schrijf op scherm
      lcd.print(" ");
    }
  }
}