RF 433 Receiver code

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//This code belongs to the following tutorial video: https://youtu.be/lGR8yVVkEL0
//RECEIVER


#include <RH_ASK.h>//RadioHead Amplitude Shift Keying Library
#include <SPI.h> // SPI Library
#include <Wire.h> //Wire Library (I2C)

RH_ASK rf_driver; //Instatiating a rf_driver for the receiver

//ASCII-based OLED library
//Its huge advantage that it is _not_ graphical, so it is using much less memory
#include "SSD1306Ascii.h"
#include "SSD1306AsciiAvrI2c.h"
#define I2C_ADDRESS 0x3C //Address
#define RST_PIN -1 //For OLED with no reset pin

SSD1306AsciiAvrI2c display;


// Set buffer to size of expected message
    uint8_t buf[24];
    uint8_t buflen = sizeof(buf);

    String receivedString; //String for long messages

//REC pin = D11
int redLED = 5; //on, when it is too loud
int greenLED = 6; //on when it is OK


void setup()
{
    // Initialize ASK Object
    rf_driver.init();

  #if RST_PIN >= 0
  display.begin(&Adafruit128x64, I2C_ADDRESS, RST_PIN);
    #else // RST_PIN >= 0
  display.begin(&Adafruit128x64, I2C_ADDRESS);
    #endif // RST_PIN >= 0
  // Call oled.setI2cClock(frequency) to change from the default frequency.

  display.setFont(System5x7);
  display.set1X();
  display.clear();

   //Managing the LEDs
   pinMode(redLED, OUTPUT);
   pinMode(greenLED, OUTPUT);

   digitalWrite(redLED, LOW);
   digitalWrite(greenLED, HIGH);
   //----------------------------

  //Serial.begin(9600);
  //Serial.println("Receiver started");
}

void loop()
{

    // Check if received packet is correct size
    if (rf_driver.recv(buf, &buflen))
    {

      for (int i = 0; i < buflen-1; i++)
      {
        receivedString += String(buf[i]); //building up a string from the received data
      }

      PrintScreen(); //print to OLED
      receivedString="";  //reset to zero, so we don't mix the messages

    }

}

void PrintScreen()
{
  display.clear();
  display.setCursor(0, 0);
  display.println("Status: ");
  display.println();

  if(receivedString == "0")
  {
    display.println("Waiting...");
    digitalWrite(redLED, LOW);
    digitalWrite(greenLED, HIGH); //Only green is on, while we are waiting
  }
  if(receivedString == "1")
  {
    display.println("Check device!");
    digitalWrite(redLED, HIGH); //Red goes on, when the trigger level is crossed on the transmitter side
    digitalWrite(greenLED, LOW);
  }


}