#define TX_PIN 21  
#define RX_PIN 20  
int led = 4;       

void setup() {
    Serial.begin(115200);       
    Serial1.begin(9600, SERIAL_8N1, RX_PIN, TX_PIN);  
    pinMode(led, OUTPUT);
}

void loop() {
    if (Serial1.available()) {
        String receivedData = Serial1.readStringUntil('\n'); 
        receivedData.trim();  
        Serial.print("Received: ");
        Serial.println(receivedData);

        if (receivedData == "0") {  
            digitalWrite(led, HIGH);
            Serial.println("LED ON");
        } else {
            digitalWrite(led, LOW);
            Serial.println("LED OFF");
        }
    }
}