#include <SoftwareSerial.h>

#define REL_PIN 12

SoftwareSerial esp(4, 5);

void setup() {
  esp.begin(9600);
  pinMode(REL_PIN, OUTPUT);
}

void loop() {
  String command = esp.readStringUntil('\n');
  command.trim();

  if (command == "turnOn") {
    digitalWrite(REL_PIN, HIGH);
  }

  if (command == "turnOff") {
    digitalWrite(REL_PIN, LOW);
  }
}