 const long interval = 1000;
 
 void setup() {
-  pinMode(ledPin, OUTPUT);
+  pinMode(BUILTIN_LED, OUTPUT);
 }
 
 void loop()

      ledState = HIGH;  // Note that this switches the LED *off*
    else
      ledState = LOW;   // Note that this switches the LED *on*
    digitalWrite(ledPin, ledState);
  }
}


 Blink the blue LED on the ESP-01 module
 Based on the Arduino Blink without Delay example
 This example code is in the public domain
 
 The blue LED on the ESP-01 module is connected to GPIO1 
 (which is also the TXD pin; so we cannot use Serial.print() at the same time)
 
 Note that this sketch uses BUILTIN_LED to find the pin with the internal LED
*/

int ledState = LOW;     

