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

const long interval = 1000;

void setup() {
  pinMode(ledPin, OUTPUT);
}

void loop()

 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;     

