char val; // variable to receive data from the serial port int ledpin = 13; // LED connected to pin 48 (on-board LED) void setup() { pinMode(ledpin, OUTPUT); // pin 48 (on-board LED) as OUTPUT Serial.begin(9600); // start serial communication at 9600bps digitalWrite(ledpin, HIGH); } void loop() { if( Serial.available() ) // if data is available to read { val = Serial.read(); // read it and store it in 'val' } if( val == 'H' ) // if 'H' was received { digitalWrite(ledpin, HIGH); // turn ON the LED delay(100); Serial.write("HIGH"); } else { digitalWrite(ledpin, LOW); // otherwise turn it OFF } delay(100); // wait 100ms for next reading }