int incomingByte = 0; // for incoming serial data void setup() { Serial.begin(9600); // opens serial port, sets data rate to 9600 bps pinMode(12,OUTPUT); } p void loop() { // send data only when you receive data: if (Serial.available() > 0) { // read the incoming byte: incomingByte = Serial.read(); // say what you got: Serial.print("I received: "); Serial.println(incomingByte, DEC); } if (incomingByte==100) digitalWrite(12,HIGH); else if (incomingByte==97) digitalWrite(12,LOW); }