All master/slave read & write examples Blink LED

examples/master_reader/master_reader.ino

@@ -12,8 +12,11 @@
 
 #include <Wire.h>
 
+int led = LED_BUILTIN;
+
 void setup()
 {
+  pinMode(led, OUTPUT);
   Wire.begin();             // join i2c bus (address optional for master)
   Serial.begin(9600);       // start serial for output
 }
@@ -22,6 +25,7 @@ void loop()
 {
   Serial.print("read: ");
 
+  digitalWrite(led, HIGH);  // briefly flash the LED
   Wire.requestFrom(8, 6);   // request 6 bytes from slave device #8
 
   while(Wire.available()) { // slave may send less than requested
@@ -30,5 +34,6 @@ void loop()
   }
 
   Serial.println();
+  digitalWrite(led, LOW);
   delay(500);
 }

examples/master_writer/master_writer.ino

@@ -12,8 +12,11 @@
 
 #include <Wire.h>
 
+int led = LED_BUILTIN;
+
 void setup()
 {
+  pinMode(led, OUTPUT);
   Wire.begin(); // join i2c bus (address optional for master)
 }
 
@@ -21,10 +24,12 @@ byte x = 0;
 
 void loop()
 {
+  digitalWrite(led, HIGH);    // briefly flash the LED
   Wire.beginTransmission(9);  // transmit to device #9
   Wire.write("x is ");        // sends five bytes
   Wire.write(x);              // sends one byte
   Wire.endTransmission();     // stop transmitting
+  digitalWrite(led, LOW);
 
   x++;
   delay(500);

examples/slave_receiver/slave_receiver.ino

@@ -12,8 +12,11 @@
 
 #include <Wire.h>
 
+int led = LED_BUILTIN;
+
 void setup()
 {
+  pinMode(led, OUTPUT);
   Wire.begin(9);                // join i2c bus with address #9
   Wire.onReceive(receiveEvent); // register event
   Serial.begin(9600);           // start serial for output
@@ -28,10 +31,12 @@ void loop()
 // this function is registered as an event, see setup()
 void receiveEvent(int howMany)
 {
+  digitalWrite(led, HIGH);       // briefly flash the LED
   while(Wire.available() > 1) {  // loop through all but the last
     char c = Wire.read();        // receive byte as a character
     Serial.print(c);             // print the character
   }
   int x = Wire.read();           // receive byte as an integer
   Serial.println(x);             // print the integer
+  digitalWrite(led, LOW);
 }

examples/slave_sender/slave_sender.ino

@@ -12,7 +12,7 @@
 
 #include <Wire.h>
 
-int led = BUILTIN_LED;
+int led = LED_BUILTIN;
 
 void setup()
 {