Upload the following sketch to your Arduino. Later on, we will see exactly how it works.
Adafruit Arduino - Lesson 5. Serial Monitor

int latchPin = 5;
int clockPin = 6;
int dataPin = 4;

byte leds = 0;

void setup() 
  pinMode(latchPin, OUTPUT);
  pinMode(dataPin, OUTPUT);  
  pinMode(clockPin, OUTPUT);
  while (! Serial); // Wait untilSerial is ready - Leonardo
  Serial.println("Enter LED Number 0 to 7 or 'x' to clear");

void loop() 
  if (Serial.available())
    char ch = Serial.read();
    if (ch >= '0' && ch <= '7')
      int led = ch - '0';
      bitSet(leds, led);
      Serial.print("Turned on LED ");
    if (ch == 'x')
      leds = 0;

void updateShiftRegister()
   digitalWrite(latchPin, LOW);
   shiftOut(dataPin, clockPin, LSBFIRST, leds);
   digitalWrite(latchPin, HIGH);
After you have uploaded this sketch onto your Arduino, click on the right-most button on the toolbar in the Arduino IDE. The button is circled below.
The following window will open.
This window is called the Serial Monitor and it is part of the Arduino IDE software. Its job is to allow you to both send messages from your computer to an Arduino board (over USB) and also to receive messages from the Arduino.

The message “Enter LED Number 0 to 9 or 'x' to clear” has been sent by the Arduino, and it is telling us what commands we can send to the Arduino which is either to send the 'x' (to turn all the LEDs off) or the number of the LED you want to turn on (where 0 is the bottom LED, 1 is the next one up right up to 7 for the top LED).

Try typing the following commands, into the top area of the Serial Monitor that is level with the 'Send' button. Press 'Send', after typing each of these characters: x 0 3 5

Typing x, will have no effect, if the LEDs are already all off, but as you enter each number, the corresponding LED should light and you will get a confirmation message from the Arduino board, so that the Serial Monitor will appear as shown below.
You can see that I am about to press send after entering 'x' again. Do this and all the LEDs will turn off.

This guide was first published on Dec 04, 2012. It was last updated on Oct 09, 2012.

This page (The Serial Monitor) was last updated on Oct 09, 2012.

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