1. The serial monitor

Keeping track of everything going on in your program can be an uphill battle. The serial monitor is a way to check in on different spots in your code by reporting back to the computer over the USB cable.

This circuit uses a button setup similar to the previous examples, but without a LED. Navigate to File > Examples > 01.Basics > DigitalReadSerial and load the example sketch.

Upload this code to your board and click the Serial Monitor button in the upper right of the sketch window or go to Tools > Serial Monitor.

To create a serial connection, we use Serial.begin(); inside setup(). The number 9600 is the baud rate, or data speed, in bits per second (bps). Inside the loop(), we use Serial.print(); to send information to the serial port. Serial.println(); does the same thing, but prints on a new line.

prints the current value of buttonState to the serial port. So with the serial monitor open, we have a live scrolling view of whatever the Arduino senses at pin 2. The serial monitor is exceptionally handy when troubleshooting, since we can easily compare what we think should be happening to what the Arduino is doing. We can also use serial communication to talk between devices and much more; read about in the Arduino reference.