This example shows you how to monitor the state of a switch by establishing serial communication between your Arduino and your computer over USB.
- Arduino Board
- A momentary switch, button, or toggle switch
- 10k ohm resistor
- hook-up wire
image developed using Fritzing. For more circuit examples, see the Fritzing project page
Connect three wires to the Arduino board. The first two, red and black, connect to the two long vertical rows on the side of the breadboard to provide access to the 5 volt supply and ground. The third wire goes from digital pin 2 to one leg of the pushbutton. That same leg of the button connects through a pull-down resistor (here 10 KOhms) to ground. The other leg of the button connects to the 5 volt supply.
Pushbuttons or switches connect two points in a circuit when you press them. When the pushbutton is open (unpressed) there is no connection between the two legs of the pushbutton, so the pin is connected to ground (through the pull-down resistor) and reads as LOW, or 0. When the button is closed (pressed), it makes a connection between its two legs, connecting the pin to 5 volts, so that the pin reads as HIGH, or 1.
If you disconnect the digital i/o pin from everything, the LED may blink erratically. This is because the input is “floating” – that is, it doesn’t have a solid connection to voltage or ground, and it will randomly return either HIGH or LOW. That’s why you need a pull-down resistor in the circuit.
In the program below, the very first thing that you do will in the setup function is to begin serial communications, at 9600 bits of data per second, between your Arduino and your computer with the line:
Next, initialize digital pin 2, the pin that will read the output from your button, as an input:
For more detail: Digital Read Serial using Arduino