Components of the Circuit

There are three essential components to this circuit:

the voltage source
For this exercise, the voltage source will be one of the pins on your Arduino board.
The sketch on the Arduino board will drive that pin HIGH to set the pin output to 5V,
or LOW to set it to 0V.
the LED
a standard 2-wire LED; this type of LED can only produce 1 color. (In contrast,
4-wire LEDs can produce any RGB color.)
a resistor
The purpose of this resistor is to limit the current through the LED. This prevents
the LED from burning out prematurely. A resistor used for this purpose is called
a “current limiting resistor”.

Layout of the Circuit

The order of the components in the circuit is unimportant, but the orientation of
the LED with respect to the voltage source is critical.
The LED must be inserted into the circuit so that the voltage at its anode is higher
than the voltage at its cathode.

LED follows voltage source Resistor follows voltage source LED is inserted backwards

Making the Connections

To assemble the circuit, you can either:

Without a Breadboard With a Breadboard

The advantage of the breadboard is that you can just plug components in and
out, taking advantage of the internal wiring of the breadboard to make the
actual electrical connections. For more detail on the internal layout of
breadboards, see this explanation of breadboards.

Both of the drawings above assume that you will also connect some source of
power to your Arduino board, e.g., the USB plug of your computer
or an external battery, as shown in the packet on using
the on-board LED.

Which Arduino pin should you use?

The choice of pin 8 in the above drawing is somewhat arbitrary. Any of the pins 0–13 will
work. However: