Tutorials/Arduino Projects/Mobile Robotics/BoeBot/What is an LED

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What is an LED?

An LED is a “Light-Emitting Diode”, which is an electronics component that emits light when it is powered. Since it is a diode, and diodes only let the current flow in one direction, for an LED to work it must be wired correctly. When connecting an LED it is important to be able to distinguish which lead is the anode (positive) and which is the cathode (negative). To make it easy to identify the leads, all LEDs are manufactured with two physical properties. The first is that LEDs have one lead that is longer that the other. This longer lead is the anode, and the shorter one is the cathode. The second feature is a small flat notch on the side of the led. The lead that is closer to the notch is always the cathode. This is important to remember since the leads may have been clipped.

Close up of an LED and one of the possible schematic symbols for an LED.

When referencing a schematic (drawing of the electrical pathways and components using symbols), the symbol for the LED shows which way the current flows and allows you to connect the LED the correct way. The cathode on the symbol is the side with the line across the point of the triangle and the anode is the other side. It is important to note that there are many variations on the schematic symbol, however, they all have a triangle with a line across the point and one or two arrows pointing out.

How to Wire up an LED

To wire up an LED you need an LED, a breadboard, some wires, and a 470Ω resistor (color code: yellow-violet-brown)

LED parts

Wiring up an LED is very simple. For this circuit, all you need to do is connect one lead of the resistor to digital pin 5 on the Arduino. Connect the other lead of the resistor to the anode lead of one of the LEDs, and then connect the cathode to ground.

LED Circuit Schematic

To make this easier, we will use the breadboard.

Virtual breadboard setup for one LED
Breadboard setup for one LED

Understanding the circuit

The Arduino digital pin in this circuit will be configured as an output, which for this setup can be thought of as connecting the circuit to the power when set to high, or disconnecting it when set to low. When it is connected the current will flow from the pin, through the resistor and through the LED, then into the ground connection. For electronics, the ground connection refers to the voltage supply connection that has 0 volts. For the Arduino circuits, the voltage supply has 5 volts. However the LED requires about 2 volts across the leads not the 5 volts output from the pin. This is where the resistor comes in to play. For a series circuit (the components are connected one after the other), the sum of the voltage drop across each component must be equal to the source voltage, which in this case is 5 volts. Since the LED needs 2 volts the resistor must have a voltage drop of 3 volts. Additionally, for series circuits, the current (amperes) going through each component is the same. Based on these properties it is possible to calculate the required resistance of the resistor using (Vs·VLED)/ILED=R, where Vs is the supply voltage, VLED is the voltage across the LED, ILED is the desired current flowing through the LED, and R is the resistance of the resistor. We already selected the 470Ω resistor because it provides a nice safety range for variances of the components.