Become Familiar with LEDs - dummies

By Cathleen Shamieh

You may be familiar with LEDs if you have an LED flashlight or use LED bulbs in your home. An LED, or light-emitting diode, is a device made of a special material known as a semiconductor. A diode is the simplest type of semiconductor device (meaning, component).

Diodes, LEDs, and other semiconductor devices have unique properties that make them useful. For instance, they don’t always allow current to pass through them. Instead, they’re picky about what’s going on in the circuit and will allow current to flow only under certain conditions.

Diodes and bicycle tires

Have you ever pumped air into a bicycle tire? The tire contains a valve that allows air to flow into the tire, but not out of the tire. You have to apply enough pressure to the pump to force air through the valve.

A diode acts like a valve for electric current. Current flows only one way through a diode (like cars on a one-way street — we hope), and only when you apply a high enough voltage (like pressure) to the diode.

Seeing light from LEDs

A light-emitting diode is a type of diode that emits, or gives off, visible light. The light emitted from an LED can be red, orange, yellow, green, blue, violet, pink, or white, as shown. The color depends on the materials and processes used to make the LED.

LED
Light emitted from an LED.

LEDs also come in several shapes and sizes. The LEDs you use in the projects in this book have round, domed cases that are either 5 mm (millimeters) or 3 mm high.

There are two types of LEDs:

  • Diffused LEDs have colored plastic cases (like tinted windows) to diffuse, or spread out, the light so it’s easier to see. The color of the plastic case is usually the same as the color of the light.
  • Clear LEDs have clear plastic cases but still emit colored light.

All the LEDs shown earlier are clear 5 mm LEDs. The following figure shows an assortment of LEDs, including a 5 mm clear LED that gives off an orange light. (It’s the unlit version of the LED that is second from the left in the preceding figure.)

Different-LEDs
3 mm and 5 mm LEDs.

You can’t tell what color a clear LED emits just by looking at it if it’s not connected in a circuit. If you buy any clear LEDs, be sure to store them in a container or bag labeled with the color they emit.

Examine your LED

Take a good look at your LED and compare it to the LEDs shown here. The actual semiconductor diode is tiny and is on a piece of metal inside the plastic case. The two stiff wires attached to the plastic case are leads that enable you to connect the tiny diode to a circuit.

Diodes
The LED diode.

Because LEDs conduct current in only one direction, you need to know which way to connect the LED in your circuit. One side of the LED is the negative side (known as the cathode) and the other side of the LED is the positive side (known as the anode). Electric current flows from the anode to the cathode of an LED but not the other way around. You can tell which side of an LED is which in three ways:

  • Compare the lengths of the leads. The shorter lead is the cathode (negative side) and the longer lead is the anode (positive side).
  • Peek inside the plastic case. The lead attached to the larger piece of metal inside the case is the cathode (negative side); the lead attached to the smaller piece of metal is the anode (positive side).
  • Look (or feel) for a flat edge on the plastic case. This flat edge is on the cathode (negative side) of the LED.

Look at the leads of your LED. Can you tell which one is the shorter lead? Now look inside the case of your LED. (You may need to shine a flashlight on the case to see inside better.) Can you spot the larger and smaller pieces of metal? Finally, run your finger around the bottom edge of the plastic case. Can you feel a flat edge?

Being able to distinguish the anode from the cathode by peeking inside the case or finding the flat edge may come in handy when you do other projects, because you may want to cut, or clip, the leads of an LED to create a neater circuit. After you clip the leads, you can’t figure out which side is which by comparing the lengths of the leads.

Orienting an LED in a circuit

When you connect an LED in a circuit, you need to orient it so that current flowing from the positive terminal of the battery flows into the positive side (anode) of the LED. If you put the LED in backward, current will not flow.

To conduct current and emit light, most LEDs require between 2.0 and 3.4 volts to be applied across the leads. The exact voltage needed depends on the color of the LED. A 9-volt battery is powerful enough to push current through any LED, but a 1.5-volt battery, such as an AA or AAA battery, isn’t strong enough. For this reason, you use a 9-volt battery rather than an AA or AAA battery for your LED circuit.

Never, ever connect a 9-volt battery directly to an LED. If you do, you may damage the LED. LEDs can handle only a certain amount of current before they have a meltdown, and a direct connection with a 9-volt battery pushes way too much current through the LED. Chances are, the LED will light briefly and then go out for good, but the LED may also melt, make a mess, and smell up your house.