Lighting A LED

Lighting A LED

Our first "project" (which is really more of a tutorial) will be about how to light a LED (Light Emitting Diode). (Like URL, LED is one of those acronyms which people aren't sure whether they should spell out or pronounce; If you spell it out, "L-E-D", it's "an LED", but if you just say "led" like I do, it becomes "a LED".)

We begin with a discussion of the humble (but amazingly versatile) breadboard. A breadboard is a panel of tiny holes into which you are meant to plug electronic components so you can connect them together in a useful way. The board is arranged so that groups of holes are connected to each other (typically in groups of 5). Thus, you can plug something into one hole, and plug something else into a hole in the same group, and thus the components will be joined together without any soldering. Breadboards are great for experimenting or prototyping circuits.

There are at least two strips of holes running down the length of the breadboard, called "bus strips". These are not joined in small groups. Rather, every hole in each strip is connected to each other. These strips are intended for supplying a power source; You plug the positive end into one strip and the negative end into the other. Some fancy breadboards may have 4 bus strips, so you can use different-voltage power supplies. *Really* fancy (and expensive, obviously) ones may have even more.

Now let us take a LED. LEDs are very commonly used for signal indicators, since they are a rather bright but low-powered little lamp. The one I have here happens to be red, which is a pretty color. If you prefer blue, or green, get one in that color instead. (LEDs are available in any primary color.) An important thing to remember about LEDs is that they are diodes. A diode is an electronic component which only lets electricity flow in one direction. A diode has a positive end and a negative end; Feed it positive power on its positive end and negative power on its negative end, and power will flow through it happily. Reverse this arrangement, and no power can flow, which really sucks. (Unless you don't want it to flow, of course; But for this experiment, we do.)

So how do you determine which end of a LED is which? If you're using a brand-new, factory-fresh LED, the easiest way is probably to look at the length of the leads (or "legs") on it: The positive one is usually manufactured to be longer (and the negative one is shorter). If you are using a reused LED which has already had the leads trimmed, or if it just happened to get mangled somehow and the legs are not in original factory condition, there is another way: The plastic on the LED is semi-transparent (it has to be, to let the light come through). If you look inside the LED through the plastic, you'll notice that the ends of the leads inside are not symmetrical; One will tend to jut out toward the other, while the other end will be basically straight. The jutting side is the negative one, while the straight side is the positive one. Just peer inside the LED and you should be able to see what I mean.

Now we shall take the LED and the breadboard, and join the LED to the bus strips, one lead to each strip. "Jumper wires" are used to interconnect components on a breadboard, so we plug the LED in so that its two leads are in different zones of the breadboard (we would not want them to be joined together!), and then join each lead to a bus strip. The following picture is what the results look like:

A LED connected to two bus strips

If we join a power supply to the bus strips now (making sure that the positive end of the power supply is on the LED's positive end, and the negative end of the power supply is on the LED's negative end), the LED would probably light. But wait! Don't try this just yet, for one simple reason: The LED will probably burn out. A LED has very little resistance, and if you just run current directly through it like this, it will probably get overloaded and burn out before very long. There are LEDs with built-in resistors to prevent just this sort of thing from happening, and if you happen to be using such a LED, go ahead and light it up right now. But if you have just a normal LED without a built-in resistor, then we will need to supply our own resistor in the circuit. This is not too hard, we just add a resistor so that one end of it is connected to one end of the LED, and the other end goes to the bus strip. (Unlike diodes, a resistor has no orientation, meaning it doesn't matter which end of the resistor goes where.) The result looks something like this:

A LED with accompanying resistor

The particular resistor that I'm using here happens to be 1,000 ohms. (If you know how to read resistor color codes, you may have already noticed that.) This is a pretty good resistance for a LED. Now we are ready to juice things up! Go ahead and join your power supply to the bus strips and turn it on. The LED should light up like a Christmas tree. Yay!

Congratulations! You have just made a light-emitting diode light up.

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