Basic electronic components, part 1

What's an LED, and how do I make it work?

Simply put, an LED is a light source.  LED stands for Light Emitting Diode.  It's a semiconductor that uses some chemical tricks to turn electricity more or less directly into light, making them much more efficient than old incandescent light bulbs.

A diode is an electronic component that allows electricity to flow in one direction but not the other.  A light emitting diode is one that happens to emit light when electricity passes through it.

I'm not going to get into how they work, because that involves quantum physics.  However, one thing to know is that due to the way that LEDs make light, the light that they make is generally of exactly one color, or wavelength.  That color depends on what kind of chemicals were used in its construction.

That color corresponds to an energy level and that energy level is reflected in the number of volts that it takes to make it light.  The closer to red it is, the lower the voltage.  Blue takes the most voltage to run.  Here are some rough numbers:

Red - 1.8 volts
Green - 2.1 volts
Blue - 3.6 volts

The other important thing to know about LEDs is that you have to protect them from using too much current.  Current is a measure of how much electricity is flowing through a circuit.  If you give a component too much current, it generates heat and it can destroy itself.  The typical way of limiting the amount of current that goes through a circuit is to use a resistor - so named because it resists the flow of electricity and thereby limits the amount of current.

Schematic diagrams

Before you even begin with electronics, you need to become familiar with what is called a "schematic diagram."  This is a way to plan and draw the circuit that you want to build in simple and easy to see sketches. Each component is represented by a symbol which hopefully helps you understand what it does.

The symbol for a diode is an arrow and a line.  The arrow indicates electricity flowing from positive to negative.
The symbol for a light emitting diode is just like a diode, but with arrows to indicate light coming out:
This is the symbol for a resistor:
This is the symbol for a battery

In order to light an LED, we would create a circuit like this:

The electricity in this circuit flows from the positive side of the battery through the resistor, which limits how much current flows through the circuit (the current anywhere along the circuit is the same), through the LED and back into the battery.

Now, if you want to actually build this, you need to know at least one important thing, which is what kind of resistor to use.  Resistors are measured in units called Ohms.  The higher the number of ohms, the higher the resistance, and the less current flows through the system.  No resistor at all is zero ohms which means any amount of current (as much as the battery can deliver) is allowed to go on through.

There are three values at work here.  Voltage, resistance, and current.  The three are related by the most basic and important formula in electronics, known as Ohm's Law.  It goes like this:


I is current (in amps), V is voltage (in volts), and R is resistance (in ohms).  You can see that if resistance goes down or voltage goes up, the current increases.  (in older references you may see voltage referred to as "E")

Here's how we work this for the LED circuit.  Let's say we are using a green LED and a 9 volt battery.  The green LED is going to "use up" about 2.1 volts (from the table above) (this is known as "voltage drop").  The rest of the 9 volts will go across the resistor (that's the resistor's "voltage drop").  We look up the data on the LED and find out that it "wants" 20 milliamps (or 0.02 amps) to operate (this is pretty much standard for common LEDs).  So we want to choose a resistor that will allow 0.02 amps of current to flow when it gets given 9-2.1, or 6.9 volts.  We rearrange ohm's law like this: R = V/I.  Plugging our number in, we find that we want 6.9 / 0.02 or 345 ohms.  In reality there's a lot of leeway in this value.  Resistors are usually only available in set values so you need to pick one that's close.  330 ohms is a pretty readily available value.

Here we build the circuit:

1 comment:

BillB said...

I purchased a few huge green, blue, white, and red LEDS at the recent Dayton Hamfest. I will try your suggestions here. Thse are really big and bright, but the dealer could not help me with specs.