| LEDs |
Light Emitting
Diodes (LEDs)
A light
emitting diode - a.k.a. an LED - is precisely what the name implies.
-
It emits light. Usually the
light is very pure - being pretty much monchromatic - and it comes in a
restricted range of colors. The most common LED color for the light is
red.
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It is a diode. That means
that current only flows through an LED in one direction. If you try to
make current flow in the reverse direction, no current will flow, and you won't
get any light either because you need current flowing in the LED to get any
light.
Goals for this lesson unit:
Here are
the goals for this lesson unit.
FACTS ABOUT LEDS
The
circuit symbol for an LED is pretty much the same as for a regular diode - since
it is, in fact, a diode. In many ways, you can treat an LED as though it
is a diode, but you do need to remember one way that it differs from a typical
silicon diode.
-
In a typical silicon diode, the
forward voltage drop will be about .6 or .7 volts. In other words, when
current is flowing forward through the diode, there will be .6 or .7 volts
across the diode.
-
In an LED the forward drop is more
like 1.7 or 1.8 volts.
The drop may also depend upon the color of the LED, but this value is typical
for a red LED.
The circuit
symbol for an LED looks much like the symbol for a regular diode. There's
usually an additional little arrow to indicate the light that comes from the
diode. Here's the symbol.
Now, let's
take a look at the LED in a simple circuit. Let's imagine that you have
the following situation.
-
You have a signal that can change.
In one state it is 0 volts - a binary zero, and in the other state it is 5 volts
- a binary one.
-
When the signal is a one, you want
an LED to light, and when the signal is a zero you want the LED off.
Here's a
circuit that will do what you want when you choose the components correctly.
The 5v source is what you have when the signal is a binary one. At that
point, current should flow through the resistor and the diode should light.
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