Light
Emitting Diodes (LEDs)
A light emitting diode - a.k.a. an LED - is precisely what the name implies.
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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.
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Given a
circuit with a light emitting diode,
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Be able to use a simplified
model of an LED to predict when current flows through the diode, and when
it does not - and when it lights,
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Be able to use information
about current flowing to predict other behavior in a circuit.
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.
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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.
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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.
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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.
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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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Let's assume that we have
1.8v across the LED.
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Then there's 3.2 volts
across the resistor.
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Then, 3.2/R amps flows
through the resistor.
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That current is what causes
the LED to light.
Using
Light Emitting Diodes
LEDs have many uses.
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Specialized LEDs can be
used in various kinds of displays.
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You can use LEDs as indicators
with logic circuits.
A typical circuit for logic signal indication
is shown below. In this circuit, you want the LED to emit light when
the output of the AND gate is a logical one.
If you have a logic signal, then the following will occur.
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If Vin is 5v (logical
1) and R is properly chosen and the LED is in the circuit correctly, the
LED will light.
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If Vin is zero, the LED
will not light.
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These statements will
be true even in the real situation where a logical 1 is not exactly 5v,
and where a logical zero may be closer to 1v. It helps that the LED
takes over 1.5v to turn on and start emitting light because a logical one
can be a voltage up to 1.5 v.
To
use this circuit there is at least one other caution.
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The LED can be inserted
into the circuit two ways. One way works, the other doesn't.
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The LED package has a
small "flat" on the flange. The flat corresponds to the end of the
diode. In the circuit model for the diode, the flat corresponds to
the bottom end in the picture below.
What
If?
What if you put 3.2W
in the circuit inadvertently? That would mean you might have had
one ampere flow through the LED. In that case, the LED will probably
never again emit any light.
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The LED has gone through
an LED to DED conversion.
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DED stands for Darkness
Emitting Diode.
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By virtue of this act
you are now permitted to go on the Gradeful DED Tour.
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That's it for this lesson,
and
Problem