- [Instructor] Light-emitting diodes, or LEDs,
are the flashiest member of the diode family.
Like normal diodes,
they consist of a semiconductor junction
that only allows current
to pass through it in one direction.
But, that junction is specially designed
so that when current does pass through it,
it'll convert some of the energy into light
by giving off photons.
The schematic symbol for an LED looks like
the symbol for a regular diode,
with the addition of two little arrows
pointing away from it
to represent the light that's given off by the diode.
Physically, LEDs come in a variety of shapes and sizes.
Many consumer electronic devices
use compact surface mount LEDs on printed circuit boards
to indicate the operating status of a device.
Those surface mount LEDs look like small rectangular prisms.
For prototyping circuits on a breadboard,
it's more common to use through hole LEDs like this,
which consist of a plastic bulb
with two long metal leads on one side.
The plastic bulb will either be clear or tinted
to indicate the color of the light it produces.
Since LEDs are a type of diode, they're polarized,
which means the direction of the current
passing through them matters.
So you can tell which side is which,
on most through hole LEDs,
the metal lead connected to the positive anode terminal
will be longer than the lead that's connected
to the negative cathode.
Additionally, there's usually a flat section
on the cathode side of the bulb,
and since I tend to get the terms anode and cathode
mixed up in my head,
I usually just remember that the flat section
on the side of the LED bulb represents
the flat R on the diode symbol.
If I ever build a circuit using an LED
and it doesn't light up,
the first step I always take
to try troubleshooting the problem
is turning the LED around,
because there's a pretty good chance I put it in backwards.
Just like all other types of diodes,
LEDs require a certain amount of forward voltage
across their junction before they allow current
to flow through them to generate light.
LEDs generally require a larger forward voltage
than normal diodes,
and it'll vary depending on the LED's color and intensity.
For example, this table shows how the forward voltage,
maximum current, and luminous intensity ratings
compare for two different colored LEDs.
The red LED will require a forward voltage drop
somewhere between two to 2.4 volts,
whereas the blue LED requires a higher forward voltage drop
between three to 3.4 volts.
Generally, LEDs that have a higher forward voltage
will also be able to handle more current.
And, on this table, I can see that the blue LED
can handle a maximum current of 30 milliamps,
whereas the red LED can only handle up to 20 milliamps.
The higher voltage and current ratings of the blue LED
mean it can dissipate more power than the red LED,
which allows it to produce a brighter light.
The blue LED has a luminous intensity of 400 millicandela,
versus 200 millicandela for the red LED.
Millicandela, or mcd,
is a standard unit for measuring
the intensity of a light source
that's commonly used to describe the brightness of LEDs.
As a point of reference, an LED with 50 to 100 millicandela
is good to use for a low-intensity indicator light.
But, if you wanted to build an LED flashlight,
you would need to use one of the ultra-bright LEDs
which have intensities of over 10,000 millicandela.
You can control the brightness of an LED
by controlling the amount of current through it.
Since the forward voltage remains constant
when an LED is on,
sending more current through an LED
will increase the amount of power it dissipates
and light it generates.
More current makes the LED shine brighter.
However, there is such a thing as too much current.
When an LED is turned on,
it allows current to pass through it
more or less unrestricted.
So, the LED needs something else to limit
the amount of current passing through it,
or else it'll try to dissipate too much power
and it'll quickly be burned up and destroyed.
The simplest way to prevent that
is to put a resistor in series with the LED
to restrict the amount of current
flowing through the circuit.
And that raises an important question.
What size resistor should I use?
As a simple rule of thumb,
for most small-scale hobbyist projects
which commonly use through hole LEDs
and are powered by 3.3 or five volt sources,
a 330 ohm resistor is generally a safe
limiting resistor value to use to keep LEDs from burning up.
However, if I want to make sure my LED
is as bright as possible,
I'll need to use Ohm's Law
to calculate the smallest resistor value I can use
that will give the LED the maximum amount of current
within its limitations.
For example, if I plan to connect my red LED
to a five volt source,
which is a common voltage for microcontroller devices,
I know that since the red LED
has a forward voltage drop of two volts,
there will be three volts left
across the limiting resistor.
My red LED is rated to handle a maximum current
of 20 milliamps.
So, to find the resistor value
that will create the maximum amount of current,
I'll divide three volts by 20 milliamps,
which gives me 150 ohms.
Now, 150 ohms is not a common resistor value
that I have in my parts kit,
so I'll round up to the nearest value that I do have,
which is 220 ohms.
I always round this resistance value up, rather than down,
because a higher resistance will restrict the current more,
so that there's less current
than the maximum of 20 milliamps.
With a 220 ohm limiting resistor,
my LED only gets about 13 1/2 milliamps of current,
which it can safely handle.
If I had rounded my calculated resistor value down,
instead of up,
that would have allowed more current
to flow through the LED and exceed its 20 milliamp rating,
which would quickly burn it out.
Once I've determined the smallest resistor value
that I can safely use
which will make my LED as bright as possible,
I can still play around with using
larger limiting resistor values
to tweak the brightness and make my LED dimmer
to meet the needs of a specific project.
For example, here are three identical red LEDs
connected to the same five volt power source,
but with different current-limiting resistors.
The LED on the left is the brightest
because it has the smallest resistor value
that I can safely use with these LEDs, which is 220 ohms.
The LED in the middle is slightly dimmer
because it's connected to a much larger
10 kiloohm resistor.
And the LED on the far right is the dimmest
because it has a 100 kiloohm resistor,
which limits the current through it
down to about 44 microamps.