Microcontroller Beginner Kit -
Learning to use LEDs and Transistors
The LED
An LED is the
device shown above. Besides red, they can also be
yellow, green and blue. The letters LED stand for
Light Emitting Diode. If you are unfamiliar with
diodes, take a moment to review the components in
the Basic
Components Tutorial. The important thing to
remember about diodes (including LEDs) is that
current can only flow in one direction.
To make an LED
work, you need a voltage supply and a resistor. If
you try to use an LED without a resistor, you will
probably burn out the LED. The LED has very little
resistance so large amounts of current will try to
flow through it unless you limit the current with a
resistor. If you try to use an LED without a power
supply, you will be highly disappointed.
So first of all
we will make our LED light up by setting up the
circuit below.
Step 1.) First
you have to find the positive leg of the LED. The
easiest way to do this is to look for the leg that
is longer.
Step 2.) Once you
know which side is positive, put the LED on your
breadboard so
the positive leg is in one row and the negative leg
is in another row. (In the picture below the rows
are vertical.)
Step 3.) Place
one leg of a 2.2k ohm resistor (does not matter
which leg) in the same row as the negative leg of
the LED. Then place the other leg of the resistor in
an empty row.
Step 4.) Unplug
the power supply adapter from the power supply.
Next, put the ground (black wire) end of the power
supply adapter in the sideways row with the blue
stripe beside it. Then put the positive (red wire)
end of the power supply adapter in the sideways row
with the red stripe beside it.
Step 5.) Use a
short jumper wire (use red since it will be
connected to the positive voltage) to go from the
positive power row (the one with the red stripe
beside it) to the positive leg of the LED (not in
the same hole, but in the same row). Use another
short jumper wire (use black) to go from the ground
row to the resistor (the leg that is not connected
to the LED). Refer to the picture below if
necessary.
The breadboard
should look like the picture shown below.
Now plug the
power supply into the wall and then plug the other
end into the power supply adapter and the LED should
light up. Current is flowing from the positive leg
of the LED through the LED to the negative leg. Try
turning the LED around. It should not light up. No
current can flow from the negative leg of the LED to
the positive leg.
People often
think that the resistor must come first in the path
from positive to negative, to limit the amount of
current flowing through the LED. But, the current is
limited by the resistor no matter where the resistor
is. Even when you first turn on the power, the
current will be limited to a certain amount, and can
be found using ohm�s law.
Ohm's Law can be
used with resistors to find the current flowing
through a circuit. The law is I = VD/R (where I =
current, VD = voltage across resistor, and R =
resistance). For the circuit above we can only use
Ohm's law for the resistor so we must use the fact
that when the LED is on, there is a 1.4 voltage drop
across it. This means that if the positive leg is
connected to 12 volts, the negative leg will be at
10.6 volts. Now we know the voltage on both sides of
the resistor and can use Ohm's law to calculate the
current. The current is (10.6 - 0) / 2200 = 0.0048
Amperes = 4.8 mA
This is the
current flowing through the path from 12V to GND.
This means that 4.8 mA is flowing through the LED
and the resistor. If we want to change the current
flowing through the LED (changing the brightness) we
can change the resistor. A smaller resistor will let
more current flow and a larger resistor will let
less current flow. Be careful when using smaller
resistors because they will get hot.
Next, we want to
be able to turn the LED on and off without changing
the circuit. To do this we will learn to use another
electronic component, the transistor.
1.6.1
The Transistor
Transistors are
basic components in all of today's electronics. They
are just simple switches that we can use to turn
things on and off. Even though they are simple, they
are the most important electrical component. For
example, transistors are almost the only components
used to build a Pentium processor. A single Pentium
chip has about 3.5 million transistors. The ones in
the Pentium are smaller than the ones we will use
but they work the same way.
Transistors that
we will use in projects look like this:
The transistor
has three legs, the Collector (C), Base (B), and
Emitter (E). Sometimes they are labeled on the flat
side of the transistor. Transistors always have one
round side and one flat side. If the round side is
facing you, the Collector leg is on the left, the
Base leg is in the middle, and the Emitter leg is on
the right.
Transistor Symbol
The following
symbol is used in circuit drawings (schematics) to
represent a transistor.
Basic Circuit
The Base (B) is
the On/Off switch for the transistor. If a current
is flowing to the Base, there will be a path from
the Collector (C) to the Emitter (E) where current
can flow (The Switch is On.) If there is no current
flowing to the Base, then no current can flow from
the Collector to the Emitter. (The Switch is Off.)
Below is the
basic circuit we will use for all of our
transistors.
To build this
circuit we only need to add the transistor and
another resistor to the circuit we built above for
the LED. Unplug the power supply from the power
supply adapter before making any changes on the
breadboard. To put the transistor in the breadboard,
seperate the legs slightly and place it on the
breadboard so each leg is in a different row. The
collector leg should be in the same row as the leg
of the resistor that is connected to ground (with
the black jumper wire). Next move the jumper wire
going from ground to the 2.2k ohm resistor to the
Emitter of the transistor.
Next place one
leg of the 100k ohm resistor in the row with the
Base of the transistor and the other leg in an empty
row and your breadboard should look like the picture
below.
Now put one end
of a yellow jumper wire in the positive row (beside
the red line) and the other end in the row with the
leg of the 100k ohm resistor (the end not connected
to the Base). Reconnect the power supply and the
transistor will come on and the LED will light up.
Now move the one end of the yellow jumper wire from
the positive row to the ground row (beside the blue
line). As soon as you remove the yellow jumper wire
from the positive power supply, there is no current
flowing to the base. This makes the transistor turn
off and current can not flow through the LED. As we
will see later, there is very little current flowing
through the 100k resistor. This is very important
because it means we can control a large current in
one part of the circuit (the current flowing through
the LED) with only a small current from the input.
Back to Ohm's Law
We want to use
Ohm's law to find the current in the path from the
Input to the Base of the transistor and the current
flowing through the LED. To do this we need to use
two basic facts about the transistor.
1.) If the
transistor is on, then the Base voltage is 0.6 volts
higher than the Emitter voltage.
2.) If the
transistor is on, the Collector voltage is 0.2 volts
higher than the Emitter voltage.
So when the 100k resistor
is connected to 12VDC, the circuit will look like
this:
So the current
flowing through the 100k resistor is (12 - 0.6) /
100000 = 0.000114 A = 0.114 mA.
The current
flowing through the 2.2k ohm resistor is (10.6 -
0.2) / 2200 = 0.0047 A = 4.7 mA.
If we want more
current flowing through the LED, we can use a
smaller resistor (instead of 2200) and we will get
more current through the LED without changing the
amount of current that comes from the Input line.
This means we can control things that use a lot of
power (like electric motors) with cheap, low power
circuits. Soon you will learn how to use a
microcontroller (a simple computer). Even though the
microcontroller can not supply enough current to
turn lights and motors on and off, the
microcontroller can turn transistors on and off and
the transistors can control lots of current for
lights and motors.
For Ohm�s law,
also remember that when the transistor is off, no
current flows through the transistor.
1.6.2
Introduction to Digital Devices - The Inverter
In digital
devices there are only two values, usually referred
to as 0 and 1. 1 means there is a voltage (usually 5
volts) and 0 means the voltage is 0 volts.
An inverter (also
called a NOT gate) is a basic digital device found
in all modern electronics. So for an inverter, as
the name suggests, it's output is the opposite of
the input (Output is NOT the Input). If the input is
0 then the output is 1 and if the input is 1 then
the output is 0. We can summarize the operation of
this device in a table.
Input
Output
1
0
0
1
To help us
practice with transistors we will build an inverter.
Actually we have already built an inverter. The
transistor circuit we just built is an inverter
circuit. To help see the inverter working, we will
build a circuit with two inverters. The circuit we
will use is shown below.
First Inverter (already built)
Second Inverter
To build the
circuit, use the transistor circuit we just built as
the first inverter. The first inverter input is the
end of the 100k ohm resistor connected to the yellow
jumper wire. Build another circuit identical to the
first (the basic transistor circuit from Section
1.6.1) except leave out the yellow jumper wire
connected to the 100k ohm resistor (the inverter
input). This circuit is the second inverter.
Connect the
output of the first inverter to the input of the
second inverter by putting one end of a jumper wire
in the same row of holes as the 2.2k ohm resistor
and the Collector of the transistor (the output of
the first inverter) and putting the other end in the
same row of holes as the leg of the 100k ohm
resistor of the second inverter (the input to the
second inverter).
Here is how to
check if you built it correctly. Connect the first
inverter input (the yellow jumper wire) to 12V (the
positive row). The LED in the first inverter should
come on and the LED in the second inverter should
stay off. Then connect the first inverter input to
0V (the ground row). (You are turning off the switch
of the first inverter.) The first LED should go off
and the second LED should come on. If this does not
happen, check to make sure no metal parts are
touching. Check to make sure all the parts are
connected correctly.
The input can
either be connected to 12V or 0V. When the Inverter
Input is 12V, the transistor in the first inverter
will turn on and the LED will come on and the
Inverter Output voltage will be 0.2V. The first
Inverter Output is connected to the input of the
second inverter. The 0.2V at the input of the second
inverter is small enough that the second transistor
is turned off. The circuit voltages are shown in the
diagram below.
When the Inverter
Input is connected to 0V, the transistor in the
first inverter is turned off and the LED will get
very dim. There is a small amount of current still
flowing through the LED to the second inverter. The
voltage at the first Inverter Output will go up,
forcing the second inverter transistor to come on.
When the second inverter transistor comes on, the
second inverter LED will come on. To find the
voltage at the output of the first inverter (10.4V),
use Ohm's law. There is no current flowing through
the transistor in the first inverter so the path of
the current is through the first LED, through the
2.2k resistor, through the 100k resistor, through
the second transistor to ground. The voltage at the
negative side of the first LED is fixed at 10.6V by
the LED. The voltage at the second transistor base
is fixed at 0.6V by the transistor. Then given those
two voltages, you should be able to find the voltage
at the point in the middle (10.4V) using Ohm�s law.
(Hint: First find the current and then work through
Form 1 of ohm�s law to find the voltage at the point
between the 2.2k resistor and the 100k resistor.)
Switch the input
back and forth from 0V to 12V and you can see that
when the first stage is on, the second stage is off.
This demonstrates the inverting action of the
Inverter.
The next project
in the series is called Pulses, Oscillators,
Clocks...
It introduces
capacitors and the LM555 timer. With these you can
make circuits with LEDs that will continually flash!
