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Electricity and Magnetism Lesson 7: High/Low LED Driver
Lesson 7: High/Low LED Driver
Let’s go back to Lesson 3 where we had the Boe-Bot simulate a shuttle launch countdown
(have the students pull up the program from their computers). At this point, your program
should count down correctly, but we want it to acknowledge the completed countdown and
the launch of the shuttle with something visual. Let’s simulate the shuttle launching by
turning on an LED.
Building the circuitry:
Before we jump right into turning on the LED, we need to understand a little bit about the
circuitry needed to make the LED turn on.
The first component of the circuitry setup is the resistor. In a later class we will go into
further detail on what a resistor is, but just know that there are many sizes of resistors that
limit the flow of current through an electrical circuit. When turning on an LED, a resistor
must be in place; otherwise, we run the risk of supplying too much current that will in turn
burn up the LED. As you can see, each resistor has a specific color coding. Each color stripe
indicates information about the total value of the resistor. The following resistor table
helps you to know the size of the resistor being used.
By using the chart, you can
determine the resistance of a
resistor from its colored stripes. The
first stripe corresponds to the first
digit, the second stripe relates to the
second digit, and the third stripe
corresponds to the number of zeros
to the right of the second digit. The
forth stripe is the tolerance of the
resistor. The tolerance stripe is
always a metallic color like gold or
silver, so it is usually easy to
distinguish which side of the resistor
should be your first stripe. For
example, a 470 (“” = the Greek
letter omega, which is used to
represent ohms, the units of
resistance) resistor would be
yellow-violet-brown with a metallic
fourth stripe, where yellow
represents a 4, violet represents a 7, and brown represents 1 zero that follows the resistor
values Putting them all together, we must have a resistor of “470” ohms.
NASA-Threads
Electricity and Magnetism Lesson 7: High/Low LED Driver
Now let’s figure out what a 220 resistor would look like.
2-Red
2-Red
1 Zero-Brown
Thus, a 220 resistor will have the color stripes of: Red, Red, Brown.
The next component that needs discussing is the breadboard. The breadboard is the white
board with all the holes in it. The following graphic shows very briefly how a breadboard
works.
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The third and final component to be discussed is the LED itself. The flow of current through
the LED is unidirectional (meaning it can only travel in one direction), thus the LED must
be positioned in the circuit properly.
How can you tell if the LED in set up properly?
There are two visual ways of determining the direction that an LED allows current to flow
in:
1)
The length of the leads
 The short lead is connected to the lower potential (ground).
2)
The LED plastic casing
The “flat” side on the base of the LED is connected to the lower potential or ground (This is
useful to know when someone has previously cut one of the leads).
NASA-Threads
Electricity and Magnetism Lesson 7: High/Low LED Driver
How do you turn on an LED?
Vss
Vdd
470 Ohm
LED
Notice how the resistor is plugged into the Vdd pin. The Vdd pin supplies the circuit with a
regulated voltage of 5V (“V” stands for volts, the standard unit for voltage). If the resistor
was plugged into a pin associated with Vin then the circuit would be supplied with the
voltage value associated with the power source. For instance if you have 4 new AA
batteries powering your BOE-Bot, each of which contains approximately 1.5 Volts, then Vin
would supply 6 Volts.
NASA-Threads
Electricity and Magnetism Lesson 7: High/Low LED Driver
What if we want to control the LED, when it turns on, how long it stays on, and so forth?
To do so we need to incorporate the circuit into a program. Move the power lead to one of
the pin numbers (The pin you place it in is arbitrary as long as you make sure you indicate
the appropriate pin in your program. If you put the lead in pin 7 make sure your program
uses pin 7). The microcontroller can now control when the pin is acting as a power source.
On the left is a schematic of a simple circuit required to light the LED. The picture on the
right is the circuit assembled on the BOE-Bot breadboard.
Vss
Pin
0
470 Ohm
LED
Notice from the picture how the resistor is plugged into pin 1. Each of the numbered pins in
this column can individually be supplied with 5V when turned on in a PBASIC program.
Let’s incorporate lighting the LED with our countdown program. Pull up the countdown
program from the previous lesson into the PBASIC editor. At the end of the countdown, add
a line of code that turns on the LED. This requires the HIGH command. The line of code
should be as follows:
HIGH #
where the # is pin number that the resistor is plugged into.
NASA-Threads
Electricity and Magnetism Lesson 7: High/Low LED Driver
This program, used in conjunction with the circuit we just built, will light an LED by
sending a high signal (5 volts) to a pin. This voltage then passes through the resistor and
the LED and then “sinks” to the ground.
LED Discussion
You may be wondering, “Why is the resistor in front? What if I put the resistor after the
LED?”
Well let’s see what happens when you hook up the following
LED Direction:
LED
Vss
Pin
0
Vss
5v
470 Ohm
You can see that all we did was “flip” the entire circuit.
Now test your program and the circuit.
LED
This diagram shows that the
current is allowed to flow from
higher potential (5v) to the lower
potential (ground). The LED
allows this current to pass
through.
What happened? Why did the LED not light up?
The real question should be: What is an LED?
LED
Vss
5v
This LED “blocks” the flow of
this current.
NASA-Threads
Electricity and Magnetism Lesson 7: High/Low LED Driver
LED stands for Light Emitting Diode. So it is a Diode that gives off light. But what is a diode?
The most common function of a diode is to allow an electric current to travel in one
direction and to block current in the opposite direction. This means that in our case, the
electricity is “blocked” because of the direction of our LED.
Pull out just the LED and “flip” the leads around so that the LED is still in front of the
resistor as shown below.
Vss
Pin
X
470 Ohm
LED
Now try your program and circuit. This time, your program should work.
This demonstrates how you can turn on an LED using a high signal.
*Just make sure you pay attention to the direction of the current flow and the LED!*
Now that we know how to turn ON an LED using the HIGH command, we need to know how
to turn the LED OFF. This is done by using the LOW command to make the pin output a
voltage of 0 Volts.