Hands-on Activity Basic Stamp Project:
Design & Build LED Traffic Lights

Learning Objectives

After this activity, students should be able to:

• Explain the importance of traffic lights and safety.
• Explain and demonstrate the use of light-emitting diodes and resistors.
• Discuss the concept of energy efficiency and how it applies to light emitting diodes.
• Describe two Basic Stamp commands and the loop programming concept.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

International Technology and Engineering Educators Association - Technology

State Standards

Materials List

Each group needs:

• 1 BASIC Stamp Activity Kit ($80 from Parallax, Inc.)
• 1 Parallax Board of Education (#28803) with a correctly installed BS2 microcontroller ($99 from Parallax, Inc)
• 1 laptop or desktop computer with the Parallax PBASIC Editor software and the USB driver installed (if using USB version of the Board of Education); software is available at http://www.parallax.com/basicstampsoftware
• red, yellow and green LEDs (1 of each color)
• three 470 ohm resistors

To share with the entire class:

• jumper wires, as needed

Worksheets and Attachments

Introduction/Motivation

Before automobiles were invented, horses were used for transportation in big cities. Without traffic rules for pedestrians and horse carriages, streets were often overcrowded, and movement of people and goods was inefficient and dangerous. The problem was especially serious at road intersections. One solution to this congestion problem was to use police officers to control the flow of traffic, which is how it is still done in some parts of the world. Another solution, which was first used in 1868, was to install traffic lights with different colors to indicate when a vehicle should "stop," use "caution - be prepared to stop," or "go" (or, go ahead).

What colors are used in traffic lights to communicate these three messages to vehicles as well as pedestrians? (Answer: Red, yellow and green, respectively.) The reason traffic lights need to have red lights (or, "stop and wait"), is because it allows pedestrians and through traffic to pass a street intersection safely and efficiently during their own green light, which means "go ahead."

Who can come up to the board and draw a typical intersection? (Select a student to draw a traffic diagram on the classroom board.) In this intersection, how many traffic lights for vehicles are needed for safety? (Possible answers: Assuming the student drew a "cross" intersection, with one street intersecting the other, one set of traffic lights is needed for each street direction; four sets of lights, one for each direction; if "hung" in the middle of the intersection, one four-way light with a directional prong for each direction.)

The design of effective traffic light systems in modern cities is a task for traffic engineers and is an important undertaking for efficiency and safety. Traffic lights must be purposely placed to result in the safest and fastest movement of people and vehicles. Also important is the timing of traffic signals. For example, a traffic signal in a quiet suburb may safely be designed to cycle quickly between red, yellow and green, whereas a traffic signal at a busy intersection in a city business district may require longer switching times to give enough timem for vehicles and pedestrians to move safely and avoid congestion.

Until recently, traffic lights contained bulky and powerful lamps, which used a lot of electrical power, which meant they were not very energy efficient. In a city with many hundreds, or thousands, of such traffic lights, this meant the use of a lot of energy. To save energy, cities replaced lamps in traffic lights with light emitting diodes (LEDs), which use very little energy. In addition to reduction in energy use, an additional advantage of LEDs is that they have a longer service life, which means they last longer than the old-fashioned lamps. LEDs were first invented in the 1920s, and are electrical devices that can generate bright light while using very little power. Cities that switch to LEDs save a lot of energy and thus use fewer natural resources from our environment.

Procedure

Background

(Note: Many excellent online sources provide additional information about the types of traffic signs and traffic signals, street lights, duration of signal cycles, and history. If supplemental information is required, either print out information in advance or direct students to conduct pre-activity research and share their findings with the class.)

It is likely that many students are unaware of the important role traffic engineers play in their everyday lives. Thanks to traffic engineering, large metropolitan areas are able to function and conduct normal operations as safely and effectively as possible. Students get to be traffic engineers in this activity and implement a scaled-down practical model of a traffic light to learn about traffic engineering. Students take on the role of engineers (see Figure 1) tasked with building a traffic light for vehicles with a number of specifications provided by a local transportation agency. Specifications (called requirements in engineering) that must be met are: the traffic light is to have three lights (red, yellow, green), be able to shine red, yellow and green for a certain number of seconds each, and be able to repeat this sequence indefinitely.

A device called a BS2 (Basic Stamp 2) microcontroller is used to control the traffic lights. A colored LED is used for each individual light (as shown in Figure 2). As illustrated in the diagram overlaid in Figure 2, an LED is represented as a triangle with a horizontal vertical bar. Notice that an LED has one wire longer than the other. The shorter wire corresponds to the left side of the diagram (the side to the left of the vertical bar), and the longer wire corresponds to the right side. Finally, in the diagram, two arrows pointing upward from the LED represent light.

In order to use an LED, it must be connected to a power source, which is usually a battery. The correct way to connect an LED to a power source is through a device called a resistor. A resistor controls the amount of current (electrical charge) that can pass through it. A helpful analogy is to think of a resistor as a pipe, and electricity as water. A wide pipe would correspond to a small resistor value (for example 10 ohms; the smaller the number of ohms, the more electricity can pass) while a narrow pipe would correspond to a large-valued resistor. In this activity, a 470 ohm resistor is used, as depicted in Figure 3. Notice in the overlaid schematic that, in contrast to an LED, a resistor is connected without regard to direction.

A resistor must be used together with an LED to ensure that the amount of current provided to the LED is not so great that it damages the LED. Moreover, the diode must be connected properly, with the vertical bar (that is, that shorter bar) connected to the ground terminal, "Vss." Another way to refer to "ground" is to use the minus sign (or, "-"). The circuit used to control the LED is illustrated in Figure 4.

To make a traffic light in which LEDs do not have to be controlled manually, students program a BS2 microcontroller to perform the function automatically (see Figure 5). A BS2 is a small computer that can perform functions programmed by engineers. This computer is a small circuit with pins for connections. Some pins are used to provide power, while others are used to program the BS2. Still other pins are used to connect the BS2 to various devices such as resistors, LEDs, or even motors and other computers. The pin labeled P0 in the figure denotes pin 0 on the BS2.

Before the Activity

• Research and copy any additional references to support class discussion.
• Set up an LED circuit using the Figure 4 diagram, as well as a working Basic Stamp traffic light, as a pre-activity demonstration.
• Install the PBASIC Stamp Editor on student computers, along with the USB drivers if using a USB version of the Board of Education.

With the Students

1. Divide the class into groups of three students each, with each group having access to a computer. Assign the following roles to each team: builder, programmer and tester. The builder wires the circuit, the programmer writes the program using the PBASIC editor, and the tester runs the system and performs needed adjustments or corrections. All three team members support one another by demonstrating teamwork.
2. Discuss the importance of traffic lights and street safety; support discussions with outside materials, as necessary.
3. Introduce and demonstrate how to use the PBASIC editor to write, save and run PBASIC programs. Demonstrate the Board of Education, in particular the on/off switch (for this activity, set the on/off switch to position 2), and the white prototyping "breadboard" area used to build circuits. Discuss circuitry safety and precautions.
4. Pass out a Board of Education to each group.
5. Hand out one red LED and one 470 ohm resistor to each group, to start.
6. After demonstrating a working LED, have grouops build and test a basic circuit, as illustrated in Figure 4, using the following sequence of steps.

• The programmer opens the PBASIC Stamp editor and enters the program shown in the right-hand window in Figure 6, saving the file as program1.bs2. The builder builds the circuit in Figure 4. (Refer to the help menu to learn about commands HIGH, LOW and PAUSE. Note the units in which time is measured in the PAUSE command. Also have students note that the LED in this example is connected to pin P0 on the BS2.)
• The tester runs the program and verifies the design, in consultation with others.

Sample discussion questions:

Looking at the BS2, how many input/output pins does the BS2 have? (Answer: 16 pins)

How many pins are used in this exercise? (Answer: 1 pin)

What are the units used in the PAUSE command? (Answer: Milliseconds)

What command would one have to enter to make the BS pause for 3 seconds? (Answer: PAUSE 3000)

• The programmer opens a new file, and writes the program illustrated in Figure 7. All three team members look in the help menu and read about the DO LOOP statement used in this program.

Sample discussion questions:

What happens when the BS2 finishes executing the commands inside a DO LOOP? (Answer: It begins executing the commands starting at the line below the DO statement.)

For how many seconds does the LED flash in this example? (Answer: For 0.5 seconds.)

7. Pass out the yellow and green LEDs along with two 470 ohm resistors. Next, each team builds a traffic light with three LEDs (red, yellow and green) with the following 45-second cycle specification:

• The red LED remains on for 10 seconds.
• The yellow LED remains on for 5 seconds.
• The green LED remains on for 30 seconds.
• The sequence repeats indefinitely.

Step 7 simulates the operation of a real-world traffic light (see Figure 8) and builds upon the program students wrote in Step 6.

Vocabulary/Definitions

Basic Stamp 2 (BS2): A type of microcontroller that can be connected to the outside world and used to come up with a solution to an engineering problem or set of specifications.

congestion: A problematic condition that occurs when movement of traffic and pedestrians has stalled due to improper function of traffic control systems and procedures, or other reasons.

conservation: Cautious and sensible use of energy and other resources that does not harm the environment.

energy: Something that can be used to perform work.

environmental impact: The effect that human activity has on the Earth, its natural processes, and all living inhabitants.

LED: Light emitting diode, an electrical device that emits light when properly connected to an electrical energy source.

microcontroller: A system that can execute user-specified commands.

resistor: An electrical component that acts to limit the amount of electrical energy flowing through it.

specifications: A set of standards that a device or system must meet in order to be an acceptable solution to a given engineering problem.

system: One or more mechanisms or devices designed to do useful work, perform a function, or solve an engineering problem.

traffic light: A system used to control the flow of vehicles and pedestrians on streets and at road intersections.

Assessment

Pre-Activity Assessment

Sharing Experiences: Lead a class discussion about student experiences with transportation and traffic safety in general. Ask students to share their own observations.

Activity Embedded Assessment

Analysis: In Steps 6 and 7, have each group try to analyze the program before loading and running it on the BS2. Instruct students to examine the DO ... LOOP statement and how it behaves. Ask students to construct practical examples of where similar functionality may be useful. Some examples of repetitive actions that can be implemented with a loop include: move the second hand on an analog clock by a given amount every one second, evenly dribble a basketball, turn pages in a book, etc.

Post-Activity Assessment

Standards: Ask students to look up traffic light standards and switching times. As an example, conduct a post-activity field trip around the school building. Using stopwatches, have students measure the traffic light patterns on a quiet residential intersection vs. a busy one. Ask students to comment on and try to explain, or "reverse engineer," the differences. Ask students to observe how each modern traffic light is composed of a number of LEDs. Challenge students to draw parallels between what they observe, the Basic Stamp program written during the exercise, and the microcontroller used. (Remind students that modern traffic lights are also controlled by microcontrollers!)

Safety Issues

• Ask students to refrain from touching the BS2 microcontroller and power circuitry on the board, as it is delicate and could break easily.
• Do not perform the activity near food or liquids.

Troubleshooting Tips

If the BS2 or the PBASIC Editor crashes, close and restart the editor, and cycle the power to the board.

Activity Extensions

Build a model of a small section of a city using LEGO® pieces, papier-mâché, or recyclable material. Use the BS2s 16 input/output lines to control approximately five 3-light traffic lights, or eight 2-light traffic lights. Have students research ways different cities control traffic, and design appropriate models for a science fair, period project, or other school event, such as the Future City competition.

Activity Scaling

• For lower grades, have students program a BS2 already outfitted with a traffic light, and provide extra assistance as needed.
• For upper grades, have students learn about the BS2 by reading the help menu provided in the PBASIC editor. Also, have students research the energy requirements of regular (incandescent) light bulbs and energy-efficient LEDs. Have students compute the percent efficiency gain of an LED vs. the light bulb. For students familiar with electric circuits, assign a research activity that involves finding the current and voltage for a regular 60 Watt light bulb and an LED. Then, have students compute the power in each case (P=IV, where I is the current and V, the voltage), and comment on the efficiency obtained in both cases.

References

Copyright

Contributors

Supporting Program

Acknowledgements

This activity was developed by the Applying Mechatronics to Promote Science (AMPS) Program funded by National Science Foundation GK-12 grant no. 0741714. However, these contents do not necessarily represent the policies of the NSF, and you should not assume endorsement by the federal government.