Using colorful light-emitting diodes to engage students in the study of electric circuits L E DS. Christopher Johnstone.

Transcription

1 Using colorful light-emitting diodes to engage students in the study of electric circuits L E DS Christopher Johnstone February

2 When learning about electricity, students are typically asked to complete a simple circuit involving a battery, wire, and lightbulb. This activity can be enhanced by adding discussion and discourse (Yang 2008) and can take on a unique final form in the case of The Friendship Detector (Cox 2012). In such electrical circuit activities, the completed circuit is usually confirmed by a glowing incandescent bulb. This article describes circuitry lessons using light-emitting diodes (LEDs) instead. These hands-on exercises help students better understand electricity by FIGURE 2 Student illustrations of successful LED circuit configurations. building multiple circuits, demonstrating their knowledge of how the electrical energy changes with each new circuit and configuration, and applying what they have learned by engineering an authentic and useful electrical device that performs a specific task. Teachers can connect these activities to aspects of the Next Generation Science Standards (NGSS) (NGSS Lead States 2013) (Figure 1). How do LEDs work? Light-emitting diodes are used in many consumer electronics products such as televisions, computers, and smartphones, and for good reason: LED arrays are 75 80% more efficient than traditional incandescent bulbs (U.S. Dept. of Energy 2012). While incandescent bulbs produce light with a glowing metal filament, and compact fluorescent bulbs (CFLs) produce light when electrons interact with mercury and phosphorus in tubes, LEDs emit light when electrons pass between semiconductor materials inside the bulb. This is called electroluminescence. When electricity is applied, semiconductor materials containing different compounds emit light at various wavelengths, producing a variety of colors (Edison Tech Center 2013). LEDs are now commonly available in red, green, blue, yellow, orange, purple, pink, and white. These small electrical devices are safer, less expensive, and more durable than incandescent bulbs and many other electrical elements used in science classrooms, and LEDs are easy to work with. (Note: Safety glasses or goggles are required for the following activities.) Lesson 1: Building LED circuits Students work with two 1.5V AA batteries, two wires, and an LED. I begin by asking them for ideas for how these materials might be arranged to create an electric circuit. Then they experiment with different configurations, working in pairs or small groups so there are enough hands to help FIGURE 1 Connections to the Next Generation Science Standards and A Framework for K 12 Science Education. HS-PS3 Energy Performance Expectation HS-PS3-3: Design, build, and refine a device that works within given constraints to convert one form of energy to another form of energy. Disciplinary Core Idea PS3.A: Definitions of Energy: At the macroscopic scale, energy manifests itself in multiple ways, such as in motion, sound, light, and thermal energy. Science & Engineering Practices: Planning and Carrying Out Investigations, Developing and Using Models, Constructing Explanations and Designing Solutions. Crosscutting Concepts: Energy and Matter: Changes of energy in a system can be described in terms of energy flows into, out of, and within that system. 28 The Science Teacher

3 Teaching Electricity and Engineering with LEDs position the batteries, wires, and LEDs that make up the circuit. Chances are that one or more students will have some experience with these materials and can help inexperienced students. Give students time to arrange the circuit materials in various configurations until some successfully light the LED (Figure 2). Safety note: Inexperienced students may inadvertently short-circuit the batteries with the wires in an attempt to complete their circuit. This can harm the battery and may cause the battery and wire to heat up. Some students will find that their LED in a proper closed circuit will not glow. They need to reverse the polarity of the LED or the circuit and realize that diodes only allow current to flow in one direction; i.e., an LED in the wrong orientation will not glow (Figure 3). This highlights circuit polarity and the overall concept that real electrical circuits are more than just simple elements connected in a general circle, but rather a sequence of relatively more complicated elements that operate under a number of finite principles that govern current and voltage. Challenge the students to re-arrange FIGURE 3 their circuit with the LED. There are multiple configurations for a successful circuit. Students should record illustrations of various configurations (Figure 2). Lesson 2: Make it quantitative Electronic devices are designed as a tool or to accomplish a task. The elements in the circuit perform the necessary task Check the polarity of the circuit. Distinguish the polarity of LEDs by examining the shape of the base, the length of the leads, or the shape and position of the electrode posts. Change the polarity by rotating the LED 180 degrees in the circuit or by changing the polarity of the voltage source or battery. but only if properly organized. Unlike incandescent bulbs, which typically glow dimly even with less than optimal electric current, LEDs are engineered to light only when a particular amount of electricity is present in the circuit. For example, many red LEDs are rated for two volts, and blue LEDs are rated for three volts. This means a red LED will only emit light when two volts are present across the February

4 diode, and a blue LED will only operate when three volts are present. This allows for some additional inquiry. For lesson 2, provide each student group with a second LED, and direct them to make both LEDs light up simultaneously while linked in series in the circuit with the two AA batteries. Ask students: How will you position the parts of your circuit? Require them to draw the circuit configuration. Students will discover that when connected in series, the two LEDs won t light with just two 1.5V batteries. Some students might begin to formulate the relationship between the number of LEDs and the amount of energy or voltage needed to make them operate singly and together. Students learn that the LEDs provide a cumulative resistance to the current and that the additive property of the batteries allows for a cumulative voltage supply when needed. How many batteries are needed to make the two series LEDs light simultaneously? Take a poll and gather student reasoning, then have the students try adding a third battery and then perhaps a fourth. They should discover that it takes only three 1.5V batteries to make the two 2V LEDs in series light. For assessment: Next to the circuit illustrations they drew, ask each student to provide reasoning for why that circuit succeeded or failed to operate the LEDs. Student responses that relate to the number of LEDs and number of batteries (or number of volts) required indicate the student has achieved a working knowledge of general electric circuits and of LEDs acting as resistors in a circuit. FIGURE 4 Using only two wires, students parallel circuits will likely resemble these two configurations. 30 The Science Teacher

5 Teaching Electricity and Engineering With LEDs FIGURE 5 Series circuit vs. parallel circuit. Series circuit Voltage is divided and shared among all LEDs in the circuit. Each LED experiences a voltage lower than that of the circuit supply voltage. Parallel circuit Voltage in the branches of a parallel circuit is shared equally among all branches. Each branch experiences a voltage equal to that of the circuit supply voltage. Tinkering time Now tell students they can make the two LEDs light up with just two 1.5V batteries and the wires provided, but only after they discover the necessary configuration (Figure 4). Let students compete to see who can meet the challenge first. Give students some tinkering time to practice critical thinking. It has been my experience that at least one student or group will eventually stumble upon the circuit configuration the parallel circuit needed to light both LEDs with just the two batteries. Encourage the successful students to help the others execute the parallel circuit. Allow for discussion on the similarities and differences between series and parallel circuits. This is also a good time for written reflection, which can include defining the relevant vocabulary: LED, voltage, current, resistance, series circuit, and parallel circuit. Ask students to propose an explanation for why the parallel circuit allows the dual LEDs to light simultaneously, while the series circuit does not. Allow students to share and critique each other s hypotheses and follow up with some formal instruction on how series and parallel circuits differ with regards to electrical current and distribution of voltage (Figure 5). Extend the students learning by asking them to consider answers to more complicated circuitry questions (Figure 8, p. 33). Lesson 3: A project-based conclusion Your students now have the necessary understanding to design a real and useful electronic device. They are ready to bolster their skills in the NGSS Science and Engineering Practices of developing a model, conducting an investigation and analyzing data, constructing an explanation and designing a solution, and eventually refining a final design. Students can use their knowledge of LEDs and electricity to design a voltmeter, which measures electrical potential difference (voltage). This exercise requires LEDs in an assortment of colors (most manufacturers create color LEDs that predictably correspond to their voltage rating). A 2V LED (red, for example) will light when two volts are provided, and a 3V LED (blue, for example) will light when three volts are provided. The students have learned that two 2V LEDs in series will operate with at least 4V provided. According to this pattern, one 2V LED and one 3V LED in series will operate simultaneously if five volts are present. If these two scenarios are wired together correctly in parallel, the different amounts of voltage can be applied, and the corresponding cumulative number of LEDs should light up when the appropriate amount of voltage is present. With an assortment of LEDs rated for 2V and 3V, students can design a voltmeter to measure voltage in 1V increments, starting with 2V. The challenge is to wire these LEDs in a single complex circuit using both series circuits and parallel circuits. Done correctly, this voltmeter, when put in circuit with a voltage source, will indicate the level of voltage. The voltage can be varied simply by changing the numbers of batteries or by using a variable low-voltage voltage source, such as those used by electrical hobbyists. Safety note: Even though working with 1.5V dry cell batteries in the classroom is not considered particularly hazardous (Roy 2007), many batteries linked together can pose a risk. In my classroom, I use plastic battery holders that allow multiple batteries to be systematically linked together, reducing the chance of mixing polarities or unintentionally creating short circuits. Students can use their knowledge of LEDs and electricity to design a voltmeter, which measures electrical potential difference (voltage). February

6 FIGURE 6 Students plan the design of the voltmeter with a schematic illustration that includes symbols and labels. Careful planning and testing This project should include a modeling phase, a time for brainstorming and planning of the design. My students illustrated their circuit schematic diagrams freehand and with the help of Google Draw (part of Google Apps) (Figure 6). Other educators recommend such tools as Google Sketchup and Dabbleboard (Brunsell and Horejsi 2012). Explanations for the operation of the students designs should accompany these planning illustrations. Students learn that careful planning and application of their previous knowledge will benefit their efforts to construct and test their product. Wiring the voltmeters is not easy, so the planning illustrations will provide a guide for them, making sure each wire and LED is placed in the correct location and in the proper orientation. I also recommend that students test their voltmeter circuit regularly in its beginning stages. Making sure each branch of the complex circuit operates before adding to it will help reduce frustrations later. FIGURE 7 A student-engineered LED voltmeter in parallel circuit with a commercially available analog voltmeter for comparison. 32 The Science Teacher

7 Teaching Electricity and Engineering with LEDs FIGURE 8 Extend the Learning Once students are comfortable with the LED system, they should be able to investigate and discuss answers to the following questions and others: Question How many batteries will it take to light up three or four LEDs in series? What will happen if three or four or more LEDs are put in a circuit in parallel? Can a circuit have LEDs that are in series and in parallel? What would this look like and how would it work? Do you think most circuits in electrical devices are wired in series or in parallel? Why? Parallel circuits seem more practical than series circuits. Why would a circuit ever contain electrical elements in series? In physics and engineering, there is always a consequence. What is the downside (electrically speaking) to wiring elements in parallel versus in series? PRACTICAL QUESTIONS Answer CONCEPTUAL QUESTIONS Depending on the types of LEDs used in the circuit, four LEDs in series will require between six and eight batteries (8V 12V). Any number of LEDs correctly wired in parallel to at least two 1.5V batteries will all light. With a sufficient voltage input, a number of LEDs wired in series combined with LEDs wired in parallel can all light up simultaneously. The required voltage will be determined by the greatest voltage combination of LEDs in series. Most elements of electronic devices are likely wired in parallel. This allows devices to multi-task. In other words, the device can perform more than one task at a time without requiring more voltage. If voltage in a circuit changes, and in particular if the voltage drops, the elements in series may no longer function. In this way, elements wired in series can act as threshold voltage indicators (i.e., if the LED does not light up, the battery needs changing or charging). As multiple elements are wired in parallel to a single voltage source (a battery for example) the current (amperage) in the overall circuit will increase. While parallel circuits can allow for multiple elements to function simultaneously, the draw on the battery is significant and will often result in a dead battery sooner than if those elements were wired in series. Resistors To successfully complete the voltmeter, students will need to use a new element in their circuits: the resistor. LEDs burn out at threshold high levels of current caused by high voltage. This usually occurs at twice the rated voltage for the LED. A 2V LED will be destroyed by 4V or more, and a 3V LED will be destroyed by approximately 6V or more. If students design a voltmeter that will be subject to 4 6V or more, small resistors will be required and need to be placed in each branch of the circuit to protect the LEDs from damage from high levels of current. Suitable resistors are usually shipped with packages of bulk LEDs or can be purchased separately. In the end, the students should be able to produce a complex circuit that when subject to various voltages is able to indicate the amount of voltage by the number and types of LEDs that glow (Figure 7). Students can collect empirical data using multimeters to test the accuracy of the voltmeters. Students may find that their voltmeters, despite careful design, are not perfectly accurate. February

8 FIGURE 9 Voltmeter scoring rubric. Circuit Schematic Diagram NGSS: developing and using models A schematic diagram was submitted Diagram indicates the position of all circuit elements. Diagram is illustrated clearly and indicates the position of all circuit elements. Diagram is illustrated clearly, indicates the position of all circuit elements, and uses standard schematic symbols. Circuit Schematic Description & Reasoning NGSS: constructing explanations and designing solutions Description of schematic is general in nature. Description of schematic addresses overall design of the circuit. Description of schematic includes thorough explanation of each branch of the circuit. Description of schematic includes thorough explanation of each branch of the circuit and the scientific concepts behind its design. Prototype Testing NGSS: planning and carrying out investigations Prototype was tested Prototype was tested over a suitable range of voltage, and relevant observations were recorded. Testing and observations (including empirical data) were completed over a suitable range of voltage. Testing and observations (including empirical data) were completed over a suitable range of voltage. Data is precise and organized. Final Design Construction and Performance Final design requires frequent adjustments. Final design requires periodic adjustments and operates over a range of voltage with mixed results. Final design requires periodic adjustments but repeatedly operates successfully over a suitable range of voltage. Final design is robust, requiring no tunings or adjustments, and repeatedly operates successfully over a suitable range of voltage. Final Design Aesthetic Final design s visual elements are minimal. Final design s aesthetics are suitable for this type of electrical device. Final design s aesthetics include elements that make using the device particularly easy and intuitive. Final design s aesthetics are particularly novel or creative. 34 The Science Teacher

9 Teaching Electricity and Engineering with LEDs While incandescent bulbs produce light with a glowing metal filament, and compact fluorescent bulbs (CFL) produce light when electrons interact with mercury and phosphorus in tubes, LEDs emit light when electrons pass between semiconductor materials inside the bulb. This is called electroluminescence. For example, the 3V LED may glow when 2.85V are provided. This amount of error provides another opportunity for students to reflect on the data and provide insight for the explanation of the various properties, benefits, and limitations of their engineering design. My scoring rubric for this project includes elements of the engineering process as well as the performance and aesthetics of their final product (Figure 9). LEDs can be a fun and engaging addition to investigations into the properties of electric circuits. Their quantitative properties allow students to explore patterns and properties of circuits and to engineer an authentic electrical tool. As an extension to these lessons, my students linked their voltmeters in circuit with some small photovoltaic panels and tabletop wind turbines to measure the voltage produced by these devices under various conditions. The students engaged in the beginning lessons because of the novelty of the LEDs and by the end were successfully operating their own voltmeters. n Christopher Johnstone is a science teacher at Proctor Junior/Senior High School in Proctor, Vermont. References Brunsell, E., and M. Horejsi Science 2.0: Engineering, modeling, and computational thinking. The Science Teacher 79 (9): 10. Cox, S The friendship detector. The Science Teacher 79 (2): Edison Tech Center Small lights with big potential: Light-emitting diodes and organic light-emitting diodes, Commercial History (1960s Today). org/led.html. National Research Council (NRC) A framework for K-12 science education: Practices, crosscutting concepts, and core ideas. Washington, DC: National Academies Press. NGSS Lead States Next Generation Science Standards: For states, by states. Washington, DC: National Academies Press. Roy, K Safer science: Circuit safety. The Science Teacher 74 (8): U.S. Department of Energy How energy-efficient light bulbs compare with traditional incandescents. gov/energysaver/articles/how-energyefficient-light-bulbs-compare-traditionalincandescents. Yang, L Lighting the way through scientific discourse. The Science Teacher 75 (9): February