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Are LEDs the Future? Energy Savings with LED Lighting

Difficulty
Time Required Long (2-4 weeks)
Prerequisites Prior experience with using a digital multimeter
Material Availability Specialty items (see Materials list)
Cost High ($100 - $150)
Safety Since this science project deals with electricity, adult supervision is recommended. The LEDs used in this project are used in traffic signals and are very bright. Do not look directly at the LED when it is in operation and be sure to wear your sunglasses. Be sure to wear safety goggles when operating power tools.

Abstract

Global warming, climate change, melting ice caps—these are all big events that have an impact our environment. What can we do to help reduce the impact? We can reduce, reuse, and recycle. What can cities do to help? Cities can eliminate waste by saving energy. Cities around the world are switching from incandescent traffic signals to LED traffic signals to save energy and money. That's because LEDs are more efficient than incandescent lamps, which means that LEDs produce more light compared to incandescent lamps for the same input power. So with LEDs, you get more for less! In this science project, you will learn how LEDs are used and why so many cities and countries are making the switch to LED technology.

Objective

The purpose of this science project is to learn about a key real-world application of light-emitting diodes (LEDs): the traffic signal. You will put together three simple circuits to study how efficient an LED is compared to a conventional incandescent lightbulb. You will then be able to determine which technology is the better choice for traffic signals.

Credits

Michelle Maranowski, PhD, Science Buddies.

Edited by Steven Maranowski, PhD, Philips Lumileds Lighting Company.

The author would like to thank Thomas Goglio for building the stand.

  • LumiledsTM is a trademark of Philips Lumileds Lighting Company.
  • Texas Advanced Optoelectronic Solutions® is a registered trademark of Texas Advanced Optoelectronic Solutions, Inc.

Cite This Page

MLA Style

Science Buddies Staff. "Are LEDs the Future? Energy Savings with LED Lighting" Science Buddies. Science Buddies, 22 Oct. 2014. Web. 21 Dec. 2014 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Energy_p003.shtml>

APA Style

Science Buddies Staff. (2014, October 22). Are LEDs the Future? Energy Savings with LED Lighting. Retrieved December 21, 2014 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Energy_p003.shtml

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Last edit date: 2014-10-22

Introduction

Our lives would be completely different without semiconductors. All integrated circuit (IC) chips are made out of semiconductors. ICs are in our cars, in airplanes, and in our kitchen appliances. But what is a semiconductor? A semiconductor is a material whose function falls somewhere between an insulator (like plastic) and a conductor (like copper). Semiconductors have the ability to conduct electricity under certain conditions, which leads to some interesting and useful devices, such as transistors and lasers.

One semiconductor you've probably heard of is a light-emitting diode (LED), which is a semiconductor device that converts electricity into light. Current in semiconductors is carried by electrons (negatively charged) and holes (positively charged). An LED consists of a layer of electron-rich material next to a layer of hole-rich material. When a current is applied to the LED in a certain direction, the electrons move toward the holes and the holes move toward the electrons. When an electron and a hole meet, they create light. The wavelength, or color of the light, depends on the materials that are used for the layers and how electron-rich layer 1 is and how hole-rich layer 2 is.

Electricity Electronics Science Project two layers in the LED
Figure 1. Here are the two layers in the LED. When the two are joined, electrons and holes meet in the middle.

This engineering science project is an excellent example of how different areas of study intersect to solve a problem. For example, how can a city save energy and save money? It costs money to run city services, such as traffic signals. That's because making power costs money. How can a city save money on traffic lights? The answer is to convert from incandescent traffic signals to LED (light-emitting diode) traffic signals. A lightbulb, called an incandescent lamp, also converts electricity into light. However, an LED produces more light with less power than a regular lightbulb does. This means that LEDs are more efficient. The wall plug efficiency is defined as the ratio of optical output power of the device and the electrical input power (i.e., the efficiency of converting electrical power into optical power). Wall plug efficiency is a measure that is used to compare LEDs to each other and to other light-emitting devices, such as the incandescent lamp.

Additional LED advantages are that they are extremely rugged, can withstand shock and vibration, are compact, last longer, and do not radiate as much heat as incandescent lamps do. In fact, about 90% of the energy that is used to light up an incandescent lamp is lost to heat!

In addition to being a more environmentally friendly option than regular incandescent lamps, LEDs improve safety on the roads. Since LEDs are so bright compared to incandescent lamps and turn on faster, they are easier to see. An LED turns on nearly instantly, about 200 milliseconds faster than an incandescent lamp. This means that a car with LED brake lights will alert other drivers behind them that they are stopping 200 milliseconds sooner. At 65 miles per hour, this provides an extra 19 feet of stopping distance for drivers looking at the LED brake light.

Electricity Electronics Science Project green LED traffic signal
Figure 2. Shown is a green LED traffic signal.

In this engineering science project you will be measuring the wall plug efficiency of the LED and comparing that to the wall plug efficiency of an incandescent lamp. To do this, you will assemble three circuits to test for wall plug efficiency. The first circuit will be a constant-current circuit that applies constant current to the LED for however long you test. The second circuit will be a light-detection circuit. The light-detection circuit contains a light-to-voltage converter that monitors any changes in the brightness of the light coming from the LED and is proportional to the output power of the light source. The third circuit applies a voltage to an incandescent lamp. Have fun and remember that you are doing your part to help the environment.

Terms and Concepts

  • Semiconductor
  • Insulator
  • Conductor
  • Transistor
  • Laser
  • Light-emitting diode (LED)
  • Diode
  • Incandescent
  • Wall plug efficiency
  • Lumens

Questions

  • What is a semiconductor? How many kinds of semiconductors are there?
  • What is the difference between an incandescent lamp and a light-emitting diode?
  • Can you confirm whether LEDs are a good choice for traffic signals?
  • What other applications for LEDs can you find besides traffic signals?

Bibliography

To read more about various LED products, see some cool pictures of applications, and read interesting case studies and white papers, go to the following website:

For a simple explanation of how LEDs work, read the following entries:

For application notes and the data sheet on the LED constant-current driver, read:

For the data sheet on the light-to-voltage converter, read:

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Materials and Equipment

Many of the following materials and equipment can be purchased from the following suppliers' websites. Part numbers are indicated in the list below.

Test Assembly

  • Safety goggles
  • Drill with ¾-inch drill bit
  • Ruler
  • Plywood, 8.5 inches x 11 inches, 1 inch thick
  • Wood dowel, ¾ inches thick, 18 inches long
  • Carpenter's glue

Constant-current Test Circuit

  • Adaptaplug "M," 5.5-mm O.D. x 2.1-mm I.D. (Radio Shack part # 273-1716); the Adaptaplug adapts the plug from the power adapter to the coaxial power jack.
  • 12 VDC - 500-mA AC-DC power adapter (Radio Shack part # 273-1774); this part converts AC electricity to DC electricity.
  • Panel-mount size M coaxial power jack (Radio Shack part # 274-1582); the power jack allows 12 V to be accessible.
  • BuckToot 350mA Constant Current LED Driver (LED Supply part # 07027-D-350)
    • Note: this is a replacement part for the PowerPuck constant current driver, part # 02008B-350, which is no longer in stock at LED Supply. At any point in the Procedure that refers to the PowerPuck driver, you should use the BuckToot driver instead.
  • Insulated test/jumper leads with mini alligator clips at each end (8) (Radio Shack part # 278-1157)
  • Digital multimeter, available at Amazon.com. Make sure that the multimeter you choose measures the variable you are interested in and has the appropriate operating range. In this science project you will need to measure DC voltage in the range of 0-4 volts (V).
  • Small piece of paper
  • Cree XPE Indus Star 1-Up Red High Power LED (3) (LED Supply part # CREEXPE-RED-1)
    • Note: this is a replacement part for the Luxeon I red LEDs, part # LXHL-MD1D, which are no longer in stock at LED supply. At any point in the Procedure that refers to the Luxeon LEDs, you should use the Cree XPE LEDs instead. These LEDs do not come with jumper wires attached. You will need an adult's help to solder jumper wires onto the positive and negative pads of the LED boards.
  • Sunglasses

Incandescent Lamp Circuit

  • Miniature lamp holder (Radio Shack part # 272-0357)
  • #1487 screw-base lamp (2 packages) (Radio Shack part # 272-1134)
  • Phillips head screwdriver

Light-detection Circuit

  • Breadboard (Mouser Electronics part # 517-922306)
  • Light-to-voltage converter (Mouser Electronics part # 856-TSL14S-LF)
  • AA batteries (2 unused)
  • AA battery holder (1) (Radio Shack part # 270-408)
  • Insulated wire, 1-3 inches long (4 pieces) (Radio Shack part # 278-1224)
  • Wire strippers
  • 10-K-ohm resistor (Mouser Electronics part # 271-10K-RC)
  • Flashlight

Additional Items

  • 2-inch clamps (2) (Home Depot part # 80002)
  • Masking tape
  • Ruler
  • Lab notebook
  • Graph paper

Disclaimer: Science Buddies occasionally provides information (such as part numbers, supplier names, and supplier weblinks) to assist our users in locating specialty items for individual projects. The information is provided solely as a convenience to our users. We do our best to make sure that part numbers and descriptions are accurate when first listed. However, since part numbers do change as items are obsoleted or improved, please send us an email if you run across any parts that are no longer available. We also do our best to make sure that any listed supplier provides prompt, courteous service. Science Buddies does participate in affiliate programs with Amazon.comsciencebuddies, Carolina Biological, and AquaPhoenix Education. Proceeds from the affiliate programs help support Science Buddies, a 501( c ) 3 public charity. If you have any comments (positive or negative) related to purchases you've made for science fair projects from recommendations on our site, please let us know. Write to us at scibuddy@sciencebuddies.org.

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Experimental Procedure

Note Before Beginning: This science fair project requires you to hook up one or more devices in an electrical circuit. Basic help can be found in the Electronics Primer. However, if you do not have experience in putting together electrical circuits you may find it helpful to have someone who can answer questions and help you troubleshoot if your project is not working. A science teacher or parent may be a good resource. If you need to find another mentor, try asking a local electrician, electrical engineer, or person whose hobbies involve building things like model airplanes, trains, or cars. You may also need to work your way up to this project by starting with an electronics project that has a lower level of difficulty.

Safety Note: Remember that you should never look directly at an LED when it is in operation. Wear your sunglasses.

While there are a lot of instructions, don't let this concern you. Here is a brief explanation of each section and its purpose. This procedure is separated into five sections:

  1. Building the Test Assembly
  2. Building the Constant-current Test Circuit
  3. Building the Incandescent Lamp Circuit

    In order to electrically test the LED against the incandescent lamp, you need to turn on the devices by applying a constant current and constant voltage respectively, which is why you first build the two circuits in sections 2 and 3.

  4. Building the Light-detection Circuit

    In this section, you will build a circuit to quantify and compare the brightness of the light coming out of the LED and the incandescent lamp.

  5. Testing and Data Collection.

    Sections 4 and 5 detail how to test the LED and incandescent lamp in order to obtain your data.

Building the Test Assembly

  1. Build a test assembly so that the light from the LED can shine directly onto the open window of the light-to-voltage converter. The test assembly needs to dry overnight, so do this step first.
  2. Drill a hole in the 8.5-inch x 11-inch piece of plywood. Drill the hole along the longer side of the piece, about 1 3/4 inches in from the edge and centered. Be sure to wear your safety goggles.
  3. Glue the wood dowel into the hole with the carpenter's glue. Make sure that the wood dowel is straight and perpendicular to the plywood. Let it dry overnight, ensuring the dowel won't move.

    Electricity Electronics Science Project wooden stand from building the test assembly
    Figure 3. Here is the wooden stand from building the test assembly.


Building the Constant-current Test Circuit

  1. Connect the Adaptaplug to the AC-DC power adapter. Be sure to follow the instructions on the package so you connect it in the correct polarity. If the AC-DC power adapter and Adaptaplug are put together incorrectly, this can result in damage to the power adapter. For this science project, align the word TIP on the adapter's input jack with the "+" on the Adaptaplug. See Figure 4.



    Electricity Electronics Science Project schematic of the power adapter and the Adaptaplug connection
    Figure 4. Here is a schematic of the power adapter and the Adaptaplug connection.


  2. Attach the coaxial power jack to the Adaptaplug. The voltage is between the center pin and shell pin (look at the diagram on the back of the package or Figure 5 to see which pin is which). For the purpose of this project, ignore the third pin. Plug the AC-DC power adapter into an outlet and test the coaxial power jack with the digital multimeter to make sure that you are getting 12 volts (V) across the center pin and the shell. Unplug the AC-DC power adapter.

    Electricity Electronics Science Project schematic of the coaxial power jack
    Figure 5. This is a schematic of the coaxial power jack.


  3. Using the alligator clip cables, clip the center pin of the coaxial power jack to the red wire of the PowerPuck and the shell pin of the coaxial power jack to the black wire of the PowerPuck. Make sure that the clips do not touch at all. The PowerPuck takes an input of 12 V and generates a constant 350 milliamps (mA).
  4. Plug the AC-DC power adapter back into the wall. You are ready to start testing the first LED.
  5. Prepare the LED for testing. Since the back is metallic, put the LED on a small piece of paper. The paper will serve as an insulator so that when you connect the LED to the current source, the device will not be shorted. A device is shorted when there is an unintentional path in which current can flow. You can also cover the back of the LED board in electrical tape.
  6. Now take an alligator cable and clip one end to V-, the green wire on the PowerPuck. Connect the other end of the alligator clip to the negative terminal of the LED. If you examine the LED closely you can see that some connections have a "-" (negative) next to them and some connections have "+" (positive) next to them.
    • Note: You may not get a good electrical connection if you try to attach alligator clips directly to the Cree XPE LED boards, because the pads are too small. If this is the case, get an adult to help you solder jumper wires to the "+" and "-" pads, and attach the alligator clips to those. If you try clipping directly to the board, make sure the back side of the board is insulated with paper or electrical tape, so the alligator clips do not short together.
  7. Put your sunglasses on. Take another alligator clip and clip one end to the V+, the orange wire on the PowerPuck. Connect the other end to the positive terminal of the LED. The LED will turn on right away. Be sure not to look directly at the LED. Looking directly at an LED can hurt your eyes.
  8. The constant-current test circuit is now complete and you are ready to apply 350 mA to your traffic signal LED. But first, unclip one of the alligator leads to turn off the LED and get started on the next step.

Building the Incandescent Lamp Circuit

  1. Take one screw-base lamp and screw it into the miniature lamp holder.
  2. Apply 12 V to the lamp. Connect the AC-DC power adapter, the Adaptaplug, the coaxial power jack and the alligator clips, as described above in "Building the Constant-current Test Circuit" steps 1-3.
  3. Using the screwdriver, slightly unscrew both of the screws on the lamp holder. Connect the red alligator clip to one screw and the black alligator clip to the other screw. The incandescent lamp will light up. Undo one of the alligator clips until you are ready to test the lamp.

Building the Light-detection Circuit

  1. The light-detection circuit is simple to make and consists of a light-to-voltage converter and a 10-K-Ohm resistor. The circuit is shown below. When light hits the light-to-voltage converter it outputs a voltage that is proportional to the intensity of the light that is shining on it.

    Electricity Electronics Science Project internal circuit of a light-to-voltage converter
    Figure 6. This is an internal circuit of a light-to-voltage converter. (TAOS, Inc., 2006.)


  2. It is easy to build this circuit on a solderless breadboard. First, place two fresh batteries into the battery holder. You can read Science Buddies Use a Breadboard to Build and Test a Simple Circuit for more information on how to use one.
  3. The light-to-voltage converter is shown in Figure 7. Push the leads of the light-to-voltage converter into three separate columns of the breadboard. Now connect the light-to-voltage converter to the power row.

    Electricity Electronics Science Project Light-to-voltage converter pin configuration
    Figure 7. Light-to-voltage converter pin configuration. (TAOS, Inc., 2006.)


  4. Take another piece of wire and strip off a ¼ cm of insulation from both ends. Push one end into the top power row and the other end into the same column as the light-to-voltage converter power lead. Ground the light-to-voltage converter with a jumper wire, which should be included in the kit (or make your own by stripping off the insulation on both ends of a piece of insulated wire). Connect one end of the wire into the GND row and the other end into the same column as the GND lead of the light-to-voltage converter.
  5. Connect the 10-K-Ohm resistor by pushing one end into the same column of the breadboard as the output lead of the light-to-voltage converter. Push the other end of the resistor into the GND row. The light-detection circuit is now complete. The output signal is the voltage drop across the 10-K-Ohm resistor. Check the voltage drop across the 10K-Ohm resistor with the digital multimeter. Test the circuit by shining light on the open window of the light-to-voltage converter with a flashlight and see if you get a change in voltage across the 10-K-Ohm resistor. You should see a voltage between 1 and 3 V. When you cover the window of the light-to-voltage converter, you should see a voltage close to 0 V.

    Electricity Electronics Science Project light-detection circuit
    Figure 8. Here is an actual light-detection circuit.




    Electricity Electronics Science Project zoomed-in view of light-detection circuit
    Figure 9. This is a zoomed-in view of a light-detection circuit.


Testing and Data Collection

  1. Find an area in your house or school that is close to a wall socket for power and in a quiet area.
  2. Clamp the breadboard to the wood dowel of the test assembly. Make sure that the rubber feet of the breadboard are lined up with the dowel. This gives the entire assembly more stability. You can use masking tape to hold the battery pack to the back of the breadboard so that it doesn't hang. This setup allows you to move the breadboard up and down. Also, make sure that the window of the light-to-voltage converter is facing the plywood base. This is where you will place the LED for testing.

    Electricity Electronics Science Project test setup
    Figure 10. Here is the test setup.


  3. Put on your sunglasses. Align the first LED directly underneath the light-to-voltage converter. Mark this position because you will have to place the LEDs and incandescent lamp in this position several times. Apply 350 mA to the first LED, wait for 2 minutes, and take a voltage reading across the 10-K-Ohm resistor with the digital multimeter. You will notice that the voltage reading fluctuates. That's because the LED is heating up. In many applications, the voltage applied to the LED is pulsed on and off to minimize heating. For the purposes of this science project, heating effects are minimized by consistently taking the voltage measurement after 2 minutes. Make sure to mark each LED with 1, 2, or 3 so that you can tell them apart. The voltage across the resistor should be approximately 2.5 V. If the reading is 3 V, move the light-detection circuit farther away from the LED. If the reading is 3 V, then the light-to-voltage converter might be saturated. Saturated means that the photo detector can't detect any more light. If the voltage is close to 0 V, then move the light-detection circuit closer to the LED. Once you see that the voltage drop across the resistor is about 2.5 V, fix the breadboard in this position using the clamps. Note the voltage drop across the resistor in your lab notebook in a data table similar to the one shown below. This voltage reading is linearly proportional to the optical output power of the LED, see Equation 1.

    Equation 1:

    Optical output power of LED (watts) proportional to Voltage drop across resistor (volts)


    So you can say that the optical output power of the LED is equal to voltage drop across the resistor, multiplied by a linear factor, as shown in Equation 2.

    Equation 2:

    Optical output power of LED (watts) = Nlinear factor × Voltage drop across resistor (volts)

    Pout = N × Vres

    • Pout is the optical output power in watts (W).
    • N is a linear factor.
    • Vres is the voltage drop across the resistor in the light-detection circuit in volts (V).
  4. This science fair project assumes that optical output power is the power collected by the detector. Using this assumption, N is the same for both the LED and the incandescent light sources, and the output power can be written in units of N in the data table (for example, write 0.75 N in the table). N cannot be determined easily, as it depends on the light emission vs. angle for each source.
  5. Measure the distance the LED is from the light-to-voltage converter with a ruler. Note the distance in your lab notebook, along with the voltage across the resistor. Use a data table like the one below.
  6. Now measure the input electrical power to the LED using the digital multimeter. Power is defined in Equation 3 as:

    Equation 3:

    Input Electrical Power (watts) = Current going through the device (amperes) × Voltage across the device (volts)

    Pin = Iin Vin

    • Pin is the input electrical power in watts (W).
    • Iin is the current going through the device is amperes (A).
    • Vin is the voltage across the device in volts (V).
  7. Take the multimeter and measure the voltage across the LED. Note this value in your lab notebook. You are applying 350 mA to the LED. To get the input power to the LED, multiply these two numbers. Remember to multiply volts by amperes. So multiply the volts across the LED by 0.350 A. Record the input power to the LED in your lab notebook. Repeat steps 2-6 for LED #2 and LED #3.
  8. Now disconnect the alligator clips from the PowerPuck and connect the clips to the incandescent lamp circuit. Place the incandescent lamp in exactly the same position as the LEDs. Repeat steps 2-6 for each of the incandescent lamps. Record the data in your lab notebook.
  9. Calculate the relative wall plug efficiency for each of the LEDs. Since you are not collecting all of the light at the light-to-voltage converter (some of the light goes off to the side), the calculation is relative. However, this measurement does mimic that of the real-world traffic light situation. When your eyes look at a traffic light they are not capturing all of the light; some of it is going off to the side of the device. Efficiency is derived by using the equations above, see Equation 4.

    Equation 4:

    Relative Wall Plug Efficiency (dimensionless) =  
    Optical Output Power
    Input Electrical Power
  10. Find the relative wall plug efficiency of the incandescent lamps using the incandescent lamp data in your lab notebook.

      Distance LED is from Light-to-Voltage Converter Voltage Reading Across Resistor (volts) Output Optical Power (watts) Voltage across LED (volts) Current Going Through LED (amperes) Input Electrical Power (watts) Relative Wall Plug Efficiency
    LED #1              
    LED #2        
    LED #3        
    Average LED wall plug efficiency=


      Distance Lamp is from Light-to-Voltage Converter Voltage Reading Across Resistor (volts) Output Optical Power (watts) Voltage across LED (volts) Current Going Through LED (amperes) Input Electrical Power (watts) Relative Wall Plug Efficiency
    Incandescent Lamp #1              
    Incandescent Lamp #2              
    Incandescent Lamp #3              
    Average incandescent lamp wall plug efficiency=


  11. Plot the efficiencies versus its respective device. What is the difference in efficiency between the LEDs and the incandescent lamps? Which one is higher? To calculate which device is more efficient, compare the relative wall plug efficiencies. When you compare the relative wall plug efficiency of the LED to the relative wall plug efficiency of the bulb (divide the LED WPE by the bulb WPE), the N factor will drop out. Which technology would you use for traffic signals?

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Variations

  • Mimic an incandescent traffic signal by placing a red "filter" in front of the incandescent lamp. You can make a red filter with cardboard and red cellophane. How does the efficiency of the lamp change?
  • Perform the same experiment using different-colored LEDs. For example, you can redo the project using yellow and green LEDs. Green LEDs are made from a different semiconductor. Compare these efficiencies with lamps that have yellow and green filters.

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