Are LEDs the Future? Energy Savings with LED Lighting

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.
   
   
   
   Figure 3. Here is the wooden stand from building the test assembly.

Building the Constant-current Test Circuit

Assemble the constant-current LED test circuit as shown in Figure 4 and outlined in these steps.

1. Plug the 12 V power adapter into a wall outlet.
2. Plug the power adapter's barrel plug into the pigtail adapter.
3. Use your multimeter to determine which wire of the pigtail adapter is positive, and which is negative. Set the multimeter to measure voltage and connect one probe to each wire of the pigtail adapter. If the voltage displays as negative, reverse the probes. Your multimeter's positive probe is now connected to the positive wire, and the negative probe is connected to the negative wire. Mark the wires (for example, with pieces of tape that you write on) so you can easily distinguish them.
4. Use alligator clips to connect the pigtail adapter to the LED driver's "Vin" pins. Connect the positive pigtail wire to the "Vin +" pin (red wire) and the negative pigtail wire to the "Vin -" pin (black wire).
5. Put your sunglasses on.
6. Cover the back side of the LED board with a small piece of paper. This is important to prevent short circuits, because the back of the LED board is metal.
7. Use alligator clips to connect the "Vout" pins of the LED driver to the LED. Look closely at the LED and you will see that it has pads labeled "+" and "-". Connect the "Vout +" pin of the LED driver (yellow wire) to a "+" pad on the LED, and the "Vout -" pin of the LED driver (blue wire) to a "-" pin on the LED.
8. The LED should turn on as soon as you connect both pads. Be sure not to look directly at the LED. Looking directly at an LED can hurt your eyes.
9. Note: Because the pads are so small, you may have a hard time connecting alligator clips directly to the LED board. 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, remember to make sure the back side of the board is insulated with paper or electrical tape, so the alligator clips do not short together.

Figure 4. Schematic for connecting the constant-current LED test circuit.

10. The constant-current test circuit is now complete and you are ready to apply 350 mA to your traffic signal LED to do your experiment. But first, you will learn how to connect the incandescent bulb circuit. Disconnect both alligator clips from the pigtail connector, but leave the other parts of the circuit connected.

Building the Incandescent Lamp Circuit

Assemble the incandescent lamp test circuit as shown in Figure 5 and outlined in these steps.

1. The pigtail adapter should already be connected to the power adapter from the previous section. Remember that you should have marked which wire was positive and which one was negative, but this actually does not matter for the incandescent lamp. Unlike LEDs (which only let current flow through in one direction), incandescent lamps let current flow through in either direction
2. Use alligator clips to connect the pigtail wires to the two leads of the incandescent lamp. The lamp should light up. Disconnect the alligator clips from the pigtail adapter until you are ready to test the lamp.

Figure 5. Schematic for connecting the incandescent lamp test circuit.

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 kΩ resistor. Figure 6 shows an internal diagram of the light-to-voltage converter. 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.
   
   
   

   A circuit diagram shows a light-to-voltage converter wired in parallel with a resistor and connected to a circuit with an LED and battery.

   
   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. You can learn how to use a breadboard in the Science Buddies reference How to Use a Breadboard for Electronics and Circuits. You can follow the written directions to insert each component, and refer to the breadboard diagram in Figure 9.
3. First, place two fresh batteries into the battery holder. Make sure the "+" symbols on the battery line up with the "+" symbols inside the battery holder. Connect the battery pack's red lead to the breadboard's power bus, and the black lead to the breadboard's ground bus.
4. The light-to-voltage converter is shown in Figure 7. It has three pins: ground (GND), power (V_DD), and output (OUT).
   
   
   
   Figure 7. Light-to-voltage converter pin configuration. (TAOS, Inc., 2006.)
   
   
   
5. Push the leads of the light-to-voltage converter into three separate rows of the breadboard.
6. Use a jumper wire to connect the "V_DD" pin of the light-to-voltage converter to the breadboard's power bus.
7. Use a jumper wire to connect the "GND" pin of the light-to-voltage converter to the breadboard's ground bus.
8. Connect one lead of the 10 kΩ resistor to the same row of the breadboard as the "OUT" pin of the light-to-voltage converter. Push the other end of the resistor into the breadboard's ground bus.
9. The light-detection circuit is now complete. The output signal is the voltage drop across the 10 kΩ resistor. Check the voltage drop across the 10 kΩ 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 kΩ 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.
   
   
   
   Figure 8. A picture of the light-detection circuit.
   
   
   
   
   Figure 9. A breadboard diagram of the 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. 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.
   
   
   

   A experiment to measure light intensity is setup with an LED shining beneath a light-to-voltage converter. The light-to-voltage converter is mounted in a breadboard which is clamped on one end to a wooden dowel. The wooden dowel is glued perpendicularly to a wooden block and a red LED is placed on the top of the wooden block. Alligator clips are attached to the leads of the LED and light-to-voltage converter to test the power output of each.

   
   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 by re-connecting the constant current LED test circuit. Wait for 2 minutes, and take a voltage reading across the 10 kΩ 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:

   $$Optical\ output\ power\ of\ LED\ (watts)\ \propto \ Voltage\ drop\ across\ resistor\ (volts)$$

   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:

   $$Optical\ output\ power\ of\ LED\ (watts)\ = N_{linear factor}\ \times \ Voltage\ drop\ across\ resistor\ (volts)$$ $$P_{out} \ = \ N\ \times \ V_{res}$$
   • P_out is the optical output power in watts (W).
   • N is a linear factor.
   • V_res is the voltage drop across the resistor in the light-detection circuit in volts (V).
   
   
   
4. This 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:

   $$Input\ Electrical\ Power\ (watts)\ = \ Current\ going\ through\ the\ device\ (amperes)\ \times \ Voltage\ across\ the\ device\ (volts)\ $$ $$P_{in} \ = I_{in} \ \times \ V_{in}$$
   • P_in is the input electrical power in watts (W).
   • I_in is the current going through the device is amperes (A).
   • V_in 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 LED circuit and reconnect 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:

   $$Relative \ Wall \ Plug \ Efficiency \ (dimensionless) \ = \ \frac{Optical \ Output \ Power}{Input \ Electrical \ Power}$$
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?