Abstract

Objective

The goal of this project is to measure the light output of an LED as a function of current through the LED.

Introduction

Today's electronic devices such as computers, handheld video games, and MP3 players are all based on components made of materials called semiconductors. Semiconductors have properties that are intermediate between conductors and insulators. Diodes, for example, are a semiconductor device that allow current to flow in only one direction. In the forward direction, diodes act like a conductor. In the reverse direction, diodes act like an insulator.

An LED (light-emitting diode) is a special kind of diode that produces light (see Figure 1).

Figure 1. A red LED (top). The longer lead is the anode (+) and the shorter lead is the cathode (−). In the schematic symbol for an LED (bottom), the anode is on the left and the cathode is on the right (Hewes, 2006).

As the current through the LED increases, the brightness also increases. Typically, the recommended current for an LED is 20 milliamperes (mA) or less. Above this value, the lifetime of the LED will be decreased significantly. Far above this value, the LED will fail catastrophically. Catastrophic failure can be defined as when the LED no longer emits light.

To keep the LED current at or below the recommended operating current level, LEDs are typically connected in series with a current-limiting resistor, as shown in Figure 2. In this circuit, the positive terminal of the battery is connected to the resistor. The resistor is connected in series with the anode of the LED. The cathode of the LED is connected to the negative terminal of the battery. In this case, the battery is providing 9 V to the series combination of the resistor and the LED.

Figure 2. Schematic diagram of an LED in series with a 1kΩ resistor and a 9 volt battery. (Hewes, 2006).

The voltage drop across an LED is about 2 V (except for blue or white LEDs, where the voltage drop is about 4 V). This means that 2 V is required for the LED to turn on and conduct or create a path for current. Voltage drop is defined as a loss in voltage across components in an electrical circuit. Of the 9 V available, 2 V is required to turn on the LED. That leaves 7 V to drop across the resistor. Think of the circuit as a waterfall loop. There is 9 V available at the top of the waterfall. Seven volts fall across the resistor, and 2 V fall across the LED. The current then proceeds in a loop. Using Ohm's law, the current, I, through the resistor will be V/R = 7 V/1kΩ = 7 mA.

Figure 3 (below) shows you how to use Ohm's Law to calculate what size resistor you need to limit the current through the LED to the desired value. The voltage drop across the resistor will equal the supply voltage minus the voltage drop across the LED (or, VS − V_L). You can then use Ohm's Law to calculate the resistance, R, needed to produce a desired current, I:

So, if the supply voltage is 9 V, what resistor would you need for a 20 mA current? R = (9 − 2)/0.02 A = 350Ω. For more details, and a set of online calculators, see the LED references in the Bibliography section (Hewes, 2006; Ngineering, 2003).

Figure 3. Schematic diagram showing how to use Ohm's Law to calculate the correct value for the current-limiting resistor (Hewes, 2006).

In this project you will make two circuits: an LED circuit and a light-to-voltage converter circuit. You will use a variety of different resistors in series with an LED to make LED circuits with smaller and larger currents. You'll use a simple light-to-voltage converter circuit to measure the output of the LED. How will LED output change with current?

Terms, Concepts, and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• semiconductor
• diode
• light emitting diode (LED)
• anode
• cathode
• voltage (V)
• current (I)
• resistance (R)
• resistor
• series
• voltage drop
• Ohm's law (V = IR, or I = V/R, or R = V/I)
• circuit
• short circuit

Note: Many of these terms and concepts are discussed in the Science Buddies Electronics Primer.

Questions

• You have a 4.5 V voltage source connected in series with a 470Ω resistor and a standard red LED. Assuming that the voltage drop across the LED is 1.7 V, how much current would you expect to flow through the circuit?
• What resistance would you need in the above circuit in order to produce a 20 mA current?

Bibliography

On this page you can build virtual circuits with batteries and resistors, then test your circuit by throwing a switch to light up a bulb. If there's too much current, the virtual light bulb blows up, too little current, and the bulb won't light. When you get the current right, the bulb glows brightly.

• Unknown, 1999. "Ohm's Law." Physics Department, University of Oregon. [accessed December 13, 2006] http://zebu.uoregon.edu/nsf/circuit.html#Ohm

These webpages have useful information on LEDs:

• Hewes, J., 2006. "Light Emitting Diodes (LEDs)," The Electronics Club, Kelsey Park Sports College [accessed December 15, 2006] http://www.kpsec.freeuk.com/components/led.htm.
• Ngineering, 2003. "LED Calculators," Ngineering.com [accessed December 15, 2006] http://www.ngineering.com/LED_Calculators.htm.

The data sheet for the light-to-voltage converter has complete specifications for these devices:

• TAOS, Inc., 2006. "TSL12S, TSL13S, TSL14S Light-to-Voltage Converters," [accessed December 15, 2006] http://www.taosinc.com/images/product/document/TSL12S-E20.pdf.

This webpage shows you how to read the value of a resistor from the colored stripes:

• Engstrom, S., 2006. "Resistor Color Codes," SamEngstrom.com [accessed December 15, 2006] http://www.samengstrom.com/nxl/3660/4_band_resistor_color_code_page.en.html.

The following websites discuss basic circuit theory. The first site is the simplest of the three. The second site is Science Buddies electronics primer.

• National Aeronautics and Space Administration. (n.d.). The NASA SCIFiles: Circuits. Retrieved November 11, 2010, from scifiles.larc.nasa.gov/text/kids/Problem_Board/problems/electricity/circuits2.html
• Science Buddies. (2010). Electronics Primer. Retrieved November 11, 2010, from www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_primer-intro.shtml
• All About Circuits. (n.d.). What are "Series" and "Parallel" Circuits? Retrieved November 11, 2010, from www.allaboutcircuits.com/vol_1/chpt_5/1.html

Materials and Equipment

To do this experiment you will need the following materials and equipment (unless otherwise specified, part numbers are from Mouser Electronics):

• light-to-voltage converter (part number 856-TSL14S-LF),
• solderless breadboard (part number 517-922306
• 6 fresh AA batteries (or freshly-charged AA batteries, if you use rechargeables),
• 2 battery holders for 3 AA batteries (part number 12BH335-GR),
• alligator clip leads (part number 13AC010),
• 1/4-watt resistors with the following values:
  • 165 Ω (part number 271-165-RC),
  • 330 Ω (part number 271-330-RC),
  • 665 Ω (part number 271-665-RC),
  • 1330 Ω (part number 271-1.33K-RC),
  • 2670 Ω (part number 271-2.67K-RC),
  • 10 kΩ (part number 271-10K-RC);
• 5 orange LEDs, (40 mcd@20 mA; e.g., part number 638-204ET),
• Multimeter, such as the Equus 3320 Auto-Ranging Digital Multimeter; available online at Amazon.com
• electrical wire, 2 feet, 30 AWG. You can purchase wire at your local electronics store
• wire cutter and stripper (part number 586-70058)

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Experimental Procedure

Building the Light Detection Circuit

1. The circuit is very simple. The light-to-voltage converter is an integrated package that contains a photodiode and an amplifier. The functional block diagram is shown below.

   Light-to-voltage converter functional block diagram (TAOS, Inc., 2006).

   Light (indicated by arrows) illuminates the photodiode sensor and generates a current. The operational amplifier (or "op amp," symbolized by the large triangle in the diagram) produces an output voltage that is proportional to the intensity of the light illuminating the photodiode.
2. Place 3 batteries into the first battery pack.
3. A drawing of the actual component is shown below. The round window contains the light-sensitive region. The component has three pins, as shown.
   1. Pin 1 should be connected to ground (black wire from the battery holder).
   2. Pin 2 should be connected to the positive supply voltage (red wire from the battery holder). The supply voltage should be between 2.5 and 5.5 V DC, so you can use either 2 or 3 AA batteries.
   3. Pin 3 is the output voltage, a signal that is proportional to the amount of light falling on the sensor.

   Drawing of light-to-voltage converter package (TAOS, Inc., 2006).

4. Here is a schematic diagram of the complete circuit. In addition to the light-to-voltage converter, there is only one more component: a 10 kΩ resistor (R_L). Connect the resistor from pin 3 to ground, as shown.

   Light-to-voltage converter circuit schematic (TAOS, Inc., 2006).

5. The output signal is the voltage drop across the 10 kΩ resistor. To read the output, use one alligator clip lead to connect the positive lead of the resistor to the red probe of your DMM, and another clip lead to connect the grounded lead of the resistor to the black probe of your DMM. Set your DMM to read DC volts.
6. You can easily build the circuit on a solderless breadboard.

   The photograph below shows a small breadboard. The breadboard has a series of holes, each containing an electrical contact. Holes in the same column (examples highlighted in yellow and green) are electrically connected. When you insert wires into the holes in the same column, the wires are electrically connected. The gap (highlighted in orange) marks a boundary between the electrical connections. A wire inserted in one of the green holes would not be connected to a wire inserted in one of the yellow holes. Integrated circuits, such as the oscillator used in this project, should be inserted so that they span the gap in the breadboard. That way, the top row of pins is connected to one set of holes, and the bottom row of pins is connected to another set of holes. If the integrated circuit was not spanning a gap in the breadboard, the pins from the two rows would be connected together (shorted), and the integrated circuit wouldn't work. Finally, the two single rows of holes at the top and bottom (highlighted in red and blue) are power buses. All of the red holes are electrically connected and all of the blue holes are electrically connected. These come in handy for more complicated circuits with multiple components that need to be connected to the power supply.

   An example of a solderless breadboard. The highlighting shows how the sets of holes are electrically connected. The red and blue rows are power buses. The yellow and green columns are for making connections between components. Integrated circuits are inserted to span the gap (orange) so that the two rows of pins are not connected to each other.

7. Build the circuit on the breadboard. If you have never used a breadboard before and need some more information, read the Science Buddies Electronics Primer: Use a Breadboard to Build and Test a Simple Circuit to further familiarize yourself with how a breadboard works.
   1. Take two 2-inch lengths of wire and strip the plastic off from both ends of both lengths. Also make three 1-inch lengths of wire and strip off the plastic from the ends.
   2. Use the 2-inch wire lengths to connect each terminal to a power bus. Connect the red wire from the battery pack to the red terminal and the black wire to the black terminal. Red indicates the positive or 'hot' end of the battery, and black is the 'cool' end of the pack or ground. Use the two to connect each terminal to a power bus.
   3. Now insert the light to voltage converter into the breadboard. Insert each pin into a separate column on the breadboard. Follow the directions in step 4 to connect the converter properly. Connect the positive power bus to a column with a 1-inch wire. Insert the power pin, middle pin, of the converter into the same column.
   4. Connect the ground wire from the battery pack into a different column. Insert the ground pin of the converter into the same column.
   5. Now connect one end of the resistor to the output pin of the converter. In other words insert both pins into the same column. Then connect the other end of the resistor to the ground column. You may need a few 1-inch wires to accomplish this.
8. Test the circuit with your digital multimeter (DMM). Use clip leads to connect the DMM across the 10 kΩ resistor, and set the DMM to read DC volts (the maximum signal will be about 5 V). When you shine a flashlight directly on the sensor, your DMM should read between 1 and 5 V (depending on the brightness of the flashlight, and how close it is to the sensor). When you cover the sensor, the DMM should read close to 0 V.

Building the LED Circuit

1. The LED circuit is very simple. As discussed in the Introduction, you should always use a current-limiting resistor in series with the LED.
2. Place 3 batteries into the second battery holder.
3. Use a clip lead to connect the red wire of the battery holder to one lead of the 165Ω resistor.
4. Use a clip lead to connect the other resistor lead to the longer lead (anode) of the LED.
5. Gently bend the ends of LED leads apart from one another so that the clip leads won't accidentally short the circuit.
6. Use a clip lead to connect the shorter lead (cathode) of the LED to the black wire of the battery holder. That's it!

Measuring LED Light Output

1. Taking care not to disconnect the clip leads, position the LED so that its top is pointing directly at the sensor window of the light-to-voltage converter.
2. Check the voltage reading on the DMM that is connected to the 10 kΩ resistor in the light detection circuit. If the LED is too close, it will drive the light detection circuit to its maximum response (about 4.5 V, with 3 AA batteries). We say that the response is saturated, because the detector cannot increase its output if it detects more light. You want to avoid this condition, because if the detector is in saturation, you will not get an accurate reading of the intensity of the LED. Move the LED away from the detector until the voltage reading on the DMM starts to drop.
3. Measure the distance between the LED and the detector, or, better yet, fix the LED in place. You want the LED at the same height as the detector window, with the top of the LED facing directly at the window. The distance between the LED and the detector should be exactly the same for all of your measurements.
4. Record the voltage reading on the DMM in your lab notebook.
5. Change the resistor in the LED circuit. Swap out the 165Ω resistor and replace it with the 330Ω resistor.
6. With the LED at exactly the same distance from the sensor, again measure and record the voltage reading on the DMM.
7. Repeat for each of the resistors (165Ω–10kΩ).
8. Remove the first LED from the LED circuit and replace with a fresh LED. Repeat steps 1–7 taking care to replace the new LED circuit at exactly the same position as the old LED circuit. Record all of your readings in your lab notebook.

Measuring LED Current

1. You also need to measure the current in the LED circuit with each of the different resistors (165Ω–2.67kΩ). If you have two DMMs, you can use one to measure the voltage of the light detector circuit, and the other to measure the current in the LED circuit. If you have a single DMM, then you have to make the current measurements separately.
2. To measure current, connect the DMM in series with resistor and LED.
   1. Use a clip lead to connect the red wire of the battery holder to one lead of the 165Ω resistor.
   2. Use a clip lead to connect the other resistor lead to the longer lead (anode) of the LED.
   3. Gently bend the ends of LED leads apart from one another so that the clip leads won't accidentally short the circuit.
   4. Use a clip lead to connect the shorter lead (cathode) of the LED to the red probe of the DMM. Note that some DMMs have separate sockets for the red probe for reading current and voltage. Make sure that the red probe is in the correct socket for reading current.
   5. Use a clip lead to connect the black probe of the DMM to the black wire of the battery holder.
   6. Set the DMM to read DC current in the 200 mA range. (For resistors > 165Ω, you will probably want to switch to the 20 mA range.)
3. Record the current reading for each circuit in your lab notebook.
4. Repeat steps 1–3 with the second LED that you used in the previous section.

Analyzing Your Results

1. Make a graph of the LED intensity, expressed as voltage output from the light detection circuit (y-axis), vs. the LED current, in milliamps (x-axis) for both of the LEDs you tested.
2. What is the relationship between LED current and light intensity? How does light intensity vary between the LEDs?

Variations

• An LED can easily be powered by 2 AA batteries instead of 3. With two batteries, the supply voltage will be 3.0 V instead of 4.5 V. If you were to use a 3 V supply for the LED circuit, can you figure out the value of the resistor you would need in order to limit the LED current to 20 mA? Which additional resistors would you need in order to replicate this experiment using a 3 V supply for the LED circuit? Try it out!
• What happens if you increase the LED current beyond 20 mA? Calculate the resistor value you would need to limit the LED current to 40 mA. Design an experiment to find out if the LED intensity at 40 mA is twice the intensity at 20 mA.
• For an experiment that investigates LED current in circuits powered by solar cells, see the Science Buddies project: How Does Solar Cell Output Vary with Incident Light Intensity?

• For more science project ideas in this area of science, see Electricity & Electronics Project Ideas.

Credits

Andrew Olson, Ph.D., Science Buddies

Michelle Maranowski, PhD, Science Buddies

Last edit date: 2011-11-07 08:45:00