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Project Summary

Difficulty	6  –  8
Time required	Short (several days)
Prerequisites	Understanding of Ohm's Law.
Material Availability	Specialty items
Cost	Average ($50 - $100)
Safety	No issues

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Sponsor

Sponsored by generous support from AMD

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 mA or less. Above this value, the lifetime of the LED will be decreased significantly. Far above this value, the LED will fail catastrophically, like a flashbulb.

To keep the LED current at a reasonable level, LEDs are typically connected in series with a current-limiting resistor, as shown in Figure 2.

Figure 2. Schematic diagram of an LED in series with a 1kΩ resistor (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). In the circuit in Figure 2, the voltage drop across the resistor will be 9 − 2 = 7 V. 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 use a variety of different resistors in series with an LED to make 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,
• light emitting diode (LED),
• voltage (V),
• current (I),
• resistance (I),
• Ohm's law (V = IR, or I = V/R, or R = V/I).

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.

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),
• for building the light detection circuit you can use:
  • solderless breadboard (part number 517-922306), or
  • you can solder the circuit together and install them in a small enclosure (you'll need to drill a hole and position the sensor so that light can reach it),
• 6 fresh AA batteries (or freshly-charged AA batteries, if you use rechargeables),
• 2 battery holders for 3 AA batteries (part number 12BH431-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 red LEDs, (40–60 mcd@20 mA; e.g., part number 638-204IT),
• one digital multimeter (DMM):
  • If you want one-stop shopping, Mouser has moderately priced (about $40) DMMs from BK Precision (part number 615-2703B) and ExTech (part number 685-MN26T), however,
  • the TM-162 is about half the price: part number TM-162 from TechBuys.Net.

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 receives no consideration, financial or otherwise, from suppliers for these listings. (The sole exception is any Amazon.com or Barnes&Noble.com link.) 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.

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. A drawing of the actual component is shown below. The round window contains the light-sensitve 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).

3. 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).

4. 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 up to 5 DC volts (usually the 20 V range).
5. 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.

6. Alternatively, if you have experience with a soldering iron, you can make the circuit in a small enclosure. You'll have to drill a hole and position the light-to-voltage converter so that light can reach its sensor.
7. Test the circuit with your DMM. Use clip leads to connect the DMM across the 10 kΩ resistor, and set the multimeter to read DC volts (the maximum signal will be about 5 V). When you shine a flashlight directly on the sensor, your multimeter 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 multimeter 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. Use a clip lead to connect the red wire of the battery holder to one lead of the 165Ω resistor.
3. Use a clip lead to connect the other resistor lead to the longer lead (anode) of the LED.
4. Gently bend the ends of LED leads apart from one another so that the clip leads won't accidentally short the circuit.
5. 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 reading on the DMM. 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.
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–2.67kΩ).

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.

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).
2. What is the relationship between LED current and light intensity?

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

Last edit date: 2009-03-15 14:27:00

Career Focus

If you like this project, you might want to think about career opportunities in Electricity & Electronics.

Electricians are the people who bring electricity to our homes, schools, businesses, public spaces, and streets—lighting up our world, keeping the indoor temperature comfortable, and powering TVs, computers, and all sorts of machines that make life better. Electricians install and maintain the wiring and equipment that carries electricity, and they also fix electrical machines. Learn more about this career: Electrician.