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Project Summary

Difficulty  7 
Time required Short (several days)
Prerequisites Understanding of Ohm's Law and an understanding of logarithms. Note: the biggest expense is a powered, solderless breadboard, which can be used for future explorations in electronics.
Material Availability Specialty items
Cost Very High (over $150)
Safety No issues

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Abstract

If you want to get your friend's attention at a crowded sporting event with lots of people cheering, you need to shout. If you're trying to do the same thing in a quiet library, a whisper works. The detection limit for each of our senses depends on the amount of "background" stimulation that is already present. This project uses an LED control circuit to investigate detection of changes in light levels.

Objective

The goal of this project is to compare linear vs. logarithmic potentiometers for controlling signals that people sense (e.g., brightness of a light, or loudness of a sound).

Introduction

Our five senses (sight, hearing, touch, taste, and smell) give us information about the environment around us. Our ability to detect small changes in these sensations (the "just noticeable difference"), is proportional to the original intensity of the sensation. For example, if you are in a very quiet room, you can hear a whisper. A second person whispering could also be heard: the added sound would be significant in relation to the existing sound level. On the other hand, if you're at a basketball game with a lot of people cheering, you would not be able to hear someone whispering two rows down, because now the added sound is insignificant in relation to the existing sound level. In other words, as sounds get louder, there needs to be a bigger change in intensity in order to detect it.

The same holds true for other sensations. On a dark night, with your eyes adapted to low light levels, you can see a single candle a mile away. In broad daylight, the candle has to be much closer before you can distinguish it, because the existing light level in daylight is so much greater.

The mathematical description of this phenomenon is called Stevens' Power Law (Staff, 2004). In words, Steven's Power Law says that the perceived sensation, R (loudness of a sound, brightness of a light), is an exponential function of the actual level of the stimulus, S, (measured sound level or brightness). In equation form, it looks like this:

Steven's Power Law, exponential form.

S0 is the threshold, or lowest detectable level, of the sensation, and k is a proportionality constant. The value of the exponent, α, depends on the particular sensation. Taking the logarithm of both sides of the equation, we get:

Steven's Power Law, logarithmic form.

So on a double-log plot, the perceived response, R, to a stimulus, S, will be a straight line, with slope equal to α (the exponent in the original equation) and y-intercept equal to log k. In this project, you will use an LED with a simple control circuit to investigate how changes in light intensity are sensed. Before getting started, you'll need some background information on how LEDs work.

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

a red LED (top) and the schematic symbol for an LED (bottom)
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).

When current flows through the diode in the forward direction, some of the current is converted into light of a specific color (i.e., wavelength). The color of the light depends on the material from which the semiconductor is made. LEDs are available in many different colors.

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.

schematic of an LED in series with a current-limiting resistor
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 − VL). You can then use Ohm's Law to calculate the resistance, R, needed to produce a desired current, I:


R = (VS − VL)/I.

So, if the supply voltage is +7 V, what resistor would you need for a 15 mA current? R = (7 − 2)/0.015 A = 333Ω. The closest standard resistor value would be 330 Ω. For more details, and a set of online calculators, see the LED references in the Bibliography section (Hewes, 2006; Ngineering, 2003).

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

For this project, we'd like to vary the output of an LED over a continuous range. Obviously, it would be a major inconvenience if we had to keep changing the current-limiting resistor for each LED! The circuit below (Figure 4) is a different approach to solving our problem.

schematic of an LED control circuit
Figure 4. This circuit uses an operational amplifier (op-amp) to control the current through an LED (Calvert, 2002).

The large triangle marked "LF411" in the center of the schematic is an operational amplifier, or op-amp, for short. An op-amp has two inputs and one output. (It also has positive and negative supply inputs, typically +/− 15 V, which are usually not shown in schematics.) In the schematic, the first input (marked +, and called the non-inverting input) is connected to a potentiometer. The second input (marked −, and called the inverting input) is connected to two circuit elements. It connects to the cathode of an LED, and to a 330 Ω resistor that connects to ground. The output of the op-amp is connected to the anode of that same LED. So the output of the op-amp is fed back to the inverting input. Op-amps are almost always used with some type of feedback.

What does the op-amp do? For a good basic understanding of how op-amps work in circuits, you just need to understand two simple rules (Horowitz and Hill, 1989, 177):

  1. "The output attempts to do whatever is necessary to make the voltage difference between the two inputs zero.
  2. "The inputs draw no current."
These rules are simplifications—for example, the inputs do draw some current. For the LF411 op-amp used in this project, 0.2 nA. You won't be able to measure that with your DMM! So these op-amp rules, as the authors state, are "good enough for almost everything you'll ever do," (Horowitz and Hill, 1989, 177).

What do the op-amp rules mean for our LED control circuit? Let's examine the inputs to the op-amp to figure it out. The potentiometer connected to the non-inverting input is a variable resistor. When wired as shown in the schematic (Figure 4), the output from the center lug of the potentiometer provides a voltage, ranging between 0 and +5 V, depending on how far the knob is turned. Simple enough. OK, what about the non-inverting input? This is the feedback connection from the output. So, by rule 1, the op-amp output should do whatever is necessary to make the voltage at the inverting input match the voltage we set with the potentiometer.

Let's consider three examples:

  1. with the potentiometer knob all the way down (0 V),
  2. half-way up (+2.5 V), and
  3. all the way up (+5 V).

Example 1 is the easiest. With 0 V at the non-inverting input, there should be 0 V at the inverting input (rule 1), so the op-amp does nothing. No current flows through the LED.

With the potentiometer half-way up, the non-inverting input of the op-amp sees +2.5 V. How does the op-amp match that at the inverting input? It does it by passing current through the LED. How much current? Enough current so that the voltage at the inverting input equals +2.5 V. Where does the current go? The current flows to ground through the 330 Ω resistor (by rule 2, we know that the inputs draw no current, so the resistor is the only path to ground). By Ohm's Law, we have I = V/R = 2.5 V/330 Ω = 7.6 mA.

With the potentiometer all the way up, there is now +5 V at the non-inverting input. By the same analysis, we conclude that the current through the LED is now 15.2 mA. Perfect! The circuit has just the right range for a typical LED. The LF411 can supply 15 mA without a problem, so the behavior we expect from our op-amp rules is exactly the behavior that we get.

You can see from Ohm's Law that the LED current increases in direct proportion to the voltage from the potentiometer. Do some more calculations for more potentiometer settings to convince yourself that this is true. Then build the circuit and see how it actually performs.

The potentiometer in the op-amp circuit gives you a way to control the current, and thus the brightness, of the LED. It turns out that there are two types of potentiometers that you could use, linear or logarithmic (also called "linear taper" and "audio taper"). A linear potentiometer changes resistance in equal steps all around the dial. A logarithmic potentiometer changes resistance in successively larger steps as you advance the dial. Which one is a better match for controls related to human senses, like brightness or volume?

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:

Questions

Bibliography

The following references are recommended for more advanced students:

Materials and Equipment

To do this experiment you will need the following materials and equipment (we've supplied part numbers from Jameco Electronics; similar parts are available from other suppliers):

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

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 don't 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 isn't 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.

Wiring the Potentiometers

  1. Here are some notes for before you start.
    1. Do your background research, and make sure that you are knowledgeable about all of the terms, concepts, and questions, above.
    2. To avoid damage to components, the solderless breadboard should be powered off when your are building or making changes to the circuit.
    3. Do not turn on the power until the circuit is completed and you have double-checked all of your wiring paths.
  2. You'll need some knobs to twiddle, so take care of mounting and wiring your potentiometers first.
    1. Wear safety glasses for this step.
    2. A tunafish can makes a good, inexpensive support for the potentiometers.
    3. Mark the center locations for the two holes. Use a center punch or tap a nail with a hammer to make a small depression at the center location so the drill bit won't slip.
    4. Drill a 5/16" hole for the shaft of each potentiometer. Go slow with the drill: the metal is thin, and the bit will tend to grab once it goes through. You'll have less problems with burrs if you go slow.
    5. Mark and drill a 5/16" hole in the side of the tuna can for running the hook-up wires.
    6. Mark the center and drill a 1/8" hole to the right of each shaft hole to accommodate the small mounting tab on the potentiometer.
    7. Carefully remove any burrs from the holes with a file.
    8. Mount the potentiometers in the can, using the provided washer and nut.
    9. Solder wires to the potentiometers (see Figure 5, below). Use red wire for the +5 V connection. Use black wire for the ground connection. For the center tap, I used a black wire for the audio potentiometer and a green wire for the linear potentiometer.

      wiring the potentiometers
      Figure 5. Wiring the potentiometers. You can daisy-chain the +5 V wire (red) between the pots (right hand lug, when viewed from beneath, as in the photo). You can also daisy-chain the ground wire (black; left-hand lug, when viewed from beneath, as in the photo). The center lug is what goes to the non-inverting input of the op-amp (pin 3). To keep your connections straight, use a wire color that resembles the color of the knob (or label the wires).

    10. Add the labels (print-out and instructions available here) and knobs (Figure 6), and this part is done.

      completed potentiometer controls mounted in a tunafish can
      Figure 6. Completed potentiometer controls mounted in a tunafish can.

Soldering Leads to the LED and Light-to-Voltage Converter

  1. Both the LED and the light-to-voltage converter can be damaged by too much heat. In order to protect these components from heat, use the needle-nose pliers as a heat sink while soldering.
  2. Put a rubber band around the handle of the pliers to hold the jaws closed.
  3. Spread the jaws open and slide the leads of the component to be soldered into the pliers.
  4. The pliers should be right next to the base of the component, with the free ends of the leads sticking out, leaving you plenty of room to solder (see Figure 7).
  5. Solder a wire of the appropriate length onto each component lead. For the LED, use a red wire for the + lead (anode, longer), and a black wire for the − lead (cathode, shorter). For the light-to-voltage converter, use a red wire for the +5 V lead, and a black wire for the ground lead. If you have another color of wire, use it for the output lead. If not, clearly label your leads so you know which is which.
  6. Check to make sure you have a good connection, then slide a piece of shrink tubing over the lead, and shrink it into place with a blast of hot air from a blow dryer. Figure 7 shows the completed LED with soldered wire leads.

completed LED with wires soldered to leads
Figure 7. Completed LED with wires soldered to leads. The yellow bar marks where the needle-nose pliers should be clamped to protect the LED during soldering. Use heat shrink tubing after soldering is completed to fully insulate the component leads.

Building the LED Control Circuit

  1. Next, you'll build the LED control circuit on the solderless breadboard. Tips for working with the breadboard:
    1. Remember that the power supply for the breadboard should always be off when you are building the circuit!
    2. Use the jumpers to make your connections neatly (i.e., use straight lines, right angles whenever possible). A neater circuit has several advantages:
      • You have easier access to circuit points when you want to make voltage measurements with your DMM.
      • The circuit is easier to understand.
      • You are less likely to make wiring mistakes, and more likely to catch them if you do.
    3. Figure 10, below, shows a detail photo of the completed circuit on the breadboard for your reference.
  2. The LF411CN chip used in this project is an example of an integrated circuit (IC). The electronics industry has several different ways of making ICs, so you can often buy the same IC in several different physicasl formats or "packages." The LF411CN part used in this project is called a "dual inline plastic" (or "DIP") package.
  3. A DIP IC has two rows of metal "pins" coming out from each side. Figure 8 shows pin numbering scheme for the LF411 DIP package. The semi-circular depression at one end of the chip marks the "top" end of the chip. In this orientation, pin 1 is at the top left. (This is the electronics industry standard for all DIP packages.) Pin 1 is often additionally marked with a small circular depression. You count pin numbers down the pin 1 side, and then back up the other side. Thus on an 8-pin DIP, pin 8 is opposite pin 1, pin 7 is opposite pin 2, pin 6 is opposite pin 3, and pin 5 is opposite pin 4.

    pin numbering scheme for the LF411 DIP IC
    Figure 8. Pin numbering scheme for the LF411 DIP IC. The small semi-circular depression marks the top end of the chip. Pin 1 is at the top left. Often, the chip will also have a small circular depression next to the pin to mark pin 1. You count pin numbers down the pin 1 side, and then back up the other side.

  4. Figure 9 shows the schematic for an individual LED control. The numbers 2, 3, and 6 show which pin on LF411 IC chip corresponds to the connection in the schematic. Each LED gets its own control, so you need to make three copies of this circuit on your breadboard. Notice that the schematic does not show the connections for power to the LF411 (V+, pin 4, and V−, pin 7).

    schematic of a single LED control circuit
    Figure 9. Schematic of a single LED control circuit (Calvert, 2002).

  5. Insert the LF411 IC into the breadboard so that chip straddles one of the gaps in the rows of holes. Make sure that each of the 8 pins is aligned with its proper hole, then carefully push the chip straight down.
  6. The first connections to make are the power supply connections for the LF411. Use jumper wires to connect the +15 V bus to pin 7 and the −15 V bus to pin 4.
  7. Next connect the 330 Ω resistor. One end of the resistor connects to pin 2, the other end connects to the ground bus. Use a needle-nose pliers to make 90° bends in the leads at the proper distance, then trim each lead to about 7–8 mm below the bend. Insert the resistor into the breadboard.
  8. Next connect the LED. Remember that the anode (+ lead) connects to pin 6, and the cathode (− lead) connects to pin 2.

    detail view of completed LED control circuit on breadboard
    Figure 10. Detail view of completed circuit on breadboard. Notice how the LF411 ICs span the gap between the holes in the breadboard. Power connections to the chip are made to pin 7 (+15 V) and pin 4 (−15 V). The 330 Ω resistor is connected between pin 2 and the ground bus. The anode (+ lead, red) of the LED connects to pin 6, and the cathode (− lead, black) connects to pin 2. The wire from the center lug of the potentiometer connects to pin 3. Don't forget to connect the +5 V and ground leads of the potentiometer to the corresponding bus lines on the breadboard.

Building the Light Detection Circuit

  1. The circuit is very simple. The light-to-voltage converter is an integrate d package that contains a photodiode and an amplifier. The functional block diagram is show n in Figure 11.

    light-to-voltage converter functional block diagram
    Figure 11. Light-to-voltage converter functional block diagram (TAOS, Inc., 2006).

    Light (indicated by arrows) illuminates the photodiode sensor and generates a current. The op-amp 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 (Figure 12). The round window contains the light-sensitve region. The component has three pins, as shown.
    1. Pin 1 should be connected to ground.
    2. Pin 2 should be connected to +5 V .
    3. Pin 3 is the output voltage, a signal that is proportional to the amount of light falling on the sensor. It should connect to a 10 kΩ resistor, which, in turn, is connected to ground (see Figure 13).

    light-to-voltage converter
    Figure 12. Drawing of light-to-voltage converter package (TAOS, Inc., 2006).

  3. Figure 13 is a schematic diagram of the complete light detection circuit. In addition to the light-to-voltage converter, there is only one more component: a 10 kΩ resistor (RL). Connect the resistor from pin 3 to ground, as shown.

    light-to-voltage converter circuit schematic
    Figure 13. Light-to-voltage converter circuit schematic (TAOS, Inc., 2006).

  4. The output signal of the light detection circuit 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).

Making Light Measurements

  1. With the power to the breadboard off, connect the two voltmeters.
    1. Use alligator clips to connect the first voltmeter across the 330 Ω resistor. The voltage across this resistor is proportional to the current through the LED (you can use Ohm's Law to calculate the LED current).
    2. Use alligator clips to connect the second voltmeter across the 10 kΩ resistor (output of the light-to-voltage converter).
  2. Position the LED and light-to-voltage converter. Here are some tips for getting your setup "just right":
    1. Each component should be securely fastened to a support (such as a block of wood), and placed on a sturdy support.
    2. Double-check all of your connections, then turn on the power to the breadboard.
    3. Let there be light! Turn up the potentiometer, and you should see the LED light up. (If not, power down and re-check your connections.)
    4. With the LED is aimed at the detector and close enough to it, you should also see a non-zero voltage reading on the DMM.
    5. The LED has a narrow viewing angle (15°), so it is important to have the LED and the light detector at the same height, and aligned with each other. You can tell when the alignment is optimal by watching the output from the light detector on your DMM.
    6. You also need to set the distance between the LED and the detector so that the output signal is just under <5 V (max. output signal) with the potentiometer all the way up. That way you'll be using the full dynamic range of the detector.
    7. If the tension from the lead wires prevents the components from staying put, tape the lead wires down to the table. This "strain relief" will make your life easier.
    8. Figure 14 is an illustration of one possible setup for making the light measurements.

      illustration of light meaasurement setup
      Figure 14. One possible setup for making the light measurements. This photo shows both the circuit assembly and the LED and light-to-voltage converter. A better way to do it would be to use the long lead wires so that the circuit and potentiometers are supported separately. That way you won't disturb the alignment of the LED and light detector when you make adjustments to the potentiometers.

  3. Measure the LED current and light output.
    1. You'll get the best results if you make your measurements in a darkened or dimly lit room. You want the baseline reading from the light detector to be as low as possible. Use a flashlight to illuminate the voltmeters to get your readings, but make sure the light from the flashlight doesn't fall on the light detector.
    2. Use the linear potentiometer first. Get at least three sets of measurements, then power down the breadboard and connect the audio potentiometer.
    3. At each setting on the potentiometer dial, observe the LED and note the change in apparent brightness—what it looks like to your eye, compared to the previous setting. Then record the voltage across the 330 Omega; resistor (you'll calculate LED current from this) and the voltage across the 10 kΩ resistor (the light detector output).
    4. Keep track of your results with a table like the one below in your lab notebook. (The first four columns are your measurements and observations, the remaining columns are calculated.)
      pot. type:___________________
      pot.
      setting      		 Subjective brightness
      (compared to previous setting)      		 Vr
      (V)      		 Vp
      (V)      		 ILED
      (mA)      		 Vp − V0
      (V)      		 ΔV
      (V)
      0
      1
      2
      etc.

    Analyzing Your Results

    1. Use Ohm's Law to calculate the LED current from Vr, the voltage across the 330 Ω resistor (column 4 in the data table). Tip: measure the actual resistor value with your DMM before making this calculation.
    2. The light detector voltage, V0 when the LED is off (potentiometer at 0 setting) is the baseline "dark" level. Subtract this reading from all of the other readings (column 5 in the data table).
    3. The final column, ΔV, is a measure of the light detector voltage change with each 30° increment of the potentiometer knob. Subtract the previous reading from the current reading. For example, to calculate ΔV for potentiometer setting "5", subtract the reading from setting "4" from the reading for setting "5".
    4. Make graphs of your results. Here are some suggestions:
      1. Graph LED current as a function of potentiometer setting.
      2. Graph corrected detector voltage (Vp − V0) as a function of LED current.
      3. Graph ΔV as a function of LED current.
    5. Which potentiometer seems to your eye to produce a constant increment in light intensity with each setting?
    6. Compare your subjective impression with what the measurement from the light detector. What does this tell you about your perception of light intensity?

    Variations

    • Try adding a 2.2 kohm resistor between the center lug of the audio taper potentiometer and ground (Elliott, 2001). What does the shape of the light output curve look like now?
    • Investigate simple audio amplifier circuits and compare using an audio taper potentiometer vs. a linear taper potentiometer for a volume control.

    Credits

    Andrew Olson, Ph.D., Science Buddies


    Last edit date: 2009-03-15 14:28:00

    Career Focus

    If you like this project, you might enjoy exploring careers in Electricity & Electronics.

    Electrician
    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.     		  Electrical and Electronics Engineer
    Just as a potter forms clay, or a steel worker molds molten steel, electrical and electronics engineers gather and shape electricity and use it to make products that transmit power or transmit information. Electrical and electronics engineers may specialize in one of the millions of products that make or use electricity, like cell phones, electric motors, microwaves, medical instruments, airline navigation system, or handheld games.
    Electrical Engineering Technician
    Electrical engineering technicians help design, test, and manufacture electrical and electronic equipment. These people are part of the team of engineers and research scientists that keep our high-tech world going and moving forward.     		  Semiconductor Processor
    What do traffic lights, lasers, and microchips have in common? They are made from special materials called semiconductors. Semiconductors have helped revolutionize technology. If you enjoy hands-on work and are interested in participating in cutting-edge semiconductor technology, then a career as a semiconductor processor maybe of interest to you!


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