How Many Letters?

Objective
In this experiment you will test how the number of letters (or characters) in a file change the size of the file.
Introduction
How many letters can you remember? You can actually remember many more letters than you think you can. The trick to your memory is the way letters are put together in meaningful ways: words, sentences, paragraphs and stories. This is called using associations. If you have memorized the alphabet, then you have memorized a pattern of 26 letters. If you have memorized the song "Twinkle, twinkle, little star," (and can spell all of the words) then you have memorized a complex pattern of 129 letters! That's a lot of information!


Twinkle, twinkle, little star,
24
How I wonder what you are.
20
Up above the world so high,
21
Like a diamond in the sky.
20
Twinkle, twinkle, little star,
24
How I wonder what you are!
20

How does a computer remember information? Since computers can't think like you and I, they can't remember things by forming associations. Instead they have to encode the information by using a pattern. One example is binary code, which is a pattern of zeros and ones that can be used to encode information and store it in your hard drive as a file.
Each piece of information that is stored in a file takes up a certain amount of space in the computer's memory. Since a computer has a limited amount of memory, the size of each file needs to be measured so that the computer can keep track of how much memory has been used and how much memory is free. The amount of space that a file uses is called the file size, and is usually measured in kilobytes (KB) or megabytes (MB).
In this experiment you will test how much memory is needed to store a simple piece of information, the letter A. Actually, one-thousand letter A's! But don't worry, I have a trick to keep you from tiring out your typing fingers.

Terms, Concepts and Questions to Start Background Research
To do this type of experiment you should know what the following terms mean. Have an adult help you search the Internet, or take you to your local library to find out more!

• letter (character)
• text
• file
• memory
• kilobyte (KB)
• megabyte (MB)

Questions
How much information is a letter?
How does the file size change as more letters (or characters) are added to a file?
How is the size of a file measured?

Bibliography
Each letter is a piece of information that is encoded and stored as a file in your computer. Find out about how information is stored and measured: T1 Shopper, 2006. "Byte Converter - File Size Calculator." [accessed: 3/10/06] http://www.t1shopper.com/tools/calculate/
In computing terminology a letter is one example of a character. Find out more about characters and how they are encoded: Wikipedia Contributors, 2006. "Character (computer)," Wikipedia, the Free Encyclopedia. [accessed: 3/10/06] http://en.wikipedia.org/wiki/Character_%28computing%29

Materials and Equipment
computer
word processing software (Text Edit, Microsoft Word, Word Perfect, Claris, etc..)
graph paper
colored pencils or markers

Experimental Procedure

1. Open your word processing program. I can be any kind of program for writing and editing text files or documents. Some examples of editing software packages are Text Edit, Microsoft Word, Word Perfect or Claris Works.
2. Open a new document. Usually this is done by clicking on "File" and then "New..." from the file menu at the top of your screen.
3. Below are 1000 letter A's:
   aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
   aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
   aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
   aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
   aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
   aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
   aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
   aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
   aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
   aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
   aaaaaaaaaaaaaaaaaaaa
4. Copy the letter A's by clicking-and-dragging to highlight all of the letters, then clicking "Edit" and "Copy" from the file menu at the top of your screen.
5. Paste the letter A's into your new file by clicking inside the new document until you see a blinking cursor, then click on "Edit" and "Paste" from the file menu at the top of your screen.
6. This will be your first file. It contains 1000 letter A's. Save the file to your computer by clicking "File" and "Save" from the file menu at the top of your screen.
7. Type a name for your file (like A1000.txt) and click the "Save" button.
8. Now you want to make a new file with 1000 more letter A's in it. Do this by clicking "File" and "Save As" from the file menu at the top of your screen.
9. This will be your second file. Type a name for your new file (like A2000.txt) and click the "Save" button.
10. Next, you want to add more letter A's to your new file. Right now it has 1000 letter A's and you want to add 1000 more. Copy 1000 more letter A's by clicking-and-dragging to highlight all of the letters, then clicking "Edit" and "Copy" from the file menu at the top of your screen.
11. Paste the new letter A's at the end of your old letter A's by clicking at the end of the last letter A (until you see a blinking cursor after the last letter A), then click on "Edit" and "Paste" from the file menu at the top of your screen.
12. Your new file contains 2000 letter A's. Save the file to your computer by clicking "File" and "Save" from the file menu at the top of your screen.
13. Repeat steps 8-12 to make files with 3000, 4000, 5000, etc. letter A's in it. Remember to save each new file with a new name that reflects the number of letters in the file (like A3000.txt, A4000.txt, A5000.txt, etc.).
14. After you have made and saved each file you can close your word processing application.
    Next, you will want to view the files you made by looking in the documents folder of your computer. Use the finder if you are using a Mac or the Start menu if you use a Windows PC.
15. Make a bar graph of your data. On the left side of the graph make a scale of file size from zero to just above your largest piece of data, in increments of 5,000 KB. For example, if my largest file size is 67,000 KB then I would make my scale go to a maximum of 70,000 KB. Draw a bar for each test file up to the number that matches the size of the file. Remember to label each bar with the number of letters in the file, label each axis, and to give your graph a descriptive title.
16. What happened to the size of the file as more letters were added? Did you see the effect right away? How many letters did it take before you saw a noticeable change in file size?

Variations

1. Try using different letters than the letter A, or combinations of letters. Try copying the alphabet over and over. Do you get a similar result?
2. Try comparing different font styles or letter sizes. Does changing the font size increase the file size? Do some font styles take more memory than other styles?
3. If you have more than one text editor or word processing program, you can compare the file sizes for the same text created by three different applications. Save the same text as a file in each application and compare the file sizes. Are the files the same size? Which application creates files that use the most memory? Which application creates files that use the least? How might this relate to the number of features that each application offers?
4. For a more advanced project, you can make a line graph of your results from each application. Put the number of letters on the X-axis and the file size on the Y-axis. Is the relationship linear? Does your line pass through the origin? What information might the Y-intercept tell you about the baseline for each application? Is the Y-intercept the same for each application? Is the slope the same for each application?

Engineering Projects

Find a science project idea.
We have developed many projects to help you find a science project idea that can hold your interest. If you have not selected an area to work in, please start searching an area of science that's best for you. You'll be spending a lot of time on your project, so you don't want it to be boring.
Browse project ideas and information by area of science.
Listed below are a few different areas of science and technology. For each of these areas Science Buddies has collected sample project ideas. All areas also contain special information, useful for any project in that area. Please read the introductory information before starting on a project because it will strengthen your fundamentals in that feild and also help further in your feild.

Applied Mechanics

Computer Science

Electrical & Electronics

Pure Mathematics

Environmental Engineering Projects

Sports Science

Some of the great students might feel these quite simple projects.
There are great projects going on in world but one thing u need to be knowing is u can only be a part of the project or just view them.
We ned to get started from smaller things to get bigger.
One of the great projects example u can see it THE ROBOCUP.
Their aim is to make robots by 2050 which wii defeat the greatest soccer team.

For some more see:

Kool Science Projects Worth Seeing

Go on !! Record on a Wire yourself !

Objective
The goal of this experiment is to learn about magnetic recording heads by building and testing a wire recorder. You'll investigate the relationship between recording current and playback voltage. Other variables to investigate are the number of turns used in the coil for the recording head, and the speed of the moving wire.
Introduction
Magnetic recording has proven to be a quick, safe, and robust method for storing and retrieving information. From the first voice recordings on Poulsen's wire recorder (Figure 1), to the tape recording machines used by radio stations in the 1940's and 1950's that freed the stations from having to produce all of their programs live, to the modern hard disk drive that can store billions of bits of digital information in an area smaller than a quarter, we can see the application of the fundamental principles of magnetism.

Figure 1. A diagram of Poulsen's design for a wire recorder.

Magnetic materials may be roughly categorized as "hard" or "soft." Magnetically hard materials are able to retain a magnetic moment even after an applied magnetic field is removed. Magnetically soft materials have negligible magnetic moment in the absence of an applied field. Both magnetically hard and magnetically soft materials have a role to play in any magnetic recording system.
At the heart of the recording system is the storage medium, where the signal is stored for later retrieval. This material is necessarily magnetically hard. The choice of material is very important. If the coercivity (Hc) is too high, then it will be difficult to record our signal. If the product of the saturation magnetization (Ms) and the volume (V) is too small, then the replay signal will be small. Additionally, one must worry about the mechanical properties of the storage medium. Corrosive resistance, magnetic grain size, and surface roughness are some examples of important material parameters to measure. Materials must be able to retain their magnetic moments for a reasonable length of time over a suitable range of temperatures and in the presence of stray magnetic fields. The materials chosen for modern tape drives and for disk drives are very different because they have differing requirements for performance and environment.
The magnetic transducer (the recording head) is responsible for writing to the magnetic medium. Reading and writing are two very different processes. However, they are sometimes handled by a single transducer, serving both functions (as will be the case for your wire recorder). Fifteen years ago, a single head both read and wrote in a disk drive. Today there are separate heads for each role. Handling each task with a separate transducer allows engineers to optimize each head for its specific purpose.
The write head needs to convert an electric current into a magnetic field. Ideally, the field generated should be as strong as possible and its strength should fall off as quickly as possible as one moves away from the head. A good basic design for a write head is a coil of wire wrapped around a soft magnetic material. This type of head is known as an inductive head because the coil of wire works as an inductor and generates magnetic flux, which is conducted and concentrated by the soft magnetic material to the pole tip(s). Modern write heads still use this principle. The most classic of head designs is the ring head, wherein the soft magnetic material is in the shape of a toroid (i.e., ring) with a small gap cut into it. The soft material has high permeability and conducts magnetic flux easily, whereas the gap has a low permeability, which causes the magnetic flux to fringe away from the gap area. This fringing field at the gap is used to write to the recording medium.
The read head converts magnetic flux into a voltage potential. It must be sensitive enough to provide a useable signal, and it must respond quickly to changes in magnetic flux. It is possible to use the same head used for writing to read the magnetic fields from the medium. In the reverse of the writing process, magnetic flux is coupled into the soft magnetic core and induces a voltage potential across the coil. In the read configuration, the voltage (v) across the coil is proportional to the change in flux (dφ/dt) experienced by the coil. This is expressed as Faraday's Law:

v = −N (dφ/dt)
N is the number of turns in the coil. The negative sign indicates that the voltage is opposite in polarity to the change in magnetic flux. Here, it is important to note that anything that increases the rate of change of the flux will generate a larger output voltage. Often the easiest method for increasing the output voltage is to move the recording medium past the head faster. Increasing the number of turns is also a convenient method for increasing output amplitude.
Currently, hard disk drives use a type of read head that has proven to have much greater sensitivity than the inductive read head. This head operates on the principle known as magnetoresistance. Here, the sensor is a piece of material whose resistance changes in proportion to the magnetic field it experiences. Magnetoresistive heads are not sensitive to the rate of change of the magnetic flux because they sense flux directly.

The Wire Recorder
There are three components to a wire recorder:

1. the wire (recording medium),
2. the read/write head (and electronics), and
3. the mechanics for moving the wire past the head (or the head past the wire). As you will see when you do this project, each poses its unique challenges!

Pictured below is a Telegraphone, the first commercially-produced wire recorder. Your wire recorder will be a greatly simplified version, but will function essentially like the Telegraphone.

Figure 2. A Telegraphone, the first commercially-produced wire recorder.

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:

1. transducer,
2. Ohm's Law,
3. magnetic coil,
4. Faraday's Law.

Questions
What is a magnetic moment?
For an inductive read/write head, how does the number of turns in the coil affect the playback voltage?
Why does a magnetic recording medium have to be magnetically "hard?"

Bibliography
This article, reprinted from The PC Guide, shows you how a modern magnetic recording device—the hard disk drive—works:Kozierok, C.M., 2005. "Hard Disk Drives," StorageReview.com [accessed March 24, 2006] http://www.storagereview.com/guide2000/ref/hdd/index.html.
The HyperPhysics site has lots of interesting material related to magnetism. Here are some relevant links:
Nave, C.R., 2006. "Magnetic Dipole Moment," HyperPhysics [accessed March 20, 2006] http://hyperphysics.phy-astr.gsu.edu/Hbase/magnetic/magmom.html.
Nave, C.R., 2006. "Hysteresis in Magnetic Materials," HyperPhysics [accessed March 24, 2006] http://hyperphysics.phy-astr.gsu.edu/Hbase/solids/hyst.html.
Nave, C.R., 2006. "Faraday's Law," Hyper Physics [accessed March 24, 2006] http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/farlaw.html.
AACG, date unknown. "Classification of Magnetic Materials," Applied Alloy Chemistry Group, University of Birmingham [accessed March 24, 2006] http://www.aacg.bham.ac.uk/magnetic_materials/type.htm.

Materials and Equipment
To do this experiment you will need the following materials and equipment:

1. microphone (e.g., Radio Shack Universal Cassette Recorder Microphone, part #33-3019);
2. simple audio amplifier (e.g., Radio Shack Mini Audio Amplifier, part #277-1008);
3. mini-phone plug (1/8 in.) for read/write head to amplifier input/output connection (e.g.,
4. Radio Shack part #42-2434);
5. approx. 6 foot length of hookup wire;
6. 2 light duty cart wheels with solid rubber tires, 14 in. diameter (e.g., Grainger.com part#2G344);
7. recording wire: solid steel wire or, even better, old magnetic recorder wire (e.g., search eBay category "Consumer Electronics/Vintage Electronics/Other Vintage" for "wire recorder", you should be able to find some for under $10);
8. at least 5 ferrite bead cores, more if you want to experiment with the number of turns on the coil;
9. magnet wire, 34 AWG, Belden 8057 or similar (e.g., Newark.com part #36F1320);
10. Teflon (or similar smooth, hard plastic) cylinder for wire guide;
11. assorted nuts, bolts, washers, lumber, and clamps for attaching wheels (see photographs in Experimental Procedure section);
12. digital multimeter;
13. heat shrink tubing;
14. tape;
15. wire cutters;
16. super glue (cyanoacrylate);
17. emery board or sandpaper;
18. soldering iron (or wire clips); and
19. small permanent magnets.

Experimental Procedure
Wire Recording Media
As was mentioned in the Introduction, a key property of the wire recording media is that it be magnetically "hard." Piano wire can be used, but will produce a low-fidelity recording. (For voice recording, everything sounds like "waw waw waaaw wa", which is obviously no good unless you're trying to sound like an adult in a Charlie Brown cartoon.)
The best solution is to look for a spool of wire specifically made for the old wire recording machines of the 1940's and 1950's. You can probably find this at vintage electronics flea market, or from an online auction site, such as eBay. We recommend trying to find some of this type of wire so that you have at least one wire type that you know should work.
See the Variations section, below, for an idea on exploring various recording media.

Fabricating the Read/Write Head
Safety Note: Remember to wear eye protection while cutting the ferrite bead cores!
1)As described in the Introduction, the recording head is made by wrapping a coil of magnet wire around a magnetically soft core, with a gap cut into it (see Figure 3, below).

2)The ferrite beads come as solid rings, so you will first have to cut the gap for the recording head. Ferrite is a ceramic material, so cutting the gap is tricky. We can suggest two methods: 1) using a diamond cutting wheel on a motorized tool (e.g., Dremel), or 2) cutting the ferrite with wire cutters and using super glue to put the pieces back together. Each method is described below. Be sure to use eye protection no matter which method you use!

If you have a Dremel tool and are willing to spend about $30 for a diamond cutting wheel, this method is probably the best. The ferrite should be securely clamped while cutting (use padding so that the ferrite is not crushed).
The second method uses tape, sharp wire cutters and super glue.

• Wrap the ferrite core in a little bit of tape just to hold the pieces together.
• Using the wire cutters, "cut" through the ferrite bead. Since the ferrite material is brittle, you will end up shattering the bead into three or four large pieces.
• The pieces are held in place by the tape, so it is relatively simple to reassemble them with super glue. Glue the pieces together in such a manner as to leave a small gap.
• Using an emery board, polish the edges of the gap.
• Note that this is a trial-and-error process, and you may have to break four or five cores before you get a nice clean break for the gap.

3)Use the magnet wire to wrap a coil around the ferrite core, on the side opposite the gap (see Figure 3, above). A coil of 200–300 turns should work well, but this could be a variable to explore. Remember that both ends of the coil wire need to be connected for the coil to work, so leave a length of wire sticking out when you start winding.

4)The coil will be connected to the amplifier output when recording, and to the amplifier input for playback. Here's how to make a cable to connect the coil to the amplifier.

• Carefully strip off about 1/4 inch of insulation from each end of the coil wire.
• Cut your hookup wire into two 3-foot lengths and strip off insulation at each end.
• Solder one piece of hookup wire to one end of the coil and the other piece of hookup wire to the other end of the coil.
• Solder the other ends of the hookup wire to the connectors of the mini phone plug.

Mechanics
Figure 4, below, shows a simple transport sytem for moving the wire past the read/write head.Figure 4. Simple transport system for moving the wire past the read/write head.

1. Two wheels are set up horizontally (as shown in Figure 4, above) to provide a method for moving the recording wire past the read/write head.
2. The wheels are 14 inches in diameter because heavier gauge, magnetically hard wires tend to break if bent at too sharp an angle. (For thinner diameter wire, such as wire for antique wire recorders, smaller diameter wheels could be substituted.)
3. The wheels have solid rubber tires, which were grooved by hand to a depth of several millimeters. This groove carries the wire.
4. The wheels are secured to separate pieces of 2 x 4" lumber to allow the wire to be tensioned between them. The lumber pieces are clamped to the table.
5. The wheels spin on threaded bolts which act as axles and allow the wheels to be secured with washers and nuts.
6. A piece of Teflon cylinder is used as a further tensioner and guide for the wire. The Teflon is grooved at a similar height to the tires and the head is placed next to the tensioner. The groove in the Teflon acts as a fixed height surface for the wire to run on despite the (sometimes pronounced) wobble of the inexpensive wheels.

Figure 5. Mechanics: detail of Teflon tensioner device. Also shows splice in heavy gauge wire.

7. The ends of the wire need to be closed to form a loop.
(a)For the wire recorder wire, due to its extremely thin diameter, we simply knot the wire ends together. Use a square knot—as any Boy Scout can tell you, hold the wire ends in each hand and then "left over right, right over left" to tie a square knot. Pull the wire to tighten the knot.
(b)If you are using heavier gauge wire which you cannot tie in a knot, the wire must be spliced. You can accomplish this by overlapping the wire ends, applying super glue, bonding the two ends by applying pressure to the joint, and sealing the joint with suitably sized heat shrink tubing. This results in a joint that has twice the width of the wire and rubs when it passes by the head and Teflon tensioner (see Figure 5, above). Careful with super glue which bonds skin easily!
8. The wire is looped around the wheels (in the grooves made in the tires), and the wheels are spread apart on the table to tension the wire.
9. The wheel assemblies are clamped to the table and the Teflon tensioner is placed against the wire, midway between the wheels.
10. The read/write head is placed against the wire, as shown below (see Figure 6, below).

Figure 6. Mechanics: detail of the recording head and Teflon tensioner device. The recording head is in contact with the wire. You can also see the cable which connects the read/write head to the amplifier input (for playback) or output (for recording).

Recording and Playback
To record, follow these steps:

1. 
   Erase any previous recording on the wire with a permanent magnet (hold the magnet in contact with the wire, and rotate the wheels to slide the wire past the magnet).
2. Connect the microphone to the input of the Radio Shack speaker/amplifier.
3. Connect the output of the amplifier to the coil on the read/write head.
4. Spin the wheels at a constant speed and speak into the microphone. You will have to experiment with the volume control of the amplifier to get the correct input signal strength (see the next section). You can also experiment with the speed of rotation of the wheels.

To play back your recording, follow these steps:

1. Connect the head to the amplifier input.
2. Rotate the wheels to move the wire past the head.

Experiments
Investigate the relationship between input current and replay voltage.

1. Use the digital multimeter to measure the output of the amplifier for a constant-intensity input (e.g., you could hum into the microphone, or you could connect a portable music player to the amplifier and use a suitable portion of a song). Use AC mode, and take an "average" reading.
2. Use the digital multimeter to measure the resistance of the coil on the read/write head.
3. Use Ohm's law to calculate the recording current (voltage/resistance).
4. Record the signal on the wire.
5. Use the digital multimeter to measure the voltage of the coil on the read/write head during playback.
6. Write down each measurement in your lab notebook.
7. Repeat steps 1–6 for at least four different input currents. The easiest way to change the current is by adjusting the volume knob on the amplifier. Keep the input signal to the amplifier constant for all tests.
8. Graph the relationship between coil input current during recording and coil output voltage during playback.
9. What happens to output voltage when the playback speed is changed? (Refer to the equation for Faraday's Law in the Introduction.) Do you notice any other changes in the output when the playback speed changes? Can you explain them?
   

Variations

1. Try changing the speed at which you record and play back the signal. Which speed sounds best?
2. Try exploring different recording media, e.g., different wire types, or magnetic tape from a recording cassette. What materials are magnetically "hard" enough to hold a recording?
3. Try changing the position of the read/write head relative to the wire. Where do you get the best signal? Where is the magnetic force strongest?
4. Experiment with the construction of the read/write head. How does performance change with number of turns for wrapping the coil? How does performance change with gap width?
5. Can you improve the mechanics of your system? For constant recording speed with a simple system, is it better to have the recording media moving with respect to the head or the recording head moving with respect to the recording media?
6. Advanced. If you have a function generator and an oscilloscope, you can test the frequency response of your system by recording and playing back sine waves. Over what frequency range can you record with reasonable fidelity? What happens to the signal as frequency increases beyond this range? Why do you think this happens? What happens when playback speed is different than recording speed? Using the oscilloscope to display the output, can you record and play back a digital signal? How many bytes of data can you record on your wire?

Make Your Own Low-Power AM Radio Transmitter

Objective
The goal of this project is to build a simple AM radio transmitter and to test its broadcast range with a radio receiver.
Introduction
Electromagnetic (EM) radiation is pretty much all around us. For example, light is electromagnetic radiation and so are x-rays. When you listen to an AM or FM radio station, the sound that you hear is transmitted to your radio by the station using EM radiation as a carrier—radio waves. Electromagnetic radiation is a propagating wave in space with electric and magnetic components. In a vacuum, electromagnetic waves travel at the speed of light.
Electromagnetic waves such as light, x-rays, and radio waves are classified by their frequency or wavelength. For example, EM radiation at frequencies between about 430 THz and 750 THz can be detected by the human eye and are perceived as light. EM radiation at frequencies ranging from 3 Hz to 300 GHz are classified as radio waves. Radio waves are divided into many sub-classifications based on frequency. AM radio signals are carried by medium frequency (MF) radio waves (530 to 1710 kHz in North America, 530 to 1610 elsewhere), and FM radio signals are carried by very high frequency (VHF) radio waves (88 to 108 MHz).
So how does a radio wave carry sounds such as voice or music to your radio receiver? The radio station broadcasts a carrier wave at the station's assigned frequency. The carrier wave is modulated (varied) in direct proportion to the signal (e.g., voice or music) that is to be transmitted. The modulation can change either the amplitude or the frequency of the carrier wave. The "AM" in AM radio stands for "amplitude modulation," and the "FM" in FM radio stands for "frequency modulation." A radio receiver removes the carrier wave and restores the original signal (the voice or music). Figure 1, below shows graphically how amplitude modulation works.

Figure 1. Illustration of amplitude modulation of a carrier wave by a signal. The top diagram shows a carrier wave at a set frequency and amplitude (green) and a signal to be broadcast (red). The signal is used to modulate the amplitude of the carrier wave. The bottom diagram shows the resulting output signal (blue). Note how the peaks of the output trace (its envelope) follow the form of the input signal. (Wikipedia contributors, 2006a)

In this project, you will make a simple low-power broadcast circuit, using a crystal oscillator integrated circuit and an audio transformer. You can connect the circuit to the headphone jack of a portable music player (e.g. mp3, CD or cassette tape player). You'll see that you can receive the signal through the air with an AM radio receiver. Although the circuits used in radio stations for AM broadcasting are far more complicated, this nevertheless gives a basic idea of the concept behind a broadcast transmitter. Plus it is a lot of fun when you actually have it working!

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:

• electromagnetic radiation and waves,
• electromagnetic spectrum,
• wave model,
• speed of light,
• wavelength,
• frequency,
• amplitude,
• crystal oscillator,
• transformer,
• amplitude modulation,
• heterodyne.

Bibliography
This site has cool way of explaining electromagnetic phenomena.Electromagnetic radiation and waves:Goldman, M.V., et al., date unknown. "Electromagnetic Waves," Physics-2000, University of Colorado, Boulder [accessed April 10, 2006] http://www.colorado.edu/physics/2000/waves_particles/index.html.
Another electromagnetic site:Butcher, G., 2003. "Electromagnetic Waves: Different Waves, Different Wavelengths," GSFC Laboratory for Terrestrial Physics, NASA [accessed April 10, 2006] http://imagers.gsfc.nasa.gov/ems/waves3.html.
Amplitude modulation:
This webpage has an applet that lets you play with carrier and modulating signal to produce AM waves:Nyack, C.A., 1996. "Amplitude Modulation," Cuthbert Nyack [accessed April 10, 2006] http://cnyack.homestead.com/files/modulation/modam.htm.
Wikipedia contributors, 2006a. "Amplitude Modulation," Wikipedia, The Free Encyclopedia [accessed April 10, 2006] http://en.wikipedia.org/w/index.php?title=Amplitude_modulation&direction=next&oldid=44559258.
Information on crystal oscillators:Wikipedia contributors, 2006b. "Crystal Oscillator," Wikipedia, The Free Encyclopedia [accessed April 10, 2006] http://en.wikipedia.org/w/index.php?title=Crystal_oscillator&oldid=46562927.
Information on AM (mediumwave) radio:Wikipedia contributors, 2007. "Mediumwave," Wikipedia, The Free Encyclopedia [accessed January 24, 2007] http://en.wikipedia.org/w/index.php?title=Mediumwave&oldid=102931548.

Materials and Equipment
To do this experiment you will need the following materials and equipment:

• 2 crystal oscillators, notes:
  Each oscillator should be at a different frequency, within the AM broadcast band (0.53 to 1.71 MHz in North America, 0.53 to 1.61 MHz elsewhere).
  For use with the solderless breadboard in this project, you want the 'full can' package.
  Suitable oscillators are available from online suppliers:
  Mouser Electronics:1 MHz, part number 520-TCF100-X1.2288 MHz, part number 520-TCF122-X.
  Jameco Electronics:1 MHz, part number 278611.2288 MHz, part number 325307.
• solderless breadboard (e.g., Radio Shack 276-175),
• 1000 ohm to 8 ohm audio transformer (e.g., Radio Shack # 273-1380),
• 1/8 inch mono phone plug (Radio Shack # 274-286A),
• a 6 V AA battery holder (holds four batteries),
• four 1.5 V AA batteries,
• a set of alligator jumpers,
• jumper wires for breadboard.

Disclaimer: We occasionally provide 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. We receive no consideration, financial or otherwise, from suppliers for these listings.

Experimental Procedure
Building the Circuit
Before we get into the step-by-step instructions for building the circuit, we'll first go over the circuit design and show you how the solderless breadboard works.

Figure 2, below, shows the connections you need to make to build the circuit. The transformer isolates your music player from the rest of the circuit, and also amplifies the signal from your music player. The amplified signal from the secondary coil of the transformer modulates the power to the oscillator chip (+ power at pin 14 and − power at pin 7). A wire connected to the oscillator output (pin 8) serves as the antenna for broadcasting the amplitude-modulated radio wave.

Figure 2. Simple AM transmitter circuit diagram. The square corner of the oscillator corresponds to pin 1. The pins are numbered according to standard positions for a 14-pin integrated circuit.

Figure 3, 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.

Figure 3. 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.

Now let's build the circuit!

1. Use two alligator jumpers to connect to the terminals of the phone plug. (If you have a soldering iron, you can solder connecting wires instead.)
2. Connect the other ends of the alligator clips to the 8 ohm side of the transformer (red and white wires).
3. Insert the oscillator across the gap in the breadboard, so that pins 1 and 7 are on one side of the gap, and pins 8 and 14 are on the other. You can identify pin 1 of the oscillator because it is next to the square corner (the other three corners are rounded). Be careful not to bend the pins.
4. Use the breadboard to connect the + (red) and − (black) terminals of the battery holder and the 1000 ohm side of the transformer (blue and green wires) as shown in the diagram and in the photo below. Note that the 1000 ohm side of the transformer has a center tap (black wire) which is not used in this project.
5. Connect a long jumper wire to the output of the crystal oscillator (pin 8). This will serve as the antenna.
6. Double-check to make sure that all of your connections correspond to the circuit diagram.
7. Figure 4, below, shows photographs of the completed setup and a detail view of the circuit on the breadboard.

Figure 4. The top photo shows the completed setup, including the music input source (portable tape player) and an AM radio receiver. The bottom photo is a detail view of the completed circuit on the breadboard. On the Radio Shack audio transformer, the blue and green wires are the 1000 ohm side, and the red and white wires are the 8 ohm side. We used small pieces of masking tape on the transformer tabs to hold it in place on the breadboard. The 8 ohm side of the transformer is connected to the phone plug (yellow oval). The 1000 ohm side of the transformer is connected to the positive terminal of the battery pack ("+6 V") and the oscillator, pin 14 (blue oval). The black wire from the transformer is a center tap from the 1000 ohm side and is not used in this project (no connection needed; we taped it off to the side to keep it out of the way). Pin 7 of the oscillator is connected to the negative terminal of the battery pack ("ground"). The wire from pin 8 of the oscillator is the antenna.

Experimenting with the Circuit
Now that you have built the circuit, here is the fun part: experimenting with it!

1. Connect the phone plug to the output (headphone) jack of your mp3 or CD player and tune your AM radio to 1 MHz. Bring the antenna within an inch of your radio antenna. Can you hear the music that you are playing on your mp3 or CD on the radio?
2. Now tune your AM radio to a different frequency say 700 kHz. Can you still hear your music?
3. Tune your radio back to 1 MHz where you can hear your music. But this time remove the 1 MHz crystal oscillator and in its place put the 1.2288 MHz oscillator. Can you still hear your music?
4. Without changing the oscillator back to 1 MHz, instead tune your radio now to 1.23 MHz. Can you hear your music?
5. Use 1 MHz crystal oscillator and tune your radio to 1 MHz. Adjust the volume control of your mp3 or CD player, is there any change in the quality of the sound you hear in your radio?
6. Until now you have kept your antenna within an inch of your radio antenna, now move your transmitter's antenna further away slowly and hear what happens. Does the quality of your sound improves or gets worse? Why?
7. Rotate the radio receiver antenna relative to your transmitter's antenna (or vice versa). Does this affect the quality of the sound? Why?
8. Try using a longer wire for the antenna. Does this affect the quality of the sound? Does this affect the broadcast range for your transmitter? Why?

Variations

1. Try receiving the signal from your AM transmitter with a crystal radio that you build yourself. You can explore how the relative placement of the receiving and transmitting antennas affects signal strength at the receiver. To see how to build a crystal radio receiver, see the Science Buddies project Rock'n'Roll Radios.
2. Try using a 9 V transistor radio battery instead of 4 AA batteries. What differences do you notice in the signal?
3. Advanced. If you have access to oscilloscope in school, try to see the signals coming out from the antenna with your mp3 turned OFF and then ON. Also connect the +6V of the battery directly to the oscillator bypassing the transformer and look at the signal. What difference or similarities do you see between these three signals?
4. Advanced. This is an extremely rudimentary transmitter and therefore the sound quality is not going to be good. However, you could add more blocks to the present circuit and make improvements. What could you possibly add to the present circuit so that you are able move your antenna further away from your radio and still hear the music? For a slightly more complex circuit, try the following link: Bowden, B., 2006. "Micro Power AM Broadcast Transmitter," Bowden's Hobby Circuits [accessed April 12, 2006] http://ourworld.compuserve.com/homepages/Bill_Bowden/page6.htm#amtrans.gif. How does this circuit compare to the simpler one in the project? How does the broadcast range compare? Can you relate the difference in performance to the difference in the circuits?

Testing a Parabolic Reflector with Light from an LED

Objective
The goal of this project is to determine the best position of a parabolic reflector for sending and receiving signals with light.
Introduction
Parabolic reflectors are used in many applications, including: flashlights, optical telescopes, radio telescopes, solar ovens, and even for picking up on-field sounds from the sidelines at football games. What is so special about the parabaloid shape that makes it useful in so many applications?
A parabola is a two-dimensional curve consisting of the points that are equidistant from a point (called the focus) and a line (called the directrix). Figure 1, below (Weisstein, 1999) illustrates the essentials. The left half of the figure shows the directrix, the vertex, and the focus of the parabola. The vertex is at the origin, (0, 0). The directrix (L) is the vertical line with x-coordinate −a. The focus, F is thus at the point (a,0). The right half of the figure shows graphically that the points of the parabolic curve are equidistant from L and F.

Figure 1. Diagram of a parabola, showing, on the left, the directrix, vertex and focus, and, on the right, illustrating that each point on the parabola is equidistant from the directrix, L, and focus, F.

The right half of Figure 1 also implies a property that makes the parabolic shape so useful in the reflector applications mentioned previously. The property is this: waves from a point source placed at the focus, F, are reflected by the parabolic curve as waves traveling parallel to the parabola's axis of symmetry (the line y = 0). So the parabolic curve is useful in flashlights because it directs the light in a strong beam out the front.
Conversely, waves parallel to the parabola's axis of symmetry are reflected to pass through the point, F. In the other applications above, the parabolic reflector is acting as a receiver, collecting parallel waves over its surface and reflecting them to the point F. Both situations are illustrated in Figure 2, below (Weisstein, 1999).
In this project you will build two cylindrical parabolic reflectors and measure how well they send light. One of the reflectors will be equipped with an LED, the other with a light detector. Before getting started, here is some information about how LEDs work.

Figure 2. Diagram of a parabola showing that rays parallel to the axis of symmetry are reflected through the focus.

An LED (light-emitting diode) is a special kind of diode that produces light (see Figure 3).

Figure 3. 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).

Figure 4. 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 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 5. Schematic diagram showing how to use Ohm's Law to calculate the correct value for the current-limiting resistor (Hewes, 2006).

The objective of this project is to build and test two cylindrical parabolic reflectors, using an LED with one reflector as the signal source and a light-to-voltage converter with the second reflector as the detector. A cylindrical parabolic curve is the three-dimensional shape swept out by a parabola as it is translated, out of the plane of the screen, along a line perpendicular to the vertex. An example is shown in Figure 3, below (Irish Solar Energy Association, Ltd). This shape is not quite as efficient as the parabolic "dish" (the figure swept out by rotating the parabola about its axis of symmetry), but it has the advantage of being much easier to make at home.

Figure 6. Example of a cylindrical parabolic mirror, from a solar heating system.

What point in front of each reflector will give you the strongest signal?

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:
parabola,
parabolic reflector,
LED (light emitting diode),
current (I),
resistance (I),
Ohm's law (V = IR, or I = V/R, or R = V/I).

Questions
With a 9 volt battery and a 300 Ω resistor in series with an LED, how much current will flow through the LED?

Bibliography
M. Erskine has a free template and building tips available on the site: Erskine, M. 2002. "Deep Dish Cylindrical Parabolic Template."http://www.freeantennas.com/projects/template/
University of Toronto, Mathematics Network, Question and Discussion Area: finding the focus of a parabolic dish:http://www.math.toronto.edu/mathnet/questionCorner/parabolic.html
Here is an article on the mathematics of parabolas (this is the source of the diagrams in the Introduction), Weisstein, E.W. 1999. "Parabola." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/Parabola.html
This is a short piece on antennas from Scientific American: Fischetti, M. 2003. "Catch a Wave." Scientific American, 288 (May, 2003):88–89.
These webpages have useful information on LEDs:
Hewes, J., 2006. "Light Emitting Diodes (LEDs),"

1)The Electronics Club, Kelsey Park Sports College [accessed December 15, 2006] http://www.kpsec.freeuk.com/components/led.htm.
2)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
For this experiment, you will build two cylindrical parabolic reflectors (one for the LED, and one for the detector). You will need:
2 cardboard squares from cereal box or equivalent, 15–20 cm (≅6–8 in);
aluminum foil to cover cardboard;
white glue;
1 styrofoam block, 16.5 × 7.5 × 2.5 cm (l×w×h) (≅6.5 × 3 × 1 in).

Note that many other choices of materials are possible. You can easily build this with materials you can find around the house (see "Deep Dish Cylindrical Parabolic Template" website in the Bibliography).

For testing the cylindrical parabolic reflector with light, you will need the following materials and equipment (except where noted, part numbers are from Mouser Electronics):

• light-to-voltage converter (part number 856-TSL14S-LF),
• 3 fresh AA batteries (or freshly-charged AA batteries, if you use rechargeables),
• 1 battery holders for 3 AA batteries (part number 12BH431-GR),
• 1 fresh 9 volt battery,
• 1 9 volt battery snap (part number 121-0622/O-GR),
• 1/4-watt resistors with the following values:
  300 Ω (part number 271-300-RC),
  10 kΩ (part number 271-10K-RC);
• 2 high-intensity LEDs, (output: 2800 mcd, peak wavelength: 660 nm, viewing angle: 30°;part number 696-LX5093SRC/E),
• one digital multimeter (DMM):
  If you want one-stop shopping, Mouser has moderately priced (about $40) DMMs fromBK 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.
• alligator clip leads (part number 13AC010),
• hook-up wire 22 AWG (get this at Radio Shack, 3 25-foot spools in different colors, Radio
• Shack part number 278-1224),
• soldering iron,
• solder,
• heat shrink tubing (optional, Radio Shack part number 278-1610)),
• blow dryer (optional, for warming heat-shrink tubing),
• long nose pliers,
• three rubber bands,
• two lengths of wooden doweling (to support LED and detector in front of parabolic reflectors),
• two wood blocks to support dowels,
• flashlight,
• room you can darken for testing.

Disclaimer:We occasionally provide 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. We receive no consideration, financial or otherwise, from suppliers for these listings.

Experimental Procedure

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, below).
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, below, shows the completed LED with soldered wire 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 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 in Figure 8. Figure 8. 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 in Figure 9. The round window contains the light-sensitve region. The component has three pins, as shown.

• Pin 1 should be connected to ground (black wire from the battery holder).
• 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.
• Pin 3 is the output voltage, a signal that is proportional to the amount of light falling on the sensor. Pin 3 should be connected to one lead of the 10 kΩ resistor; the other lead of the resistor should be connected to ground.

3. Figure 10 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 (RL). Connect the resistor from pin 3 to ground, as shown. Figure 10. 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. 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 + terminal of the 9 volt battery (red wire from battery snap) should be connected to the + lead of the LED (anode, longer lead).
2. The − terminal of the battery should be connected to the 330 Ω resistor. Place the resistor near the battery so that it does not interfere with mounting the LED in front of the antenna.
3. Connect the other end of the resistor to the − lead of the LED (cathode, shorter).
4. To turn on the LED, connect the battery snap to the battery. Disconnect the battery when not in use.

Experimental Setup and Measurement

1. Build the two antennas (one for the LED and one for the detector) using the template and instructions on the webpage Deep Dish Cylindrical Parabolic Template (Erskine, 2002).
2. Tips:
   Use the styrofoam block as a guide to shape the antenna, but do not permanently attach it in front of the finished antenna. If your material does not hold its shape on its own, then glue the styrofoam supports to the top and bottom, not the center, of the antenna.
   Cover the antenna surface with aluminum foil before shaping.
3. How far from the detector can you still pick up a signal without the reflector? Set the reflectors up facing each other farther than this distance.
4. Using the wood dowels and blocks, rig up a support in front of each reflector to carry the LED or the detector. Mark off the dowels in one cm increments so that you can easily measure the position of the LED and the detector with respect to the reflectors.
5. Use rubber bands to attach the LED and the light detector to each dowel.
6. Make your LED light measurements in a darkened room. Use a shaded flashlight to check the reading on your DMM. Make sure that light from the flashlight does not fall on the light-to-voltage converter.
7. Find the points in front of each reflector where you get the maximum response from the light detector.
8. Move the LED in 1 or 2-cm increments, checking the reading on the light detector at each position.
9. Move the light detector by 1 or 2 cm, and repeat.
10. What happens if you block off a portion of the reflector in front of the LED?
11. What happens if you block off a portion of the reflector in front of the detector?

Variations

1. You can use parabolic reflectors to enhance the performance and security of your home wireless network. See the Science Buddies project The Point of a Parabola: Focusing Signals for a Better Wireless Network.
2. Design an experiment to see how much further you can detect a light signal from an LED using a pair of parabolic reflectors.
3. LEDs have different viewing angles, some narrow, some broad. Can you design a method for testing the intensity of light from an LED at different viewing angles? (The response from the light-to-voltage converter depends on its viewing angle. See the graph "Normalized Output Voltage vs. Angular Displacement," on page 4 of the light-to-voltage converter data sheet (Taos, Inc., 2006)). Online electronics suppliers such as Mouser Electronics, and Jameco Electronics have many different LEDs to choose from. Test your parabolic reflectors with several different types of LEDs, with a variety of viewing angles. If you block part of the parabolic reflector, can you measure a difference in the amount of light reaching the detector? Do you measure the same decrease for LEDs with wide viewing angles as for LEDs with narraow viewing angles?
4. More advanced students might want to try designing an experiment to answer one or more of the following questions:

• Using similar construction methods as for the project described above, can you build an approximation of a 3-D parabolic antenna?
• How much further can you send a light signal with a 3-D parabola?
• Parabolas can be narrow and deep, or wide and flat. Which shape works best for sending a signal with an LED? Do some research on the "f/D" ratio for a parabolic antenna for this one.