Title: Communicating with Light: From Telephony to Cell Phones Revision: February 1, 2006 Authors: Jim Overhiser, Luat Vuong Appropriate Physics, Grades 9-12 Level: Abstract: This series of six station activities introduces the physics of transmitting "voice" information using electromagnetic signals or light. Students explore how light can be modulated to encode voice information using a simple version of Bell's original photophone. They observe the decrease of the intensity of open-air signals by increasing the distance between source and receiver, and learn the advantage of using materials with different indices of refraction to manipulate and guide light signals. Finally, students are introduced to the concept of bandwidth by using two different wavelengths of light to send two signals at the same time. Special Kit available on loan from CIPT lending library. Equipment: Time Required: Two 80-minute periods NY Standards 4.1b Energy may be converted among mechanical, electromagnetic, Met: nuclear, and thermal forms 4.1j Energy may be stored in electric or magnetic fields. This energy may be transferred through conductors or space and may be converted to other forms of energy. 4.3b Waves carry energy and information without transferring mass. This energy may be carried by pulses or periodic waves. 4.3i When a wave moves from one medium into another, the waves may refract due a change in speed. The angle of refraction depends on the angle of incidence and the property of the medium. 4.3h When a wave strikes a boundary between two media, reflection, transmission, and absorption occur. A transmitted wave may be refracted. Communicating with Light: From Telephony to Cell Phones Teacher Information Section Objectives: • To learn about the different components of an optical network, how they function and work together to build a telecommunications system. • To gain familiarity with basic optics concepts including wavelength, amplitude, refraction, reflection and total internal reflection within a technological context. • To understand that light guides (such as optical fibers) prevent signals from dispersing and allow them to be transmitted over large distances. Historical Background: Alexander Grahm Bell invented and tested a device called a photophone in 1880. Bell's device consisted of a cylinder with at piece of light-reflecting metal foil cover one end. Bell spoke into the other end of the cylinder and directed sunlight reflected off the foil on to a crude selenium cell (precursor to the modern photocell) that was connected in series to a battery. The battery provided the voltage to power one of Bell’s telephone receivers, and the selenium cell provided the voltage spikes necessary to produce intelligible sound. Changes in the intensity of the light beam were used to transmit the information. Modern telecommmunication systems that use light change (modulate) the intensity of the beam. However, light was not used to carry voice and other signals until recently. Bell's invention, which he considered to be the most ingenious of all his inventions, never took off. It could transmit voice only 200 meters through free space, which was a critical limitation. It would take the invention of optical fibers and fiber amplifiers, capable of guiding light thousands of kilometers, before light was again used to carry communications signals. Meanwhile, radio was invented and held the technological upper hand in communications for decades. Reginald Aubrey Fessenden’s (1866-1932) is given credit for the first attempt to transmit voice via a radio signal (telephony). He employed a spark transmitter (invented by Heinrich Hertz) operating at about 10,000 sparks/second. To modulate his transmitter he inserted a carbon microphone in series with the antenna lead.... He experienced great difficulty in achieving intelligible sound. On the 23 December 1900, Fessenden, after many unsuccessful tries, transmitted words without wires. The first radio transmission was the following: "Hello test, one, two, three, four. If it's snowing where you are, Mr. Thiessen. If it is, telegraph back and let me know." Mr. Thiessen telegraphed back immediately. It was indeed snowing where he was. After all Mr. Thiessen and Professor Fessenden were only 1 mile apart. But not withstanding, the short distance, and the poor quality of the transmission, this date heralded the beginning of radio telephony, which dominated distance communication for several decades. Instructions to build your own light radios: Materials List for Light Emitter Number Catalog Total Item Source Cost per Number Cost Assembly Red LED 92F465 $0.38 $0.38 OR OR OR OR Newark Blue LED 34C1518 $0.97 1 $0.97 OR OR OR OR Yellow LED 34C1534 $0.44 $0.44 Aluminum Newark 95F2580 $5.36 1 $5.36 enclosure box Switch (SPST) Newark 61F1245 $3.59 1 $3.59 $2.99 1/8" Mono plug Radioshack 274-286 1 $1.50 (pkg of 2) Ribbon cable 2 pieces $13.33 for (14 strand, Newark 89F4840 1.5' long $0.06 100' spool 0.05” pitch) 2 strand Pocket radio Radio Shack 12-467 $16.99 1 $16.99 9 V Battery clip Newark 16F433 $0.53 1 $0.53 Resistor (300Ω) Newark 84N1622 $0.12 1 $0.12 Total cost for project: $29.12 Materials List for Light Receiver Number Catalog Total Item Source Cost per Number Cost Assembly DigiKey 425-1004-5-ND $2.27 $2.27 Photodiode OR OR OR 1 OR Hamamatsu S2386-45K $11.64 $11.64 $2.99 1/8" Mono plug Radioshack 274-286 1 $1.50 (pkg of 2) Ribbon cable 1 piece $13.33 for (14 strand, Newark 89F4840 1.5' long $0.03 100' spool 0.05” pitch) 2 strand Amplifier/speaker Radio Shack 277-1008 $12.99 1 $12.99 Total cost for project: $16.97 OR $26.16 Suppliers: • Newark website: www.newark.com • Radio Shack website: www.radioshack.com • DigiKey website: www.digikey.com • Hamamatsu website: www.hamamatsu.com Tools needed: • Soldering iron, rosin core solder, electrical tape • Drill and drill bits • Pliers, wire stripper Instructions for Light Emitter 1. Find two of the two-stranded 1.5' pieces of ribbon cable. At each end separate the strands for about 2" and strip the insulation from about 1/4" of each strand. If the strands are the same color, then mark one of them red at each end so you can tell them apart. Do this for both pieces of ribbon cable. 2. Find a mono plug and unscrew the black cap. Take one of the pieces of ribbon cable and solder one end of the strand marked red to the short terminal of the mono plug. Solder the other strand to the long terminal. The plug and wire should look like the picture below. Screw the black cap back on the mono plug. 3. Find a LED and the other piece of ribbon cable (not the one used in step #2). Solder one end of the strand marked red to the long led of the LED. Solder the other strand to the short leg of the LED. The LED and wire should look like the picture below. Wrap the legs of the LED and any bare wires in electrical tape to prevent them from touching and causing a short circuit. 4. Find the 9V battery clip and the 300Ω resistor. Solder one leg the 300Ω resistor to the red wire from the 9V battery clip. Solder the other leg of the resistor to the unmarked strand of the ribbon cable attached to the mono plug. Wrap the 300Ω resistor in electrical tape so that no bare wires are exposed. 5. Find the switch. Solder the black wire from the battery clip to the middle terminal of the switch. Solder the unmarked strand of the ribbon cable attached to the LED to one of the side terminals of the switch. Finally, solder together the strands from each ribbon cable marked red. The circuit will look like the picture below. Wrap the solder joint in electrical tape so that no bare wires are exposed. to mono plug to LED 300Ω resistor (wrapped in tape) to 9V battery clip switch 6. Drill a ¼” hole on the top side of the box for the switch. Insert the switch through the hole and tighten the nut on the threaded portion of the switch to lock it in place. Install a battery to the batter clip and tape the battery to the inside of the box to prevent it from rattling around. Your box should look like the picture below. 7. File two dents, each in opposite edges of box, to allow the wire to the mono plug and the wire to the LED to enter the box without getting pinched. Put the box together. 8. Turn on the radio and tune it to a station. Then plug in the mono plug of the light emitter assembly and flip the switch to determine which position is on and which is off. Label the switch positions. Instructions for Light Receiver 1. Find a two-stranded 1.5' pieces of ribbon cable. At each end separate the strands for about 2" and strip the insulation from about 1/4" of each strand. If the strands are the same color, then mark one of them red at each end so you can tell them apart. 2. Find a mono plug and unscrew the black cap. Take one of the pieces of ribbon cable and solder one end of the strand marked red to the short terminal of the mono plug. Solder the other strand to the long terminal. The plug and wire should look like the picture below. Screw the black cap back on the mono plug. 3. Find the photodiode.