Laboratory: Modulated Laser

1995 The Regents of the University of California


Electromagnetic radiation is used in communication. You are more familiar with radiowaves, microwaves, and TV signals. Visible light is also electromagnetic radiation and can be used to transmit a signal. It is a whole new technology called fiber optics.

A discussion of this laboratory can be found in the Exploratorium Science Snackbook. We have modified the snackbook's "modulated laser" by substituting an LED (light emitting diode) for a flashlight bulb. By using an LED instead of a flashlight bulb, we can get a cleaner signal. Unlike incandescent light bulbs, the LEDs have a directional polarity. You must connect the positive and negative leads to them correctly, or they will not work.


In this laboratory, you will convert sound into electrical pulses. The electrical pulses will be converted into rapid fluctuation in the brightness of a flashlight bulb. This light will be transmitted through the air or fiber optics to a solar cell and converted back into electrical pulses. The electrical pulses will be fed into an amplified speaker and turned back into sound. The apparatus is shown in figure 1.

Here's a link to figure 1


  1. Using the example provided and circuit shown on the right side of figure 2, connect the wires to the transmitter or light modulator.
  2. Next, connect the wires to the receiver as shown on the left side of figure 2.
  3. Check to make sure that your tape recorder is putting out sound. Plug your tape recorder in your sound modulator.
  4. Plug the solar cell into the amplifer/speaker. Turn on your amplifier/speaker. Test your system by shining the light on the solar cell.
  5. Get together with another lab group to transmit and receive sound. How far will your system transmit the signal?


  1. Your communication system uses visible light to transmit a signal through the air. Explain how your communication system (transmitter and receiver) works.
  2. How far will your system transmit the signal? How can you increase the transmission distance of your signal?
  3. Most communication systems use radiowaves or microwaves. What are the advantages of using radiowaves or microwaves over your visible light system?

What's Going On?

"The induction coil acts as a short circuit for the direct current from the battery. As a result, when the flashlight is turned on, current flows through the coil and the lamp is lit.

The induction coil acts as an open circuit for the alternating current from the tape player. As a result, the electrical pulses that would power the headphones must flow through the flashlight bulb. The "sound vibrations" recorded on the tape are transformed into electrical vibrations that produce rapid fluctuation in the brightness of the flashlight bulb. These fluctuations are picked up by the solar cell and are turned into electrical pulses, which are amplified by the speaker or tape recorder and turned back into sound.

Laser beams, radiowaves, and flashlight beams are electromagnetic waves, not sound waves. In this Snack, these electromagnetic waves are modulated to carry the pattern of the sound waves.

For the reader who is electronically more sophisticated, the circuit diagram for the modulated flashlight is given in figure 2."

Parts List and Cost:

(part numbers are from Radio Shack)

Transmitters (Light Modulator)

Super-Bright LED
Part Number: 276-086
$5.19 each

2 D battery holder
Part Number: 270-386
$1.59 each

2 D batteries
Part Number: 23-550
$2.59 / two

1 SPST slide switch
Part Number:275-406
$   .99 / two

1 47 Ohm resistor
Part Number: 271-009
$   .39 / two

1 6' audio cable w alligator clips
Part Number: 42-2421
$2.39 each

Misc. nuts, bolts, and wire


1 silicon solar cell Part Number: 276-124 $4.19 each 1 6' audio cable w alligator clips Part Number: 42-2421 $2.39 each 1 mini audio amplifier/speaker Part Number: 277-1008 $11.99 each 1 9V. battery Part Number: 23-553 $1.99 each

One laboratory unit will cost $36.75, not including the tape recorder. The prices are from Radio Shack. The cost will be less if you shop around and buy in bulk.

For more information, email your comments to or contact Regan Lum

All text, images, and other resources in this page are Copyright 1995, The Regents of the University of California. All rights reserved. For permission, email

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