Electronics Puzzler: Electronic Voice Transmitter

By Forrest M. Mims III

How did D.J. send his voice down to the ground?

D. J.'s engineer knew that in 1880 Alexander Graham Bell and Sumner Tainter invented and demonstrated the Photophone, the world's most advanced wireless communications system since smoke signals, signal flags, semaphores, signal mirrors and flashing lights. In the simplest implementation of Bell's Photophone a beam of sunlight was reflected from a thin mirror to a parabolic reflector hundreds of feet away. When words were spoken into a tube directed at the back side of the mirror, the mirror vibrated in step with the sound waves, thus, amplitude modulating the beam of sunlight reflected from the front surface of the mirror. The beam of reflected sunlight carrying the voice signal was focused by the reflector onto the light-sensitive surface of a selenium photocell that Bell fabricated himself. The photocell was connected in series with a battery and a telephone receiver, which formed the electronics of a receiver that transformed the modulated beam of sunlight back into voice.

As instructed by his engineer, D. J. begin making a Photophone transmitter by using his pocket knife to remove both ends of a plastic water bottle.

D. J.'s food supply provided plenty of Photophone mirrors in the form of the shiny foil packages in which his meals were packaged. He placed a shiny piece of foil from one of the food packets over one end of the water bottle and secured it tightly in place with a rubber band.
where to cut the water bottle
Figure 2. Where to cut a plastic water bottle to begin making the Photophone transmitter.
Completed photophone transmitter
Figure 3. The completed Photophone transmitter.

In only minutes, D.J. had fashioned a transmitter capable of sending his voice several hundred feet or more over a beam of reflected sunlight. Meanwhile, down below the engineer was finishing up a receiver to intercept and demodulate the sunlight signals. The receiver consisted of a a small solar panel connected to the input of an audio amplifier through a 0.1 uF capacitor. The engineer taped the solar panel to the end of a one-foot long square tube that he made from cardboard and duct tape. He sprayed the inside of the tube with black paint. The tube provided a shield to keep direct sunlight away from the solar panel. The engineer then mounted the collimated solar panel and the amplifier on an adjustable tripod that he placed on a shaded sidewalk south of the tower crane. He attached a large bicycle reflector just below the collimator tube and pointed its open end in the general direction of D.J.'s perch high in the sky.

Following the engineer's instructions sent from the station to his portable radio, D.J. used his Photophone transmitter to reflect a beam of sunlight down to the sidewalk below. When the reflected sunlight formed a splotch on the sidewalk, he carefully adjusted the Photophone until the reflected sunlight reached the receiver. He instantly knew when the Photophone was properly pointed, for the reflectors glowed bright red. By holding the Photophone very still against the side of a window in his little cabin, D.J. could again send his voice to his listeners, the number of which more than tripled when the news media learned how the charity stunt was salvaged by the quick-thinking engineer.

How The Transmitter Works

A suitable receiver circuit for D.J.'s Photophone is shown in Figure 1 in Jameco Recipe 6 (Infrared Remote Control Tester). All that's required is to substitute a single solar cell, a series connected array of two or more solar cells or a low-voltage solar cell panel for the photodiode in Figure 1. Jameco sells a variety of low-voltage solar cell modules with which you can experiment (see Part Nos. 227985, 227993, 171061, and 228013). You can use a pre-built audio amplifier instead of building your own. Just be sure to insert a 0.1 or 0.01 uf capacitor between the solar cell(s) and the amplifier. Test the receiver by pointing an infrared TV remote control unit at it. You should hear a clear buzz or tone when you press a button on the remote control.

CAUTION: Always wear dark sunglasses when experimenting with a Photophone. Persons at the receiver end must never look directly at sunlight reflected by the Photophone transmitter as this might cause eye damage. Instead, those at the receiver end should simply point their receiver in the general direction of the transmitter and look at the receiver instead of the transmitter.

Photophone Background

While the scenario of this Puzzler is fictional, the method really does work. In fact, I've talked over sunbeams many times. In 1980, my wife Minnie and I celebrated the centennial of Bell's invention of lightwave communications at the site of his former lab at 1325 L Street in Washington, DC. We were accompanied by representatives from the National Geographic Society, the Smithsonian Institution and Bell Labs. There we spoke to one another over a beam of sunlight using one of my homemade Photophone. We also spoke over a beam of red light transmitted by a red LED through a 100-meter optical fiber and detected at the end of the fiber by an identical LED.

Going Further

Building a working Photophone is an ideal experiment for young people. The exercise teaches basic principles of wireless communications, modulation, optoelectronics, amplification and solid state electronics.

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