|
Arabic
Bengali
Chinese
English
French
German
Hebrew
Hindi
Italian
Japanese
Korean
Malay
Polish
Portuguese
Spanish
Turkish
Ukrainian
Vietnamese|
ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Photo background. Transmission of sound using a beam of light. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Alternative energy sources Not everyone knows that Alexander Graham Bell did not consider the telephone his most important invention. Indeed, Bell favored another invention that he predicted would revolutionize the means of communication. Bell was obsessed with the idea of transmitting voice with a beam of light! Turning to the Sun as the only reliable source of high-intensity light he had at his disposal, Bell tried to use it as a multi-purpose communications medium. He called his invention the photophone. Bell spent most of the last years of his life in unsuccessful attempts to expand the scope of the photophone. Before Bell's death in 1922, the photophone had only limited military use.
Ironically, his dream of transmitting messages using light finally came true more than 100 years after the birth of the idea. No, we do not use sunlight to any great extent for communication, but we have learned how to use the energy of the sun to excite emitters called lasers and direct laser beams along a glass fiber, the thickness of which is no more than the thickness of a hair. The era of fiber optics has come, and the idea of comprehensive communication, first expressed by the brilliant inventor of the telephone, is becoming a reality. Isn't it exciting to follow in the footsteps of the famous inventor and rediscover the photophone? So let's do it. Memories of the past All this happened one fine day in 1878, but we are getting ahead of ourselves in our story. Bell was very interested in the means of communication, as evidenced by his numerous inventions. But, in addition, he admired the light, which fascinated him. Already in Bell's time, solar electricity was known. Phenomena associated with it were first observed by Edmond Becquerel in 1839, i.e., 8 years before Bell's birth. While conducting a series of experiments on electricity, Becquerel immersed two metal electrodes in a conductive solution and exposed the apparatus to sunlight. Much to his surprise, a small electrical voltage developed between the electrodes. This discovery went largely unnoticed until 1873, when Willoughby Smith discovered a similar effect by exposing a piece of selenium to light. The effect was insignificant, but this moment should be considered the real birth of solid-state solar cells. Why did things like this happen at all? It was inexplicable from the point of view of classical physics! But Bell didn't care. He was a practical man, and his imagination was occupied by the idea of creating a telephone set powered by light. Over the next few years, he followed with great interest the slow progress in the field of photoelectricity and photographic instruments. In 1878 he had the idea of a photophone. Working with selenium detectors, Bell designed and experimented with many variants of this instrument. Although the early experiments were fairly simple, they were nonetheless successful. On April 1, 1880, Alexander Graham Bell listened to the voice of his assistant, Sumner Tainter, as his words were carried by a beam of light over a distance of over 200 m. Dr. Bell's light intercom became a reality. It was on these successive successes that Bell based his predictions of the further development of communication technology, which then seemed fantastic. For example, he was a firm believer that in the future people would only travel with the help of light. Photo background Developing numerous devices for the photophone and improving its design, Bell noticed that the most sensitive were devices in which selenium resistance was used as a light detector. Of course, he worked without electronic amplifiers. Instead, he used light focusing to amplify the signals. In search of the best optical system, Bell designed a variety of lens and mirror systems. One of Bell's detectors consisted of selenium elements arranged in a circle, onto which light was focused using a collecting lens. In another design, the detectors were located on a cylindrical surface and placed at the focus of a parabolic mirror. In all his devices, selenium detectors were connected in series with a battery and a high-resistance telephone capsule. When modulated light fell on the surface of the selenium, it caused a change in its resistance, which was converted into sound waves by the telephone capsule. You can easily replicate his early experiments. Take out the photodetector first. Of course, they are now made differently from what Bell used to use, but the Vacte model VT312 / 2 photodetector is very similar to Bell's. It is a selenium photoresistor with a small amount of cadmium added to improve performance. It actually has two detectors. Bell often used multiple detectors to increase sensitivity. The detectors are connected in series and placed at the focus of a parabolic reflector. Any size reflector will do, however, the larger the bowl, the greater the range. View the Edmund Scientific Co. catalog. (7785, Edscorp Bldg., Barrington, NJ 08007). They have a wide range of parabolic and Fresnel reflectors. The detector can be mounted at the focus of the reflector using a star holder like the one shown in fig. 1. The detector forms a common electrical circuit with the battery and the high-resistance telephone capsule. A 12-volt battery, such as a car battery or several flashlight batteries connected in series, is suitable for this purpose. The magnitude of the voltage does not play a role here. On the other hand, the phone capsule is not so easy to find. The capsules used in modern phones, unlike their predecessors, have low resistance and do not work well in our case. You can turn to radio amateurs who have a pair of old high-impedance headphones. At the very least, they know where to get them. As you can imagine, these headphones are not as popular now as they used to be.
All these parts, connected in series, make up the receiving part of the photophone. Now it's up to the transmitting part.
In many of his early studies, Bell did not try to optimize the transmitting part of the photophone. He focused his attention on improving the optoelectronic circuit of the receiver. As a result, many of his early designs are simple in the best sense of the word. Among the interesting designs was a metal tube 2,5 cm in diameter and 5 to 7,5 cm long. At one end of the tube he attached a mirror, as shown in fig. 2. When the tube is spoken, the sound waves cause the mirror to vibrate and modulate the light from the source. You can go further by replacing the hard mirror at the end of the tube with a piece of metallized film. Now the most exciting moment has come - testing the photophone. This must be done by at least two people. Have your friend hold the transmitter to his mouth, stand facing the sun, and adjust the angle of the transmitter's mirror so that some of the light is reflected back to your receiver. While your friend is talking into the phone, move the parabolic reflector until it crosses the light beam and focuses it on the detector. Be careful when pointing the reflector. Do not point the receiver directly at the sun, as concentrated sunlight can quickly damage your detector. Make the first test at a short distance, because the slightest movement of your friend at a long distance greatly affects the signals amplified by the photophone, and makes tuning difficult. After setting up, listen to your friend's voice on your phone. Increasing the range of the photophone There are several ways to increase the range of the photophone. One of them is based on increasing the size of the parabolic reflector, the other is based on amplifying the transmitter signal by increasing the size of the mirror attached to it. You may be able to stretch aluminized Mylar film over one end of a large tin can. You can increase the sensitivity of the detector. You will probably want to experiment with different photosensitive elements, changing their location, as Bell did. Changing the battery voltage and earphone resistance will also change the sensitivity of the receiver. Of course, modern electronics can be used in the photophone circuit. The parameter limiting the sensitivity of the receiver is the output voltage of the photodetector. The best way to increase the output voltage is to run it through an amplifier. On fig. 3 shows how this can be done. First, replace the photoresistor with a small solar cell. It is somewhat more sensitive under these conditions and is obviously less prone to damage when exposed to direct sunlight.
Circuit IC1 is a preliminary stage for amplifying a small signal from a solar cell. The element is connected by a variable component to the input of the circuit through the capacitor C1. Thanks to this connection of the photovoltaic cell, it is possible to "cut off" all the light, with the exception of the modulated one. Resistors R1 and R2 determine the preamplifier gain equal to the ratio of R1/R2. As the distance between the transmitter and receiver increases, the values of these resistances should be changed. However, don't set the gain too high or the circuit will self-excite. You can suppress parasitic generation by connecting capacitors in parallel with resistors R2 and R3, but this will worsen the frequency response of the receiver. By changing the value of R2, it is necessary to change the value of R3 by the same amount, since the values of these resistances are always equal. The signal from the preamplifier output is fed to the volume control R4, from where it goes to the final amplifier IC2. This amplifier raises the signal level to the level needed to drive the loudspeaker. Pretty good compared to what it was without an amplifier. When making the circuit, note that two power supplies are required, +9V and -9V. 9V batteries for the transistor receiver will do. However, the magnitude of the supply voltage is not critical, and any available power supply in the range of 6-15 V can be used. Improving Transmitter Performance It is possible to improve the sensitivity of the photophone by attaching an amplifier to the transmitter, the circuit of which is given in Fig. 4. It uses the same integrated power amplifier LM386 as in fig. 3, however, its input receives a signal from a microphone, and not from a solar cell.
The output of the power amplifier is driven by a small 5 cm speaker, similar to those used in pocket transistor receivers. A piece of aluminized Mylar film is stretched over the speaker. When you speak into the microphone, your voice is amplified and sent to the speaker. In turn, the speaker makes the film covered with a mirror layer vibrate and modulates the sun's beam. To further increase the communication range, it is necessary to increase the size of the speaker, and hence its reflective surface. I have observed experiments in which small fragments of a mirror were glued directly to an oscillating speaker diaphragm. However, I cannot vouch for the effectiveness of such a device, as I have never tested it. It probably acts like a bowl-shaped reflector. In the process of improving the photophone, Bell and Tainer found more than 50 ways to modulate the light beam with the voice, including the variable polarization circuits currently used in sophisticated laser communication devices. Conclusion If you once got carried away with the creation of an optical communication system, it is difficult then not to think about this exciting problem. In the later years of his life, Bell predicted a great future for her. Optical communications projects initiated by Bell's experiments are coming to fruition. Unfortunately, the inventor's projects were not implemented during his lifetime. Author: Byers T.
Alcohol content of warm beer
07.05.2024 Major risk factor for gambling addiction
07.05.2024 Traffic noise delays the growth of chicks
06.05.2024
▪ Another form of ice discovered ▪ A new type of nanorefrigerator ▪ There are more cell phones than toothbrushes
▪ section of the site Reference materials. Article selection ▪ article And my Matryona became neither Pava nor Crow. Popular expression ▪ article Which metal can get plague? Detailed answer ▪ Article Orientation by natural phenomena. Travel Tips ▪ article Imitator Cuckoo. Encyclopedia of radio electronics and electrical engineering ▪ Article Vanishing Warrior. Focus Secret
Home page | Library | Articles | Website map | Site Reviews
www.diagram.com.ua |
See other articles Section 
Latest news of science and technology, new electronics:
All languages of this page