Advanced Projects in Laser Communication

  
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The last section presented a number of basic free-air laser light communications projects. You learned how to modulate a He-Ne laser beam using a transformer, transistor, and even a piece of Mylar foil stretched in a needlepoint hoop. You also learned various ways to electronically modulate laser diodes and recover the transmitted audio signal. Th
Advanced Projects in Laser Communication - schematic

is section presents advanced projects in free-air laser-beam communication. Covered are methods of remotely controlling devices and equipment via light and how to link two computers by a laser beam. Everyone is familiar with Touch-Tone dialing: pick up the phone and push the buttons. You hear a series of almost meaningless tones, but to the equipment in the telephone central office, those tones are decoded and used to dial the exact phone you want out of the millions in the world. You can use the same technique as a remote control for actuating any of a number of devices, such as motors, alarms, lights, doors, you name it. The tone signals are sent from transmitter to receiver via a laser light beam. With the right setup, you can remotely control devices up to several miles away, and without worry of interference or FCC regulations. The Touch-Tone (or more simply tone control ) system supports up to 16 channels. Each channel is actuated by a pair of tones. Tone selection depends on the buttons pressed on the keypad. Dividing a common 16-key keypad into a matrix of 4 by 4, as illustrated in ill. 14-1, shows how the tones are distributed. E. g. , pressing the number 5 key actuates the 770 Hz and 1336 Hz tones. Pressing the number 9 key actuates both the 852 Hz and 1477 Hz tones. The dual tones help prevent accidental triggering, but they also present a somewhat difficult decoding dilemma. The first tone dialing circuits used...



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