Finally, an audio/video amplifier is used to boost the level to whatever is needed todrive the speaker, teleprinter, monitor, or instrumentation at the receiver output.A modulated-light receiverModulated light beams can be demodulated easily using photodiodes or photovoltaiccells, as long as the modulating frequency isn’t too high. At audio frequencies, a circuitsuch as the one in Fig. 27-14 will provide sufficient output to drive a headset. Thisdemonstrates the method by which fiberoptic signals are detected.Quiz51727-14A simple modulated-light receiver.For use with a speaker, another bipolar amplifier stage can be added following thestage shown. To increase the sensitivity of the receiver (that is, to allow reception ofmuch fainter modulated-light beams), an FET stage can be added between thephotovoltaic cell and the bipolar stage shown.Modulated-light experiments can be done with simple apparatus, costing only a fewdollars. If reasonably large lenses and/or reflectors are used, line-of-sight voice commu-nications can be had for distances up to several hundred feet, using circuits based onthe transmitter of Fig. 26-17 and the receiver of Fig. 27-14. With additional amplifica-tion and very large reflectors, the range can be increased to several miles on clearnights. This could make a great science-fair project.QuizRefer to the text in this chapter if necessary. A good score is at least 18 correct. Answersare in the back of the book.1. The reflected wave in a radio signal:A. Travels less distance than the direct wave.