Jameco Electronics Puzzler Solution: Transparent MicrophoneBy Forrest M. Mims III
How did Buzz Masters design a transparent microphone?
Pop singer Twanger McTune wanted to get more of the recognition that she and her remaining fans thought she deserved. Her idea was a transparent plastic microphone with no internal wires or electronics that would faithfully reproduce her voice while attracting considerable attention due to its novelty. Buzz Masters, an optical fiber nerd with a strong interest in the history of electronics, quickly thought of a solution.
A Transparent Microphone.Buzz was very familiar with Alexander Graham Bell's invention of the photophone in 1880. The photophone was the first system for transmitting the human voice over an amplitude modulated beam of sunlight. The simplest photophone transmitter developed and tested by Bell was simply a flexible mirror against which voice was directed. The thin mirror vibrated in step with the sound of voice directed against it, thus amplitude modulating a beam of sunlight reflected by the mirror.
Buzz knew that a microscope cover slip or even a piece of clear, flexible plastic could serve as the diaphragm in a transparent microphone. To test the idea, he attached a piece of clear plastic film over the end of a one-inch length of PVC pipe taped over the speaker of a portable transistor radio. When the plastic film was illuminated by a near-IR LED, about 8% of the near IR was reflected from the film. He connected a silicon photodiode to an audio amplifier, pointed the photodiode toward the film and switched on the radio. The sound from the radio could be clearly heard via a headset connected to the output of the amplifier.
Buzz planned to use a pair of large-core optical fibers to connect the microphone to the outside world. One fiber would carry near-IR light from an LED to the microphone diaphragm and the second would carry near-IR reflected from the diaphragm to an audio amplifier. The light source was a high-power 880 nm LED operated continuously through a series resistor that limited current to 100 mA. To reduce interference from stage lights at the receiver end, Buzz used a photodiode installed inside an IR transmitting encapsulant like those used in near-IR remote control receivers. The photodiode signal would be weak, so he designed a simple audio preamp stage to beef up the signal before it reached the main amplifier. A small capacitor (C1) was inserted between the silicon photodiode and the preamp stage to avoid saturation from stray IR emerging from the transmitter fiber before it was reflected from the diaphragm.
Buzz knew everything would work in theory, but building a prototype was not simple. The main problem was the need to carefully place the ends of the two optical fibers close enough to the diaphragm to provide adequate signal strength at the receiver fiber. After several tries, he transformed a cylinder of transparent plastic, two clear inserts, a small piece of plastic film and two large core optical fibers into a transparent microphone that met Twanger's requirements. Even the microphone cord was transparent, its two fibers were installed inside a clear plastic tube.
Twanger was very happy with her transparent microphone. So was Buzz, who received orders for transparent microphones from a dozen other singers.
Twanger wasn't happy that Buzz was selling transparent microphones to her competition, so she asked Buzz to add something new and spectacular to renew the novelty of her microphone. Twanger wanted her microphone to sparkle as she sang.
Buzz thought of several ways to add a sparkling light effect to the microphone. The one he preferred used three additional optical fibers that carried light from blue, green and red LEDs to the interior of the transparent microphone housing. He designed a transparent conical insert that he installed in the handle portion of the microphone with the conical portion facing down. The two microphone fibers passed through a hole in the insert to reach the diaphragm. The three new fibers were terminated below the new insert. Light emerging from the three new fibers was sprayed outward when it struck the plastic cone and appeared to sparkle when the microphone was moved. The blue, green and red LEDs at the opposite end of the new fibers could also be modulated by Twanger's voice or by her musical accompaniment.
This Puzzler is based on a transparent microphone I proposed in my lab notebook on 6 September 1977 and a working model that I built and tested on 10 January 1979. The design was very simple. A plastic film was secured across one end of a 1.75 inches inches long plastic cylinder by means of a plastic insert. Two additional inserts with drilled holes were inserted into the open end of the cylinder to receive the single end of a bifurcated optical fiber cable. A near-IR LED was connected to one of the Y ends. The second Y end was connected to a photodiode and an audio amplifier. This ultra-simple arrangement worked quite well. It was not fully transparent, since the diaphragm film was aluminized, but with sufficient amplification the microphone will work without aluminization since the front and back surfaces of the film will together reflect about 8% of the oncoming light.