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Jameco Electronic Puzzler Solution: Minimalist

By Forrest M. Mims III

How did consultant Ed Brown solve an assignment from the Minimalist Electronics Society?

Engineer Ed Brown was assigned to design a minimalist voice intercom that would link the two tiny structures that served as the offices of the Minimalist Electronics Society.

Minimalist Intercom
Figure 1. The two offices of the headquarters of the Minimalist Electronics Society need a minimalist intercom.


The solution had to be based on minimalist electronics. Thus, tin cans linked by a string or a speaking tube made from a garden hose were not eligible. Standard electronic solutions such as a two-conductor or power line intercom or a cell phone or radio link would not meet minimalist specs. The dry, rocky soil precluded a single wire link with an earth ground at each end.

Brown considered an optical link with an LED and silicon photodiode serving as, respectively, a source and detector at each end. But this, too, would be too complex to meet minimalist specs. Brown was in a quandary until he remembered reading that a light-emitting diode can both emit and detect light. This meant he could design a lightwave communicator that would provide a 2-way intercom with only a single dual-purpose light source and detector (an LED) at each end of the link. Figure 2 shows how this solution could connect the two offices with a single optical fiber.

Connecting the Offices
Figure 2. Connecting the Minimalist Electronics Society offices with a single optical fiber having a single dual-purpose (send-receive) LED at either end of the link.

The Minimalist Electronics Society liked Brown's approach – but they preferred a full-duplex intercom rather than a half-duplex link in which only one party could speak at a time. Brown explained that while full-duplex is more convenient, half-duplex is half as minimalist as full-duplex. This logic prevailed, and the society voted 12 to 0 against the half-duplex solution, which meant that the half-duplex idea was unanimously approved. (In the the Minimalist Electronics Society, the minimum number of votes always wins. Thus, those in favor of a proposition always vote against its passage.)

Brown's minimalist intercom would work through free space or a single strand of optical fiber, and each method had minimalist issues. In the free space mode, airborne dust would scatter a good many photons, thus compromising minimalist ideals. An optical fiber would add a component while better fulfilling minimalist ideals by confining their communications inside a fiber instead of broadcasting them over stray photons. The minimalists selected the fiber method by voting 10 to 4 against it.

Circuit Block Diagram
Figure 3 is a block diagram of the circuit at each end of Brown's minimalist intercom. The system works best with either a red or near-IR AlGaAs LED connected as a dual-purpose emitter-detector. Ordinarily both sides of the system are in receive mode. A push-to-talk switch allows either side to begin talking.

A prototype was assembled and tested. Standard optical fiber connectors were not sufficiently minimal for Engineer Brown, who became a dedicated MES member while working on this project. Instead, he bored small holes into each of two super bright red LEDs. He then inserted one end of the fiber into each hole and cemented them in place. Instead of soldering the LEDs into the circuit at each end of the link, he inserted the LED pins into the fully recyclable, reusable – and therefore minimalist – solderless breadboards on which he assembled the two circuits.

Going Further

Should your chapter of MES want to duplicate the headquarters minimalist intercom, you might want to consider going free space instead of fiber. (But minimize your communications with headquarters and don't tell them). For best results, mount a suitable lens before each LED to both focus its emission into a narrow beam and collect the IR from the opposite LED. The two lenses will need to be pointed exactly at one another, so it's best to use red AlGaAs LEDs in order to see the beam. This greatly simplifies aligning the system.

BACKGROUND

For many years I've experimented with and written about the dual nature of many solid state devices that can both emit and detect light. During high school in 1962, I found that a cadmium sulfide photoresistor designed to detect light would emit a soft green glow and bright green flashes when excited by an automobile spark coil. On March 14, 1966, I connected a 2-transistor pulse generator to a silicon solar cell. An identical solar cell nearby detected infrared pulses emitted by the first cell and converted them into a pulsed photocurrent that was amplified and transformed into an audio tone by a simple amplifier. In 1969 and again on October 18, 1972, I used an IR image converter to observe IR emission from forward biased silicon diodes (1N3070 in the latter experiment). During 1972-73, I experimented with a wide variety of light wave communication systems in which a single LED served as a dual-purpose light emitter and detector at each end of both free space and optical fiber links. On February 19, 1980, exactly 100 years after Alexander Graham Bell and Sumner Tainter used Bell's Photophone to become the first people to send their voices over light, my wife Minnie and I joined representatives from the Smithsonian Institution, the National Geographic Society and Bell Laboratories at the site of Bell's former laboratory at 1325 L Street in Washington, DC. Our purpose was to commemorate the Photophone Centennial, an event I had proposed to Melville Bell Grosvenor, Bell's grandson, during a visit to his National Geographic office in 1977.

While my wife Minnie took pictures, I sent my voice to the others over a beam of sunlight reflected from a homemade photophone made by cementing a thin circular mirror to one end of a short aluminum tube. The modulated sunlight was received and converted back to voice by a silicon solar cell connected to an audio amplifier, (Bell used a homemade selenium photocell connected in series with a battery and a telephone receiver). We then brought the occasion up to date by sending our voices both ways through a 100-yard spool of optical fiber with a red LED at each end of the fiber–much like the system proposed here for the Minimalist Electronics Society.

More details about the use of LEDs as dual-function emitter-detectors can be found in references 1-7.

REFERENCES

1. F. M. Mims III, Bidirectional Optoisolator, Electronics, 127, May 10, 1979.

2. Ibid., Experimenting with a Two-Way Optoisolator, Jameco

3. Ibid., Light Emitting Diodes, Howard W. Sams & Co., 1973, pp. 118-119.

4. Ibid., Communicate Over Light Beams with the First Single-LED Transceiver, Popular Electronics, March 1976, pp. 66.

5. Ibid., Using LEDs as Detectors, Modern Electronics, February 1988, pp. 62-68.

6. Ibid ., Sun Photometer with Light-Emitting Diodes as Spectrally Selective Detectors, Applied Optics, 31, 33, 6965-6967, 1992

7. Ibid., LED Sun Photometry, Optics & Photonics News 20, 32-38, 2009.