The Photophone

In Februrary of 1880, Alexander Graham Bell and his assistant Charles Sumner Tainter invented a device that allowed the transmission of sound via a beam of light. It was called the Photophone. Bell believed the Photophone was his most important invention! Indeed, 100 years later in the 1980s fiber optics came into widespread use. It is noted on a plaque that on June 3rd, 1880 the first wireless transmission of a telephone message using the Photophone took place. This year marks the 131st anniversary of this event.

Bell’s Photophone used crystalline selenium cells at the focal point of its parabolic receiver. My version uses a solar cell as the receiver. The transmitter is a mirror attached to a speaker aimed at the solar cell. The mirror is aligned so that it reflects the Suns rays onto the solar cell. The solar cell is mounted at one end of a black 4″ ABS pipe about 2 feet long. The light reflecting off the mirror is modulated by the vibrating speaker. The Solar cell receives this modulated wave of light and outputs a corresponding modulated voltage that can be fed into an amplifier and output through a speaker to produce audio. A 0.1 uF capacitor is connected in series between the solar panel and the amp.

The whole project is quite simple and fun to build. Enjoy the video and.. as always, Keep On Hackin!

5 thoughts on “The Photophone

  1. If you had described this idea to be before you built it then I would never have believed it would work. If there was an award for “Cool shit you can build in 10 minutes with $10 of parts” then this wins it.

    And it seems to cope best with higher-frequencies, judging by the rather good sound transmission of the Doors song. I reckon it would be even better if you use a smaller mirror because there would be less inertia for the speaker coils to overcome and the reflected light beam would be narrower thus amplifying the signal seen by the solar cell.

    And I never knew solar cells had such a fast response time. This reminds me of a laser microphone which uses very similar techniques to your photophone.

    Impressive.

  2. Hate to nit-pick, but just one correction… in your video you say that it’s a 0.1uF cap in parallel on the solar cell, but it’s in series (you have the text above the video correct saying it’s in series).

    Other than that, great project! I agree with you that Forrest Mims has been an inspiration to many in the way that he presents things where most anyone can understand.

    • Thanks, good catch. I’ll put an annotation on the video.. meant to say series… DOH!

  3. Not really relevant to the classical photophone, but if you use a laser connected to the audio source you will get a much clearer sound at a further distance when shined onto the solar panel.

    Nice job though.

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