The photophone is an easy to make system for transmitting sound using sunlight that was invented by Alexander Graham Bell back in the 1880s. One side transmits sound using sunlight and the other side receives the sunlight and turns is back into sound.

You can see and hear it in action in the video below.

Receiver on the left and transmitter on the right.

Receiving sunlight and turning it back to sound.

Transmitting sound via sunlight.

How does the photophone work?

As shown below, to transmit it requires a sounds source, a speaker and a small mirror. The sound source is a radio in this example, but it can be anything, even an iphone, as long as you plug an amplifier into it so that you can play the sound to a speaker. As the sound plays, the speaker vibrates, making the attached mirror vibrate too. An easy place to get the small round mirror is to take one from a woman's compact mirror normally used for checking and fixing makeup. Cheap compacts can be found at dollar stores.

Photophone transmitter.

Closeup of the mirror on the speaker.

To receive it requires a box, a solar cell, a capacitor, an amplifier and a speaker, as shown below. The box can be any longish tube. Its purpose is only to keep extra light from getting to the solar cell. Ideally only the transmitted light would get to the solar cell. I also added a 22 nanofarad capacitor (22nF or 0.022 microfarads or 0.022uF) but any capacitance in that range will work as long as it's not an electrolytic capacitor.

The amplifier I used was my homemade amplifier in a jar but you can also use any commercially made amplifier such as a small portable one or one built into some PC speakers.

The diagram below shows how it works.

1. The sound source plays music or anything else to the speaker.
2. That makes the speaker vibrate, that's how speakers produce sound. The small mirror is attached to the speaker and so it also vibrates.
3. Sunlight reflects off of the mirror and goes to the receiver. Since the mirror is vibrating, that causes the intensity of the light arriving at the receiver to fluctuate in time to the music.
4. The receiver is a box with a solar cell in it. Solar cells turn sunlight into electricity. Since the incoming sunlight is fluctuating in time to the music, so is the electricity that's produced by the solar cell.
5. That fluctuating electricity is input to the amplifier which makes the electricity stronger.
6. The end result is loud music coming out of the speaker at that end.

The capacitor is there just to help block any electricity that the solar cell is producing that has nothing to do with the fluctuating sunlight, the DC electricity.

How the photophone works.

Video - Transmitting Sound using Sunlight - The Photophone

The following short video shows how to make the photophone along with a demonstration of it in action.

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