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