[I made some changes to also support a microphone]

In this tutorial, I will demonstrate how audio (music or sound) can be transmitted over a light beam generated by a LED. This project includes 2 parts:

The transmitter, that takes the audio and modulates it using the LED
The receiver, that reconstruct the audio from the LED's light

This method and project can be expanded to a LED bulb as well.

How does it work?

The input audio is amplified and then modulated using a LED by changing the current that lights the LED based on the audio wave. The receiver translates the light into voltage and reconstructs the audio wave.

Part List

R1 = 100 Ohm
R2 = 1.5K
R3 = 3.3K
R4 = 1.1K
R5 = 1.1K

R6 = 10K (required only if you use a microphone)

C2 = 100 nF (104)
C3 = 10 uF
C4 = 10 uF

D1 = 1N4733A or other

Q1 = 2n222A

L1 = Basic LED (any color)

Electrect microphone OR a mono audio cable

Solar panel Speaker with amplifier

Step 1: Building an Audio Amplifier

The audio amplifier circuit is based on the LM386 audio amp chip. Vss (source voltage) is 9V although the 386 may require less depending on the IC type (see https://en.wikipedia.org/wiki/LM386). The audio source can be either an electrect microphone or a mono audio cable (3.5mm) that connects to a phone / PC / radio audio out jack.

The blue part in the schematic diagram is required only if you use a microphone.

Step 2: Step 2: Building the LED Modulator

The transistor and its resistors were selected so that the LED would have sufficient current to be turned on all the time (~20mA) while the audio is modulated over this current, causing it to fluctuate. R2 and R3 are acting as voltage dividers (so that the voltage on the emitter (and collector) is around 2.5V.

Step 3: Step 3: the Receiver

The receiver is composed of a solar panel (in which light is converted into voltage) that is connected to a speaker with an amplifier (otherwise the audio will not be heard). Alternatively, you can build another audio amplifier (with LM386 IC) and feed it with the solar's voltage. In that case, connect the output of the amplified audio to the speaker (make sure the solar panel's output voltage is not high enough. If so, use a voltage divider to bring it to a reasonable level suitable for the amplifier circuit). It's very important to filter the DC voltage received from the solar panel by putting a small capacitor (104 will do) in series with the solar panel's positive cable, before you feed it into a speaker or another amplifier.