DIY Photophone (transmit Sound Over a Beam of Light)

Hello everybody, hope you're doing well

in this instructable I'm gonna show you how to make a photophone with a slightly modern twist, I'm also gonna show you how to send sound via light and how to communicate using light. And all of this portrayed here in this instructable can be done with simple household materials and in a few different ways, so that's great as well.

The photophone was invented by Alexander Graham Bell and his assistant Charles Sumner Tainter in 1880. Unlike a regular contemporary telephone, photophone used modulated light as a means of wireless transmission instead of modulated electricity carried over a conductive wire.

This is an easily reproducible experiment/project to do with kids and also to explain to them more about light, wireless telecommunication, and optics in physics.

Here is a Bell's description of the light modulator:

We have found that the simplest form of apparatus for producing the effect consists of a plane mirror of flexible material against the back of which the speaker's voice is directed. Under the action of the voice, the mirror becomes alternately convex and concave and thus alternately scatters and condenses the light.

And also don't forget that this is a telecommunication device maybe not that practical nowadays, but still a great experiment, make two of these and you got yourself a two ways communication. I will also share with you some tips and tricks on how to check if everything works properly and do some troubleshooting if you decide to give this project a go yourself.

For transmitter:

Speaker

Selfie stick

Popsicle stick

Small mirror

Amplifier

Switch

Few screws

Duct tape

Batteries (power supply)

Small L bracket

Some kind of board to mount all of these components on.

For receiver:

Solar panels

Cardboard

PVC pipe (or some kind of long tube)

Speaker

Amplifier

Tools: Screwdriver, pliers (wire cutters), soldering iron (as well as some solder), utility knife, multimeter (optional).

Ok, what do we want to achieve here?

You are searching for a speaker and a mirror, a small makeup mirror should do the trick. I would suggest you to first get a mirror and then a speaker. So the mirror needs to be positioned in front of the speaker, on the photo above you can see several different speakers, and while all of them should work fine for this project the one on the left is ideal because the mirror simply drops in place without touching the speaker cone, so the speaker works as intended and mirror is close to the sound source. In my case, it was a lucky coincidence, but don't worry if, for example, you have a square-shaped mirror or different dimensions speaker, the main goal here is simply to position the mirror in front but still really close to the speaker, and there are many ways you can achieve that so just be creative :D.

In this step attach the mirror to the speaker, as I mentioned before that the mirror just drops in place in my case so I used a bit of hot glue to secure it.

After you glued the mirror to the speaker use 1/2 of a popsicle stick (cut popsicle stick in half) to attach a speaker to the top of a selfie stick. The speaker that I used here had a hole in the magnet on the backside so once again that was a lucky coincidence. In my case, this was an easy way to attach a speaker to the selfie stick.

It may seem pretty unusual at first to use a selfie stick, but as it turns out it is a really easy way to make the speaker and mirror move up and down. These were popular back in 2014 so maybe you have one of them laying around collecting dust.

Note that for this project the most important part of the selfie stick is a stell monopod.

Why use a selfie stick here in this project?

Since this is a telecommunication device selfie stick makes it easy to move the speaker and the mirror up and down so the sunlight could hit a receiver (reflecting from the mirror).

It depends where did you get your speaker, for example, if you reused your speaker from old desktop speakers you should be able to reuse the amplifier as well and if that is the case skip this step. However, if that is not the case you still have few options and all of them are perfectly valid, for example, you can make your own amplifier (which I will show you later in this instructable), or you could use one of those amplifier boards.

Unlike the original photophone, this one uses sound that is coming out of the speaker, so for this to work you need an amplifier. Since there is only one speaker used here you need a mono amplifier, although if you are reusing an older board you could either use only one channel or maybe even bridge that amp.

If you are using one of those mono amplifier boards follow this procedure:

If you look at an amplifier board you should see some letter s, they tell you what goes where.

IN stands for input (this is an audio input, this is audio that photophone should transmit later)

5v +/- stands for power supply input,

OUT stands for output (this is where the speaker connects to) In my case for the power supply I used 4 AA batteries placed in the battery holder, there are two wires coming out of the battery holder, one of them (it isn't important which one, although polarity matters so don't reverse it) goes to the board, and the other one goes to the switch and from the switch to the board, and that is it for power supply:D.

For the audio input however you would need to convert stereo to mono (if for example you're gonna use your smartphone to transmit audio), but if you're gonna use something like a microphone you could skip this part.

If you're planning on using your smartphone (or something else with a 3.5 mm audio output) as an audio source you're gonna need a 3,5 mm audio jack (you can even recycle one from old headphones). Now if you're using old headphones follow this procedure: first cut them at a desirable length and remove isolation. After that you should find 4 wires, these wires have isolation on them and the easiest way to remove that isolation is simply to burn it off with a lighter. So one quick stroke over the flame should do the trick. At this point, you should have 4 wires with isolation removed, twist together 2 of those that are the same color, and connect 2 100 ohm resistor to the other 2 wires ( 1 resistor at each wire), then twist other ends of resistors together and that is it :D.

And if you're not using old headphones, but rather an audio cable with a 3.5 mm connector at each end follow this procedure: first, cut your cable at a desirable length and remove isolation. After that unlike before in this case you should find 3 wires, and you need to discover which one is ground. You could get this by trial and error or an even better (and easier) way would be to use a multimeter (continuity tester more precisely). If you're using a multimeter simply turn the knob to a diode symbol and hook up one lead to the ground of the headphone jack and the other on one of those 3 wires (one by one) when you hear a beeping sound you found your ground wire. And like before connect 2 100 ohm resistors to other 2 wires ( 1 resistor at each wire), then twist other ends of resistors together and that is it.

At this point you connected all of those components that are needed to make the transmitter, now the only thing left is to mount them on the board, after that the transmitter is completed.

I hot glued the battery holder and a switch, and also for a selfie stick I used a small L bracket and few more screws for extra support.

Make sure to keep long wires because the mirror and a speaker should move up and down on a selfie stick.

The original photophone receiver used a simple selenium cell photodetector at the focus of a parabolic mirror, here we are going to use solar panels (you could use one, but in my case, I used two connected in series).

If you are using more than one solar panel start by connecting them in series (+ of one solar panel goes to – of the other).

To prevent any extra light that could interfere with audio reproduction we are going to place the receiver in a pipe (or some kind of box). Start by tracing a pipe on a piece of cardboard and then cutting it out. After that hot glue, the solar panels somewhere in the middle then use some tape to glue it to the pipe. Use some kind of tape to attach cardboard with solar cells to a PVC pipe, but make sure to cover all holes to prevent additional light from coming in.

For this project to work we need two amplifiers and two speakers (in the case of a one-way communication). Now you could use more or less the same setup you used when making a transmitter, and while that would work great I'm not going to do that here because I'm gonna show you how to make an amplifier, so you can see that there are a lot of good ways on how you can make a fully functional photophone.

To pull off this build with easy-to-find materials I used a good old LM386 IC.

Supplies :

LM386 IC

Capacitors: 470n, 100n, 47n, 33n 220µ, 10µ

Resistors: 10k, 10

Speaker,

Wires,

Potentiometer (optional),

Some kind of a box (here I used an electrical junction box)

Start by placing IC somewhere in the middle of a PCB board and solder it from the other side of the board. Follow the schematics and do the same with other components.

While making this try to place all electronics components as close as you can to the IC itself (especially capacitor that goes to pin number 6 of LM386 IC) in the case of an LM386 this is important.

And also I made a short and simple circuit on a breadboard to test it out. That circuit works, but with just a few more components it works even better and it is simply more reliable.

This step is easy if you did previous steps, in this step connect the - from the solar panel to the - of an audio amplifier (at the input) and connect the + of the solar panel to the 1,5μF capacitor (this capacitor is here simply to filter out any DC that's coming out of the solar panels so if you don't have a 1,5uF capacitor use a smaller value capacitor but don't use electrolytic ones) and connect another end of a 1,5uF capacitor to an input of an audio amplifier and that is it.

Well, there is a really easy way to test this type of receiver, and that is simply to take a TV remote and point it into a solar panel and press some button on it (also don't forget to turn ON an amplifier), after that if everything works properly you should hear some ttt-ttt-ttt-ttt sound. Note that we as humans can't see IR light (IR LED is found inside of a TV remote) but a camera can so that is also a neat little hack to check if batteries inside of a TV remote are still working.

This is an experiment that is pretty much the same in its concept as a photophone, but here I'm gonna show you how to use an LED to transmit audio instead of that mirror and a speaker setup as seen in steps above of this instructible.

So essentially receiver stays the same, but the transmitter is a bit different.

Supplies for a transmitter

LED,

Transformer (audio transformer is ideal for this experiment, but that one is a bit harder to find, although a regular transformer from old power plug wall adapter should do the trick)

Batteries (and a battery holder)

Audio source (for example smartphone)

3.5 mm audio jack

You could get a transformer if you have some of those old wall plug adapters simply cut open the casing (if there aren't any screws holding the case together, if there are unscrew those) and get a transformer. Note that you may find some additional circuit, that circuit is called a diode bridge (Graetz circuit) or a full bridge rectifier, we don't need that for this project, so if you do this you are only looking for a transformer.

And if you're not sure whether there is a transformer inside those power adapters, you could feel it by weight, because transformers are made mostly from iron and copper so they are heavier than those other SMPS-s (Switch Mode Power Supply-s).

Start with a transformer, connect one side (two wires that are coming out of the transformer) to the audio jack I showed you how to do this in previous steps, and one wire on the other side of the transformer goes to one wire of the battery holder, from battery holder to the LED, and from LED to the other wire of a transformer, and that is it. And also if your LED doesn't light up try changing her polarity.

The transformer is a device that transfers electrical energy from one circuit (primary winding) to another (secondary winding) through the process of electromagnetic induction.

So what's happening here is that when the sound changes the current on the primary side of the transformer it changes the current on the secondary side as well, which means that the sound on the primary side of the transformer modulates the output of the LED. That modulated light hits the solar panels (which are inside of a PVC pipe), and then the signal that is coming out of the solar panel goes through a capacitor into an amplifier where it gets converted back to sound again. That is how photophone works! So for example you can hear light that is coming out of IR LED that is inside of a TV remote. And if you're wondering how the first transmitter works (the one with a selfie stick) it works on exactly the same principle, the difference is that the light (sunlight that is reflecting from a mirror) gets modulated when the speaker vibrates. So actually mirror is vibrating and the light reflecting from it gets modulated. The first transmitter has one obvious flaw which is that it doesn't work on a rainy day, since there is no sunlight that will reflect from a mirror into a receiver. In the first video I demonstrated this principle, but the sound coming out from the speaker is a bit low (although it is still hearable), so in the second video I added one really simple preamplifier that I put together on a breadboard. On the video, you can hear that there is no audio being played when I block the light that is coming out of the LED to hit solar panels (which are inside of a PVC pipe), although there is still a bit of static sound which is coming out of the solar panels. If you want this to work at a greater distance try using a laser instead of a LED.

And if you're wondering how to set up a preamplifier, this is how it goes: at the input of a preamp connect solar panels, and the output of the preamp goes to the input of an amp, and that's it!