Transmitting Audio Wirelessly Through Light

I love listening to, and playing music. I also enjoy playing with lasers, and using wireless technology! Why not combine these geeky interests to make my own wireless audio transmitter? You can wirelessly transmit music/data using all sorts of other technologies, but this project will utilize a portion of the visible light spectrum through a laser. This is a perfect project for kids wanting to clinch a science fair ribbon too. Wow the judges by using a laser pointer to transmit your favorite song from across the hall! I ran tests in the video at a short distance, but it works flawlessly for at least several tens of meters. If you want to know how the system works, scroll down to the Explanation section.

Transmitting Audio Wirelessly Through Light from Treehouse Projects on Vimeo.

Resources Required

  1. Laser Module
    I used a 5mW 650nm laser module, purchased from eBay. It only cost me $2.00!
  2. Audio Output Transformer
    I used a 8 ohm : 1K ohm transformer, also purchased from eBay. It cost me roughly $6.00, but you can also purchase the same thing from Radioshack (part #273-1380) for less.
  3. 3V & 1.5 Power Sources
    The 3V will be needed for the laser pointer, and the 1.5V for the headphones. I ended up requiring slightly more to get everything working better (~3.3V & ~2V respectively). Just use AA batteries if you don’t have a lab power supply.
  4. Photoresistor
    These are dirt cheap – only a couple of cents.
  5. Pair of Headphones and Audio Source (i.e. mp3 player)

Although things don’t have a tendency to always work out right the first time, if all goes well this project should take 1-2 hours to setup, and about $10 in materials.

Images

audiolight audiolight
audiolight audiolight
audiolight audiolight

Process

The process is actually best described with a simple circuit diagram. Let me know if you have any questions.
audiolight
Explanation

*Jan. 2013: Note that I have realized that my explanation is not entirely accurate. I sincerely apologize, and will be updating this post with a more accurate explanation soon. Again, I sincerely apologize, I’m learning too :)

I’ve been waiting to make this for a while, and as you may know, I like to know how things work and just not how to use them. So let’s put that notion to use here. It may seem odd that you can transfer music/audio using light, but the easiest way to think about this is to consider the audio being transmitted as just information – raw, hard, unforgiving information. It is simply because we know the data represents meaningful sound, that we attach a device (i.e. speakers/headphones) which vibrates in response to the waveforms of data being received to create the sounds we want to – and can – hear. We know we should interpret the data as audio, and not something else. As an aside, an electromagnetic microphone (most common one), works in the exact opposite manner. Theoretically, you can attach any signal source to a speaker – it just may not sound like music, or anything at all. In fact, that’s what an oscilloscope does; but I digress.

To clarify this early on, this project is dealing only with an analog input source. So we know my mp3 player transmits analog data, and the analog component of the waveform has to be either the voltage, current, or resistance (max two). This means that one or two of the three variables fluctuates to create the waveform. From my research, I have found that for my case the voltage and current are the analog components – which makes sense when I explain what the audio output transformer (yellow cube in the transmitter circuit) is meant to do. The famous equation V = IR can thus be re-arranged to R = V / I. Notice that the fluctuating voltage and current that I just mentioned results in the resistance of the load changing. The load is usually your pair of headphones, but for this project, it is a laser. As you may imagine, a laser requires more power than a pair of typical headphones to operate, which is precisely why we need the audio output transformer.

You may be familiar with voltage transformers: they step down or step up the voltage depending on the number of turns on the primary and secondary coil of the transformer. You find these transformers in most of your electronic devices which you plug into your wall sockets. An audio output transformer is the same thing, except it steps up/down the resistance instead of voltage. So why would we need this? Recall that the laser pointer requires more power than a pair of headphones, and my mp3 player isn’t meant to have laser pointers plugging into it ;) To account for this, we need to make sure that the resistance signal is strong enough to impact the intensity of the laser, the same way it would impact the intensity of the vibrations in a pair of headphones. See the diagram below for an analogy:

audiolight

What this horrible drawing is trying to illustrate is the resistance of the circuit. The above stream of water is much smaller, and therefore requires a smaller dam (resistor) to block (or at least slow down) the flow of water (electrons). This wave represents the raw output from the mp3 player intended for the headphones. Now, the wave below is much more powerful, and the same tiny dam (resistor) would impact the flow a lot less. This is what the second wave represents: the laser. So the solution is an audio output transformer, which brings the signal to a level which will have an impact on the larger wave.

The rest is simple. The laser is now transmitting the same signal by varying in intensity based on the signal it is now receiving thanks to the transformer, but this happens so fast that it looks like the laser’s dot is not changing at all! Talk about cool! The photons from the laser hit the photoresistor on the receiving end of the circuit, and converts that energy back into a corresponding resistance for the headphones. Things don’t seem so much like magic anymore, eh? Let me know if you have any questions :)

References

Posted in Experiments, Projects
30 comments on “Transmitting Audio Wirelessly Through Light
  1. superkittens says:

    very cool! “De-magicking” makes things so much more interesting.

  2. Ahmed Al-Amir says:

    This is really great. I cannot wait to try it out. Keep up the good work.

  3. SpK says:

    This is an awesome demo project, kids love it and adults can’t believe what they’re seeing. Use two transmitters with different audio sources to show that the beams are not just turning the output on and off (this is how the viewer’s mind typically explains what they are seeing), they are actually carrying the information.
    If anybody even remembers AM radio anymore, this project works on exactly the same principle (amplitude modulation), just using a laser beam as the carrier instead of a fixed-frequency radio carrier.
    There is another description of this principle at:
    http://scitoys.com/scitoys/scitoys/light/light.html.
    This link describes another very easy way to build the receiver, by using a solar cell instead of a photoresistor (instead of the radio or amplifier, you can use an amplified speaker like the kind you use with a desktop computer). In this case, no other power source is needed on the input (but you still need to power the radio/amplifier/speaker as usual). Fluorescent lights will cause a hum, and camera flashes will make a loud unpleasant snap (just like engine noise used to be a problem for AM car radios, and why FM is so much cleaner), but daylight and incandescent sources should not cause any interference (the DC offset they produce may or may not do some damage to the radio/amplifier/speaker over time though, I haven’t had a problem but YMMV).
    I would highly recommend using the bipolar LED for protection at the transmitter, as described in the scitoys link, especially if you use a very cheap (dollar-store) laser pointer. These cheap laser pointers have no protection for the laser diode against static electricity. That’s OK when everything is enclosed, but to build this project you have to open up the pointer to access the power terminals. My experience has been that if you happen to touch any bare metal in the laser power path while the laser is being powered, chances are that the residual static charge on your body will instantly destroy the lasering ability of the laser diode, turning it into a very weak and useless LED. Maybe this problem does not occur if you are using a higher-quality enclosed module as shown here, but we destroyed a lot of cheap laser pointers before we discovered this tip.
    Thanks for posting this great project. People’s mouths literally fall open when they see it working.

    • Awesome comment, I learned a lot from it! The solar cell is a really cool idea, I might try that.

      Hey, I wonder what the sun sounds like? What if I pointed the solar cell towards the sun, and listen to it in the exact same manner? Do you think I would hear anything? I think I should be able to pick up noise, but perhaps not understand its significance.

      Your bit on AM and FM got my gears turning too. I really wish I had an oscilloscope, hopefully someday soon – they are very expensive.

      Interesting you mentioned the sensitivity of laser diodes to static electricity. I think I actually destroyed one that way, and thought it was faulty. It works just like an LED now.

      Thanks for your great comment, this is exactly what I was hoping for when I started this blog :)

      • superkittens says:

        You wouldn’t hear much if you put a solar cell to the sun and amplify the resulting signal…it would be more of a crackle. That was actually one of the things that really put curiosity into Carl Sagan as a kid when his parents took him to the World’s Fair.

        As for oscilloscopes, you’re right they are bloody expensive. You’re a student right? Just use the scopes they provide in labs – that’s what I did. Put them tuition dollars to full use!

  4. Julian Perkins says:

    Hi, I am using this project for my science fair, I was wondering if you could help me a little on setting it up and finding/making the jack that you used for your MP3 device to plug into the breadboard.
    My science fair is coming up soon, and I have all the materials I will need (save the audio input). Thanks, Julian

    • Hi Julian,

      Sounds awesome! Great project for a science fair.

      So just to make sure I understand you correctly, you have everything else done, but are having trouble finding a headphone jack to plug your mp3 player into?

      If that’s the case, there are several ways around it. I will name a few ways for you:

      1. Buy a jack. Although I don’t buy from Sparkfun, but because you need the piece fast, you can order it from them. Here is a link to what you need If you have a local hobby shop, they are bound to have this as well.

      2. Buy a female 3.5 mm audio cable. This can be found almost anywhere, you might even have one lying around. It looks like this. You need to cut off the female end of it. Then, you strip the wires at the end of the cables, attach them to some terminal blocks or pin headers (like I did for the transmitting end of my project), and you’re done!

      3. Probably the cheapest way is to salvage a pair of broken headphones. All you need is the male headphone jack, cut with whatever length of wire you want at the end. The concept is exactly the same as #2, just strip the wires at the end, and connect it to your project.

      I always like salvaging parts. It will make your project look a lot more genuine too, as making use of what you have is definitely an impressive skill. It’s a lot easier to buy than it is to be creative. I don’t know if you watched the video, but I got my headphone jack from a sound card. Since I don’t have the time to desolder more, I ordered several online for only $0.12. That’s a great store for most of your electrical needs (shipping is also incredibly cheap), but your order can take up to two weeks to arrive.

      I hope that helps. If you need more detail/help, please let me know!

      Best of luck with your science fair project, I’d really like to know how it goes! :)

      • Julian Perkins says:

        Hi,
        First, Thank you so much for responding!
        So, yes that is the problem I’m having is actually plugging the MP3 into my circuit board. So what you are saying is that I can cut, (I’m using my iPod touch), a jack that plugs in where the headphones do and cut that, and strip it to fit in my circuit board socket? That is really the last part I need, I have all the boards, power sources, etc. that I need. Again, thank you very much! -Julian

      • Precisely. Look at my pictures, I have done the exact same thing. I stripped the cable, and attached them to some pin headers. On the receiving end, I could have done the same thing instead of using a jack, but just wanted to emphasize the point of being able to salvage useful components.

  5. Julian Perkins says:

    Ok, Thank you so much! You have really saved my butt. And if there is anything else i need help with, may I contact you again?

    • Awesome!

      And absolutely, contact me any time. Leave a comment, or e-mail me via the contact form on this website.

      If I had a penny for the number of times people I don’t even know have helped me, I’d be filthy rich :)

      So helping you is the least I could do, Julian.

  6. Peter Wexel says:

    Hey I am 16 and I am wanting to do this for a hobby but I am also looking into selling this to a couple of my friends is there anything I could do to make the quality of it any better than it already is just Not trying to make it like super quality but could this take a constant use on it? If you could email me your response that would be best for me.

    Thanks
    Peter

    • Hiya Peter,

      That’s great. It’s good that you’re getting into these things now, and looks like you have a hint of that entrepreneurial spirit in you too :)

      To answer your question, I am actually working on a way to make this project better and cheaper. I just have to find the time for it, and will update my blog when I’m done it.

      With regards to the quality, I’m not quite certain about what you mean. It works quite well, though the only thing is that with one laser you will only get mono sound. I don’t know if that is what you mean. Otherwise, as long as you have the laser pointed at the sensor, the device itself is perfectly consistent.

      Please let me know if I didn’t answer your question accurately.

      I have also e-mailed you this response, as per your request.

      Thanks.

  7. A.S.A says:

    Hey

    Thanks a lot for this great project, it’s really helpful and useful

    But can you please give me its applications in our daily life, I mean for example a certain instrument which use this concept

    Another thing, can you please give the circuit diagram, I mean the normal circuit diagram

    Thank you a lot again

    • Hi,

      My pleasure, and forgive me for my delayed response.

      The applications are numerous. As far as we know, nothing travels faster than the speed of light. Although we can’t process signals at light speed, if we can send them at light speed, it certainly makes communication and data transfer much faster, and reduces latency issues. My project obviously is not optimized for that, but it is just showing how photonics (a part of it at least) works.

      This kind of a solutions actually exists already. Have you ever heard of fiber optic technology? Well, this is pretty much the same thing. Only difference is that transmitting light in a straight line is impractical for the way our infrastructure is designed (i.e. optimized for flexible wires to carry electrical signals), so fiber optic cables refract light at certain angles inside the wire to mimic the physical properties which make wires convenient.

      I don’t have a “normal” circuit diagram for this. I mean, it is so simple. If you must have a circuit diagram, please let me know and I’ll try and find some time to whip one up for you.

      Thanks!

  8. Hi,
    this is very intresting to transmit the signal by using laser.
    i also want to imply this technique but i need of it’s circuit diagram. can you send me please.thank you.

    • Hi Sarfraz,

      Hmm, unfortunately I have not made a circuit diagram for this. If you really need one, let me know, and I’ll make one next week. But the circuit is actually extremely simple, and I think it is well described by the diagram.

      Nevertheless, if you still would like one, I can put one together. I just need some time as I am busy for the next few days.

      Thanks!

  9. bassam says:

    Hi ,
    I did everything that illustrated in the video ,but i have a little problem that the output volume is very law
    so, if u have any idea to make a higher volume i will be grateful.
    thanks .

    • Treehouse Projects says:

      Hi Bassam,

      At least you have it working! The first thing which comes to mind is turning up the volume on your input (i.e. mp3 player or whatever you are using). Try that. Make sure your output is not being supplied with too low a voltage either. Keep me posted!

  10. Pattaya kuruma says:

    can we make our camera on mobile act like the receiver in this experiment?

    • Treehouse Projects says:

      Seems possible (and certainly very interesting), but it would take a whole lot more work. The frame rate on your phone’s camera would probably be insufficient to pick up the audio signals, unless the sounds being transmitter were specifically designed to be simple enough for transmission.

  11. angelica says:

    Hi!

    I have the same problem. Everything works, but the output volume is too low. I have speakers for my output device but I have to literally put the speakers to my ear in order to hear the output. My cellphone, which is what I use for the input, is on max volume already. Also the voltage for the output is already at 5 volts.

    Thanks in advance.

    • Treehouse Projects says:

      Hi,

      Perhaps the voltage source is too low? What voltage are your speakers supposed to run at, how much power do they require, and is your power source sufficiently matching those requirements?

  12. Kaitlyn says:

    Do you have to use an mp3 player?

  13. ryan says:

    Hello, my son, Ryan, and I are doing your project for his school science project. Do you have a schematic of how all of the parts connect to the bread boards? There are a couple of additional pieces that we didn’t see on the parts list so if you had a more complete one that might help us as well. We aren’t overly technical so any additional assistance would be appreciated.

    Thanks,
    Stan and Ryan

    • Treehouse Projects says:

      Hi Stan,

      Hmm, I am quite certain that all the parts have been listed. The circuit is honestly as simple as what is shown in my drawings, there is nothing to add or remove from it to make it more complete. Could you please be a little more specific with regards to what you feel is missing? You could also send me pics of your setup that I can try and help you debug at contact [at] treehouseprojects [dot] ca. Thanks!


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  3. [...] someone else to listen in. Instead of sending radio waves bouncing all over creation, this uses the focused light of a laser to transmit audio. In the image above you can see the silver cylinder which houses the laser diode. It is focusing [...]

  4. [...] someone else to listen in. Instead of sending radio waves bouncing all over creation, this uses the focused light of a laser to transmit audio. In the image above you can see the silver cylinder which houses the laser diode. It is focusing [...]

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