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.
- Laser Module
I used a 5mW 650nm laser module, purchased from eBay. It only cost me $2.00!
- 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.
- 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.
These are dirt cheap – only a couple of cents.
- 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.
*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:
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
- I was inspired to make this thanks to Free Info Society – thanks!