By Aditya Gautam

Transmitting music wirelessly is not nearly a new thing, and while their are somewhat  more practical ways to do it, there’s something damn cool about doing it using a LASER beam.

Not to mention, it’s way simpler than designing a transmission system based on radio-waves, modulation, demodulation, et al. And by simple, I mean really, really simple. Using mostly stuff you’ll easily find around, or over-the-counter in departmental stores. And on the cheap. Probably the single most expensive thing in the setup is the LASER pointer, which you might already have.

Basically, we’re amplitude modulating the LASER beam in accordance with music on your music device. Think you’re great at amplitude modulation, modulation, demodulation techniques, coherent, square law detectors et al? Great. We’ll be using none of that. This uses a very simple circuit, and as long as you can assemble a circuit as per the schematic – although I’ll be walking you through the steps – you’re great to go!

We’re going to be needing a LASER pointer (duh), cells, a potentiometer (47K or 22K should do fine), some wire (wires stripped from an Ethernet cable would be the best), a toggle switch, an audio transformer (one salvaged from an old tape deck/hi-fi system/TV/flicked from your school laboratory should do fine), and a 3.5 mm audio jack.

For the receiver, we’ll be needing a phototransisitor, another 3.5 mm audio jack, a magnifying glass, and a high-gain amplifier (a laptop a mic input or a mic with preamp plus the amp, or a guitar preamp plus a speaker system, or basically any thing that comes with a mic input and speakers should do fine)

I suggest your salvage as many things as possible from old rusty electronic gadgets around your house, flick the rest from your school or college or laboratory. Buy these only if you think it’s absolutely necessary, and if it is worth it .

In addition to all these, you might also want to use a wire cutter and stripper, some insulating tape, soldering iron and solder, a breadboard, alligator clips, and a friend to help you out, though none of these are absolutely necessary. Except maybe the friend, ‘coz it’ll be great fun, then.

So, if you’ve got the things ready, or just want to read this, let’s move on. A disclaimer before you get started, though.

This workbench exercise includes the use of LASERS. Now while diode LASERS used in LASER pointers are relatively safe, do not point it at your eyes. Not even for fun. Take it from someone who’s been lased in the eye. Feels like phosphenes on steroids, for hours together!

Another thing, I’d like to add: while you might as well just connect wires by twisting their ends together, I suggest you use a breadboard, especially if you’re an electronics hobbyist or junkie. A breadboard is to an EE what a slate is to a kindergartner. Plus, it makes everything much more simple, and makes reusing possible, and easy.

So now, to get started (no, actually)

Firstly, we need to modify the LASER pointer. The button cells are probably not going to be sufficient, so we’ll be replacing them with normal AA cells. So, first see what’s the operating voltage of the LASER. It usually should be around 3 to 6 volts. So get as many cells, preferably with a battery compartment/holder, so that its leads would be more convenient to use.

Next, get two leads running out from the positive and the negative battery terminals of the pointer. Connect the wires to the terminals using the alligator clips, or clear tape, or solder them.

And then, we’re going to be externally switching the LASER ON and OFF, so we need to ensure that the local switch on the pointer is always ON. you could do this by simply fastening a tape around the switch, ensuring it is depressed. A slightly more preferable method, in case it’s an old LASER with no use), to hack it open, and short the terminals across the switch, so that it’s always ON.

Next, assemble the circuit as per the following schematic:

                                                                

View the schematic in full size

Here’s a brief explanation of the setup: To start with, you should do well with any audio transformer, but its the all the more suitable if you use a 8 ohm – 1K-ohm transformer, as its the most easily available audio transformer, as well as the most preferred and widely used audio-transformer, but then again, as I said, any audio-transformer should do just fine.
Connect one of the ends on the 8 Ohm side to the negative terminal of the battery compartment, and also to one terminal of the pot. Connect the other end of the pot to the one terminal of the toggle switch, and the other end of the transformer, on the same side. You could also connect it to the centre tap of the transformer on the same side, as shown in the figure.. Connect the other end of the toggle switch to the the negaive terminal of the LASER. Connect the positive terminal of the LASER to the positive terminal of the battery. This completes the circuit on the primary side of the transformer. On the secondary side, the connections are easier. Connect one end of the transformer to the left terminal of the 3.5 mm jack, the other to the right terminal of the jack. Leave the centre tap terminal of the transformer alone. If your jack comes with a ground terminal as well, just short it with the left terminal of the jack. As simple as that.

The receiver end’s as simple as the secondary side of the audio transformer. Connect the collector of the phototransistor to the left terminal of the audio-jack (shorted with the ground terminal, if it has one), and the emitter terminal to the right terminal of the right terminal of the jack. Done!

That’s it!
Now position your transmitter and receiver across a distance you want to transmit (you can position them as much as a kilometre apart, with excellent quality). Connect the audio jack on the receiving end to the amp. Note that if your pre-amp has a 6.25 mm port, then you’ll have to use a 6.25 mm audio jack on the receiving end instead of the 3.5 mm jack. Now align the transmitter and receivers so that the LASER’s falling positively on the receptor of the photo-transistor. Use the magnifying glass to focus the LASER on to the receptor. Connect the jack on the transmitting end to your iPod, mobile-phone, computer, or anything that can output music through a 3.5 mm jack. Turn up the volume on your music player and the pot, play some bassy music, and stream it across on a LASER beam!

This kind of setup is ideal when your source of music is  on end, and the the listener/speaker at another end, and you don’t want wires all over the place (although you could easily hack this to transmit it through an optical fibre cable).

So what exactly are we doing here? Basically, The transformer modulates the power going to the laser. The signal from the radio is added to and subtracted from the battery power, and the laser gets brighter and dimmer along with the volume of the music or voice in the signal. That’s all the rocket science involved in this!

This workbench exercise is originally the idea of user navaburo, found on instructables.

Head over to his blog and check out some cool electronics and Linux hacks!

So what next?

Well, this is a relatively simple and basic setup for streaming music over a LASER beam, which pretty much any one who can read a simple schematic can pull; for a slightly more advanced version of this, one with much enhanced fidelity, I’m working on a variant of this – except that it’s all digital, so instead of varying the intensity of the LASER, it’s all going to be terms of ON or OFF. So while it does the same thing, it’s actually the whole thing redesigned from near-scratch. Right now, its all in my head now, I’ve got to figure and work out on a few kinks – I seem to have ironed out most, as of now, but I’ll try to still work on it. So in the event that it does work out, I’ll push it here, so do come back and check! See you till then!

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