Send Music over a Laser Beam

WARNING: this project involves the use and modification of laser devices. While the lasers I suggest using (store-bought red pointers) are relatively safe to handle, NEVER LOOK DIRECTLY INTO A LASER BEAM, BEWARE OF REFLECTIONS, and be EXTREMELY CAREFUL when MODIFYING a laser product. Also, I am not liable for anything stupid you do.

Here is yet another thing to do with those promotional laser pointers: send music (or data) from point A to point B over the laser beam using amplitude modulation. All it takes is pointing the moded laser at a detector, and music can be heard from an attached amplifier. The range and quality (or data speed) can vary, but I have gotten a HALF MILE of range with excellent audio quality and around 300bps of throughput.

The image shown here is the transmitter and receiver working across my desk during a test.

TO SEE HOW TO USE TWO LASERS TO TRANSMIT TWO MUSIC CHANNELS AND MIX THEM WITH A PAIR OF SUNGLASSES, check out the blog post here.

A video of the system working can be found here: http://video.google.com/videoplay?docid=6895048767032879458&hl=en

Much inspiration for this project came from http://sci-toys.com/scitoys/scitoys/light/light.html#laser_communicator

Step 1 Gather Materials

To send music over a laser beam you will need the following parts, most of which can be gotten for less than 5 dollars at radioshack total (besides the pointer, which probably costs $15). If you are on a tight budget, try replacing the laser with a red LED and a 100ohm resistor attached in series.

for the transmitter:
a laser pointer
batteries (D-cell work best)
potentiometer (variable resistor) 50k ohm or less
audio source (iPod, cd player, mic preamp, PC line-out, etc.)
some wire (cat5 aka ethernet cabling works best)
toggle switch (a turbo switch from an old PC works well)
audio transformer (can be pulled from audio equipment)
1/8" audio jack (can be gotten from the end of a headphones cable)

for the reciever:
phototransitor (photodiodes or IR detectors also work)
1/8" audio jack
some more wire
high-gain amplifier (laptop with mic input, or mic preamp plus amplifier)
magnifying glass (helps at great distances)

tools:
wire cutter/stripper
soldering iron and electronics solder
tape (clear and/or electrical)
digital multimeter (can be useful... not realy required)
tripod (helps for aiming laser at a distance)
empty pizza boxes with white backs (for finding beam and for adjustments)
some assistants

Comments

[NGinuity]

I assume you're using the transformer in that manner to "step down" the level of the input signal as well as modulate the DC going to the diode. Is that correct?

[ALogan97]

Is the audio transformer necessary? I forgot to buy one and don't want to go back out to radioshack to get one.

[Zack1296]

YA CAKE! btw the project is awesome

[chubyatyahoo]

can you replace the laser with an led

[Swishercutter]

That would be an Optoisolator...at least by the time you actually got the led close enough to eliminate noise and get an effective signal...but basically an optoisolator uses an LED coupled with a phototransisitor in order to electrically isolate the control circuit from the circuit being controlled.

[raykholo]

i would seriously not recommend doing that will it work? yes is it effective? absolutely not an led is not a focused beam, cannot be used over long distances like a laser can, and it is more prone towards the distortion as the light is more spread out. The laser points light precisely at the photo diode, so that is the light which is converted to audio. Theoretically, using an led would require total darkness so as to eliminate that interference. Give it a shot and tell how it went!

[XOIIO]

Could you just hook a laser diode to the audio output and a reciever to a microphone input? IT seems like it would make sense.

[navaburo]

Short answer, no.

This is mainly because laser diodes only conduct electricity in one direction - that is, they work with Direct Current (DC). However, (most) audio outputs have alternating directions of current (AC). So when you hook up a laser diode to the audio output, you only get half of the waveform.

Furthermore, laser diodes have a minimum voltage required to turn them on. So you will actually get less than half of your waveform if you attach the laser diode directly to the output.

That is why there is a battery pack involved: the batteries provide the direct current bias needed to keep the laser on. The audio output (via the transformer) subtly changes the strength of that direct current, and is therefore manifest in the laser's output brightness.

Hope that helps.

[Warlrosity]

what is a duh? is it an iPod knockoff?

[GameNox]

No. It is an IPod. It means that you need music. Your'e a duh! =P

[jelte1234]

actually: would ripping apart a wireless cheapy headphone not also work, and then replacing the leds with a laser? or is this a bad idea?

[inquist]

No the LEDs are in the transmitter, and they have a wide angle so your headset can receive from anywhere in the room. A laser transmitter has to point straight toward its receiver, in this case William Tell style

[ep0317]

Could you please specify the type of audio transformer that you used? thanks..

[caseyC088]

would it be possible to make a two way radio sort of thing if you made two of these??

[lazer155]

 Is the potentiometer absolutely necessary?  I dont have one on hand so I want to know if I can use just a resistor.  Can I use one of those photocells instead of the phototransistor?

[navaburo]

Short answer: YES.

Long answer: if you knew exactly what resistance(s) to use, you could substitute the potentiometer(s) for fixed resistor(s). However, these resistances depend on your laser pointer and the voltage of your battery pack, so it is really convenient to use the pots. (They are inexpensive too.... and useful in other projects, so I'd recommend buying/salvaging some.)

[lazer155]

 What is the potentiometer for?  I got it working without a pot or a resistor.  Does the pot filter out static and background noise?

[navaburo]

Lasers often have a non-linear response. That is, their brightness depends on the power-supply voltage in a complicated way. The potentiometer(s) along with a control of the input audio amplitude (mp3 player volume) allow you to select which section of the psv-brightness curve to use. So, you should be able to select a linear portion, and get great audio quality. Using a non-linear portion will result in nasty sounding distortion.

Also, I realize now I didn't address your second queston "can you use a photocell instead of a phototransistor". I assume you mean either a Cadmium-Sulfide passive cell or an active solar panel (generates electricity from light). The issue with the photo-cells are that they (typically) have a slower responce-times as compaired to a phototransistor or photodiode. The result is that a photo-cell-based circuit does not pick up on the higher frequencies as well, if at all. It might sound like music through a wall (only bass).

But, you can always try origional designs! Do let me know how you get along with this!

Chris

[lazer155]

 Also how would I hook this up directly to a speaker instead of having to put it in a mic port?

[ayman deabes]

nice project

[adhwaith]

hey... i don't seem to be able to get any music across... is there any way of testing if the transmitter works???? and i'm using a 10k ohm potentiometer, could that be a reason???

[Firecrow]

when i point my laser beam at the reciever i get no sound.. Does this mean that my laser beam is not good enough? and if so could i just connect it to a infrared led?

[navaburo]

Firecrow, You can test the function of your receiver by holding the laser close to the receiver and quickly passing the laser beam across the photodetector. You should here a low-pitched WompWompWomp. If you are using the mic port on a computer to receive and amplify the signal, you can use a recording program (like Audacity) to see if the laser is being detected. If you hear/see the signal from flicking the beam across the detector, then you know the detector works, and you have to work on the transmitter. Best of luck and let me know how it goes! - Chris

[Gamernotnerd]

Can you post some pics of the sensor lab?

[UbuntuNinja]

My laser uses 3 button cell batteries (4.5 volts) so can i replace the 3 volts in the diagram with 4.5 volts to give the laser a reasonable brightness?

[navaburo]

Yes. Although remember that you want to maximize your signal to noise ratio (SNR), and that the raw brightness is not the signal strength. The signal strength is roughly proportional to the range of brightnesses the laser exhibits during operation. For a digital signal where the laser goes full-on and then full-off, the raw brightness is the brightness range. However, in an analogue scheme such as this one, one must closely consider the modulation to determine what parameters will maximize signal strength. Put briefly, a dimmer laser beam may result in better transmission because the laser is more sensitive in that brightness region (it can get dimmer and brighter from that mid-point). Hope that helps, Chris

[UbuntuNinja]

Can you help me to understand how i should wire up my LM386 amplifier IC? Here is the schematic of the IC:

[navaburo]

UbuntuNinja,
I assume you wish to use the LM386 as part of an audio amplifier on the recieving end? I would recommend building an audio amp with it along the lines of MAKE's Crackerbox Amp: http://makezine.com/09/crackerboxamp/

You will need to then use a battery in series with the phototransitor (unless you use a small solar cell, which provides its own power), and hook this up to the audio amp.

Viel Spass!
Chris

[UbuntuNinja]

Yeah i think it is actually better to use less voltage than the laser takes for normal operation (not in audio circuit) because it will be receiving extra voltage from the ipod which which would create a voltage greater than what the laser is build for. Since we are talking about lasers, overvoltage could damage the laser.

[navaburo]

Yes, overvoltaging the laser could also be a concern! If you burn out a laser let me know! (I think it is harder than you might think)

[ashes63]

hope this one works!!

[UbuntuNinja]

That schematic is very helpful. I like how you correctly represented the different ways to wire the transformer. I was actually having difficulty figuring out which side of the transformer was supposed to be wired to what looking at the authors diagram.

[UbuntuNinja]

Here is the schematic for the circuit without the black background, for easy and efficient printing: http://www.instructables.com/file/FYUF1DKFVW22CNV/
Here is that same file in Inkscape SVG (scalable vector graphics) format:
http://www.instructables.com/file/FW4ZN91FVW22CNW/

[gentry]

Check out this IR sound transceiver from 1961.

[alex-sharetskiy]

nice magazine!

[astro boy]

hey they stole my idea! gir!!!!!!!!!!

[brian108832]

sooo. i am completely new to this... i have only the basic understanding of circuitry so please excuse my question but. on the schematic it shows the output from the mp3 player with 3 lines. i understand the left and right but what is the ground?

[navaburo]

There are several ways to answer that question, I will give two of them:

1) The academic answer: Voltages are only defined between TWO points. A single point only has a voltage insofar as a reference (ground) has been established. Any device "communicating" with the mp3 player needs to have a reference to make sense of the left and right signals.

2) The simpler but not totally correct answer: The signal needs to "return" to the source to complete the circuit. The ground allows for this completion.

---

Electronics is a great hobby to get into. I would recommend getting yourself a copy of the classic intro "Getting Started in Electronics".

[trebuchet03]

Very cool :D There's software out there to send morse code through your speakers and also interpret code brought in through your microphone/line in... I made a really cheap serial 'radio' that did just this... but the range was not so hot :P

[navaburo]

could you post the url for the software?