LASER COMMUNICATOR

[Swordofsalvation]

Hey laser people!!

Just a few days ago, I began work on my newest project, a laser communicator. Basically, it sends sound across a room on a beam of light by synchronizing the light intensity with the frequency of the music. A solar panel attached to a speaker on the other side of the room picks up the light and turns it back into pulsing ac current and sound.
Wat I need to know is what wavelength of visible light would work best for this project. Also, I was thinking that using a DDL style driver might be less expensive, since I don't need to fit the whole device in a small host.
I tested the concept out using an LOC diode , and it worked great. Unfortunately, the next day I killed my diode by forgetting to regulate my power supply. Would it be betetr to use 532nm, 660nm, or 405nm if I am using a solar cell as my recievor?
Any other advice would be appreciated!!!!

 

[Asherz]

pretty much exactly the same concept here http://www.youtube.com/watch?v=-R9M4qy37Fs&NR=1 and I don't think the laser would matter as far as I know? In theory IR should work aswell because it doesn't matter if it is visible light just that it contains energy, which all lasers do.

I am probally wrong though But the video is good, I was thinking of doing it.

 

[Bionic-Badger]

Yup, you can do that, you can also use it as a remote microphone that transmits the vibrations of whatever surface it strikes back to a receiver. Might want to use a photodiode or photocell though, because they're made specifically for detecting light, not just absorbing light for power in the case of the cell. I doubt the specific wavelength will matter much.

 

[GoRdOn_B]

pretty much exactly the same concept here http://www.youtube.com/watch?v=-R9M4qy37Fs&NR=1 and I don't think the laser would matter as far as I know? In theory IR should work aswell because it doesn't matter if it is visible light just that it contains energy, which all lasers do.

I am probally wrong though But the video is good, I was thinking of doing it.

Wow... never seen anything like that before

Thats awesome

 

[Warske]

Wat I need to know is what wavelength of visible light would work best for this project... I tested the concept out using an LOC diode , and it worked great. ... Would it be betetr to use 532nm, 660nm, or 405nm if I am using a solar cell as my recievor?
Any other advice would be appreciated!!!!

As you can see from the response curve that Zom-B posted here http://www.laserpointerforums.com/forums/YaBB.pl?num=1217029972/448#478

you will get more signal the farther into the red, and even into the IR that you go.  But you specifically said "visible," so I would go with the 660 nm LOC that you used, since it gives you a lot of power for the buck.

The above response curve should be essentially the same for silicon solar cells as for silicon photodiodes.  My understanding is that the two are essentially the same thing anyway.  

I would think you are going to get some 100 or 120 Hz hum from the lights in the room, depending on where you live.  If that's a problem, you might want to put a notch filter ahead of the amplifier.  http://www.google.com/search?hl=en&q=notch+filter&aq=f&oq=

Also if you put some red plastic over the detector, that will filter out some of the room light and sun light while letting the signal through.  That should improve the hum mentioned above and make it less likely that your detector will saturate in strong light.  With some research, you can do better than red plastic and find an optical filter that is quite narrow for the wavelength laser you are using.  That is more expensive, though, and might be overkill.

Another trick is to stick the detector at one end of cardboard tube, to help shade it from extraneous light.

You can improve the linearity of your detector, and thus reduce audio distortion, by reverse biasing the solar cell (just like with a photodiode).  As an experiment, hook the solar cell, a 9 v battery, and a 1 k ohm resister all in series.  The solar cell should be wired in so that the voltage across it is negative compared to what it usually puts out.  It should be about -9 v in the dark, and go toward 0 volts when you shine light on it.  If it goes above 0 volts, use a lower resistance.  If it doesn't approach zero volts in the strongest light, use a higher resistance.  Now hook your amplifier across the resistor (use a DC blocking capacitor in series, maybe a few uF, if the amp doesn't have one built in).  That should give you a stronger signal with less distortion.  

 

[Swordofsalvation]

Yes, thanks for all the ideas. I wanted to get a photo diode, but the reason why i used a solar cell was because this way I could directly hook the cell up to a speaker because the solar cell creates its own current, and because sound is just pulses of electricity, i odnt need to have any circuitry between the cell and the speaker.
My next step is to be able to transmit my voice using a microphone. Wat surprises me tho, is that its amazing too hear the various frequencies of the light coming from all the various bulbs through my house. LED's give off a very weird frequency!!! and flourescents are just plain annoying.
another question: would a ddl circuit work fine for this project? its the cheapest

 

[Benm]

I suppose the transmitter/driver part is the easy one - getting good noise isolation and s/n ratios over distance is another matter entirely, but it would help to start with a sensor that is only sensitive to the desired wavelength.

 

[DayOfShadows]

not meaning to thread jack but man look at this http://www.youtube.com/watch?v=MVMxm4WjY4Y&feature=related , same guy as before, check around 3:50, NICE work this guy does, even with cheap lasers.

 

[Swordofsalvation]

hey benm, how do you aquire such a sensor, that is sensitive to only 660nm?

 

[Benm]

You don't, but it would suffice to have a sensor that is somewhat sensitive to 660 behind a proper filter - the point is mostly to avoid interference from light at other wavelengths... this is not unlike what is done with IR sensors for tv remote controls.

 

[laserpyro]

Yup, you can do that, you can also use it as a remote microphone that transmits the vibrations of whatever surface it strikes back to a receiver. Might want to use a photodiode or photocell though, because they're made specifically for detecting light, not just absorbing light for power in the case of the cell. I doubt the specific wavelength will matter much.

hey would this work fine? Photo Transistor

 

[Benm]

Looks usable to me. I would just give it a try, i'm fairly certain it will work as a proof of concept with very little circuitry.

Building a hifi-quality link that will span large distances is something else however, if you just set it up using the phototransisor and an audio amp after that, the system will pick up 100/120 Hz rum from lightbulbs and such.

There are means to get much better sound quality through an optical link like this, but those involve a fair bit of electronics experience to design and construct. To get the basic operation, the approach in the YouTube - Laser linked Ipod. .

 

[laserpyro]

anyone know where to get an audio isolator. the thing you put in the circuit with the laser?

 

[Benm]

You mean the little transformer used in the youtube story? Those you can order from electronics stores, but if you have an old modem (the dial-up kind) stored away somewhere, chances are there will be something usable on that. I'd need to see detailed pictures to confirm though, but nearly all modems had some kind of line seperation transformer, which suit the bill exactly for this application.