Instructables

Step 6: How does that work? And where do I go from here?

This circuit works using amplitude modulation, exactly like AM radio, except using a visible light wavelength instead of a radio frequency. The audio signal leaves the iPod as a varying voltage which forces a varying current through the laser. Then the laser's varying brightness conveys the musical information. Finaly, the phototransistor varies in resistance as the brightness on it changes. The mic amp applies a small voltage to the phototransistor and amplifies the resulting current.

A problem with this system is that at each step there is a non-linear transfer function, that is, there is distortion that occurs because the brightness changes are not always proportional to the change in voltage applied. See the screenshot below for an example, and listen to the attached audio sample.

The next step in this project would be to use pulses (like fast, computer operated morse code) to convey digital information like text, crystal-clear audio, or even video. One could even network computers with laser beams in a manner similar to fiberoptics but in open air. I will post C code for my transmitting and recieving programs.
 
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Zack12963 years ago
YA CAKE! btw the project is awesome
Qthedude6 years ago
have you tried this with higher frequency lasers? such as green or blue lasers? I realise that they are more expensive, but also may be able to transmit very high amounts of information compared to red lasers, to check out these go to thinkgeek.com/5a47
ashes636 years ago
hope this one works!!
circuitpaintII.bmp
ashes636 years ago
I used this project idea for my senior project at school and I must say, this was the best experiment. Although it didn't take me a mere couple hours, instead it turned into a two week extensive study session learning about electronics. I do have one thing to add. Instead of the schematic drawing on page 3, I made one:) I attached the photo if you want to add to the whole project page! Again, thank you for your insight and direction; I found much help reading from this.
circuitpaint.BMP
navaburo (author)  ashes636 years ago
Excellent! Thanks for the schematic (added to page).

How was the quality on the resulting sound?

Also, you may consider trying a similar instructable I wrote on making a laser microphone to hear through external windows HEREHERE. You have all the equipment for it right now, you just need to turn off the music, and turn up the gain on your amplifier (or maybe use a two-stage amp for more gain). (and of course bounce the laser off the window).

Enjoy!
lol @ the first song
micronxd7 years ago
rofl interesting music selection... awesome project... i like the surveillanceone too