Small solar panel to pick up sound waves

Thread Starter

Rolland B. Heiss

Joined Feb 4, 2015
236
Today I read about sending sound through light (in this case a laser beam) and on the other end they were using a small solar panel one would remove from a garden light or something similar. The solar panel was connected to an input jack on a receiver. So I began to wonder, although I haven't yet tried it... what would happen if I hooked up a solar panel to a receiver outside? Could I possibly pick up stray communications floating around in the sunlight? Has anyone here tried that and if so, what were the results?
 

Papabravo

Joined Feb 24, 2006
13,923
It seems highly dubious. Communication using light is normally done with fiber optic cables that preserve signal integrity by means of total internal reflection. In free space and in the atmosphere a beam of light will diverge rapidly in inverse proportion to the square of the distance, leaving so little energy at the receiver as to be indistinguishable from noise and other thermal vibrations.

That said if you want to do the experiment - knock yourself out.
 

Papabravo

Joined Feb 24, 2006
13,923

wayneh

Joined Sep 9, 2010
16,398
Could I possibly pick up stray communications floating around in the sunlight?
The only signals would be ones riding on light in the visible spectrum, and you'd have to be able to see the source directly or reflections of it. I can't think of any normal use of visible light for communication of audio signals, and if they exist at all they would be an extremely low percentage of the available power.

Think is terms of signal to noise ratio. Your solar cell will pick up and generate noise. (I'm not sure which is the bigger source.) To be useful, any signal would have to rise above that noise in order for you to find it. That means it would need a certain level of power. Now look around outside. How many lights do you see? Some, to be sure, but not much compared to sunlight. Now, how many of those lights are communicating a signal? None, as far as I know.
 

Thread Starter

Rolland B. Heiss

Joined Feb 4, 2015
236
The only signals would be ones riding on light in the visible spectrum, and you'd have to be able to see the source directly or reflections of it. I can't think of any normal use of visible light for communication of audio signals, and if they exist at all they would be an extremely low percentage of the available power.

Think is terms of signal to noise ratio. Your solar cell will pick up and generate noise. (I'm not sure which is the bigger source.) To be useful, any signal would have to rise above that noise in order for you to find it. That means it would need a certain level of power. Now look around outside. How many lights do you see? Some, to be sure, but not much compared to sunlight. Now, how many of those lights are communicating a signal? None, as far as I know.
It was just an idea I had. Once common light was considered just that. Light and little more and yet we use light to capture images in time via photographs which are now mainly captured in a digital form. So I still can't help but wonder what else light can do. Does everything we say get captured somehow by light or some other means? After all, when we speak it generates a sound wave in the form of energy and energy never dies, but it does change form. How to tap into it after that energy leaves and changes form interests the heck out of me!
 

nsaspook

Joined Aug 27, 2009
7,344
It was just an idea I had. Once common light was considered just that. Light and little more and yet we use light to capture images in time via photographs which are now mainly captured in a digital form. So I still can't help but wonder what else light can do. Does everything we say get captured somehow by light or some other means? After all, when we speak it generates a sound wave in the form of energy and energy never dies, but it does change form. How to tap into it after that energy leaves and changes form interests the heck out of me!
You can decode sounds from light/video but you need a very high frame or sample rate to extract a high-fidelity hidden audio signal from the noise.
http://forum.allaboutcircuits.com/threads/the-visual-microphone-passive-recovery-of-sound-from-video.100194/

In the case of my solar panel experiment the original signal was too noisy to see small changes from cloud cover during the eclipse so I added a noise reduction filter to the raw digital signal to extract the lower level variations by the analyzing a large number of samples over a period of time into one data value that averaged out the thermal noise from the panel.
Original signal (oversampled 128X) with noise:
http://forum.allaboutcircuits.com/attachments/moon_chart-png.71607/
 
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Wendy

Joined Mar 24, 2008
22,145
A laser bouncing off a pane of glass (or mirror) into a solar cell should be able to pick up sound just fine. It is one of several active eavesdropping techniques that are out there. Getting it off a recording is a different matter, though it is theoretically possible it is not very likely.
 

alfacliff

Joined Dec 13, 2013
2,458
there have been experiments with using lasers to downlink satelites to earth, and putting green lasers on satelites to see through sea water to locate subs and communicate with subs too. some hams I know have been using optical communications as point to point links, so I ddont see why comercial or government users havnt too.
 

Thread Starter

Rolland B. Heiss

Joined Feb 4, 2015
236
A laser bouncing off a pane of glass (or mirror) into a solar cell should be able to pick up sound just fine. It is one of several active eavesdropping techniques that are out there. Getting it off a recording is a different matter, though it is theoretically possible it is not very likely.
I don't see why it would matter whether it is 'live or memorex' if anyone remembers that old commercial? Vibrations are still vibrations and the light carries the vibrations. Can the light differentiate between a live vibration and a recorded one and thus choose to only pass the live one?
 

nsaspook

Joined Aug 27, 2009
7,344
I don't see why it would matter whether it is 'live or memorex' if anyone remembers that old commercial? Vibrations are still vibrations and the light carries the vibrations. Can the light differentiate between a live vibration and a recorded one and thus choose to only pass the live one?
In effect yes. A video recording is a series of snapshots of reality. If the random vibration (say 1000hz) is much faster than the typical HD camera frame-rate (30FPS) you lose the ability to reconstruct the signal by looking at the light from the video. All you would see is a set of changes in signal with large gaps that can create artifacts of the original signal.

At about 3:00 you can see the camera and strobe start to actually see the signal in the wafer from the speaker.
 
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Thread Starter

Rolland B. Heiss

Joined Feb 4, 2015
236
In effect yes. A video recording is a series of snapshots of reality. If the random vibration (say 1000hz) is much faster than the typical HD camera frame-rate (30FPS) you lose the ability to reconstruct the signal by looking at the light from the video. All you would see is a set of changes in signal with large gaps that can create artifacts of the original signal.

At about 3:00 you can see the camera and strobe start to actually see the signal in the wafer from the speaker.
Very informative nsaspook! Thanks for helping me understand a few things here that I couldn't grasp initially. Yet, we are talking about frame rates in a video. Is the same true in relation to an audio recording?
 
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