Audio transmission using a 5 mW laser and a solar panel!

Thread Starter

AnthonyKpS

Joined Apr 12, 2019
16
Hello fellow circuiters!
I'm building a project and I need your help!

I'm trying to transmit audio using a laser. Main idea:

Sound source: smartphone
Transmitter: Laser beam
Power source: 5V/9V
Receiver: Solar panel
Sound reproduction: Speakers

There are many implementations in the web but none suits me nor helps me solve my problem.

There are 2 versions that seem to be close the desired outcome. I provide schematics here: https://easyeda.com/AnthonyKpS/Laser-Transmission-with-an-Audio-Amp.

First scenario-Transformer-Only amplification:

Using an audio jack, we take the AC signal from a mobile phone playing music, connect the two ends to the small side of a transformer and receive a higher Voltage signal. Then connect in series the laser, and battery and the two ends of the transformer.

Result: medium volume distored sound

Fix: Place a trimpot between the battery and the laser, lower the voltage to around 2.1VDC


The problem with this fix is that 2.1VDC is marginally close to the minimum Voltage that the laser needs to work properly, thus in any sudden drops of Voltage the laser lights off. Also, considering that the Supply Voltage of the laser given by the manufacturer is 3~5VDC, the laser is essentially underfunctioning.


Second scenario-Adafruit 2130 amplification (Class D Audio amplifier):

Using an audio jack, we take the AC signal from a mobile phone playing music, connect the two ends to the differential input of the Amplifier, connect the power to it and also the connect the two ends of the laser to the amplifier's output.

Result: medium volume distored sound

Fix: Place a trimpot between the battery and the laser, lower the voltage to around 2.1VDC

Same problem.


Third scenario: Tranformer+Adafruit 2130 amplification (Class D Audio amplifier):

I was told that I should try connecting the audio jack to the tranformer and THEN to the amplifier.

Result: very high satisfying volume but also distored sound

Also, on all the scenarios i tried pluging in the system a 9V battery. The only difference was that there were less voltage drops.

As you can clearly see, in the first two attempts the common denominator of the problem is that damn 2.1VDC that if i surpass the sound becomes distorted or not able to hear at all. Why is that? Also, in the third scenario why do you think the sound is distorted?

Any ideas? Any mistakes I've made?
Please let me know!
Don't hesitate to ask for anything that I haven't provided.







 

Yaakov

Joined Jan 27, 2019
2,541
Welcome to AAC.

Thanks for a very detailed e description, but there are a couple of things that could help.

1) "Distortion" isn't one thing. Characterizing the distortion may be critical to eliminating it.

2) Drawings/Schematics/Datasheets In particular, the datasheet on the solar cell, which, it is my surmise, is simply incapable of producing an undistorted waveform at the frequencies you want. Though it could also be the transformer, or the laser diode.

Transformers are common sources of distortion at audio frequencies.
 

sghioto

Joined Dec 31, 2017
2,276
Using an audio jack, we take the AC signal from a mobile phone playing music, connect the two ends to the small side of a transformer and receive a higher Voltage signal. Then connect in series the laser, and battery and the two ends of the transformer.
This is the conventional method of a simple laser modulator with the proper transformer.
What transformer are you using? Most likely you are over modulating the laser.
SG
 

danadak

Joined Mar 10, 2018
4,057
Is your laser beam hitting the solar cell focused, a spot ? Because
that tells me your output will be quite small,

Might be better to do this with a phototdiode, which means you have to aim
precisely. Or use a photocell with filter "tuned" to the laser frequency and defocus
the laser somewhat.


Regards, Dana.
 

Thread Starter

AnthonyKpS

Joined Apr 12, 2019
16
Welcome to AAC.

Thanks for a very detailed e description, but there are a couple of things that could help.

1) "Distortion" isn't one thing. Characterizing the distortion may be critical to eliminating it.

2) Drawings/Schematics/Datasheets In particular, the datasheet on the solar cell, which, it is my surmise, is simply incapable of producing an undistorted waveform at the frequencies you want. Though it could also be the transformer, or the laser diode.

Transformers are common sources of distortion at audio frequencies.
Thanks for the quick reply!

1) I can provide a .wav file so you can hear youself. I'm not a able to characterize distortion unless "GHHHHHHHHH" is a valid characterisation.

2) I've posted some schematics on the original thread. Here they are again: https://easyeda.com/AnthonyKpS/Laser-Transmission-with-an-Audio-Amp. As far as datasheets I'll try my best to post them aswell.
 

Thread Starter

AnthonyKpS

Joined Apr 12, 2019
16
This is the conventional method of a simple laser modulator with the proper transformer.
What transformer are you using? Most likely you are over modulating the laser.
SG
Exaclty. The principle I'm taking advantage of is Amplitude Modulation. I'm using 2 generic tranformers, bought from a local electronics store. I tried both and the results are generally the same. Most guides had an Audio-Output tranformer, i couldn't find one, nor salvage one.

Il post the measurements I did on the 2 tranformers.
 

Thread Starter

AnthonyKpS

Joined Apr 12, 2019
16
Is your laser beam hitting the solar cell focused, a spot ? Because
that tells me your output will be quite small,

Might be better to do this with a phototdiode, which means you have to aim
precisely. Or use a photocell with filter "tuned" to the laser frequency and defocus
the laser somewhat.


Regards, Dana.
Thanks for the reply!

My beam is quite wide. I observed that by widing the beam to some extent (5~6 cm diameter) the output volume was higher.
 

Thread Starter

AnthonyKpS

Joined Apr 12, 2019
16
Measurements and datasheets:

Laser's Specifications:
  • Model: LD-G650A03
  • Supply voltage: 3~5VDC
  • Absorbed current: 40mA
  • Wavelength: 650nm
  • Colour: red
  • Projected beam type: point type (.)
Solar panel's Specifications:
  • Material: Polycrystalline Silicon
  • Max. Power: 2W
  • Operating voltage: 6V
  • Current: 0.3A
  • Open circuit voltage: 7.2V
  • Short-circuit voltage: 0.38A
  • System voltage: 750V
 

Attachments

Thread Starter

AnthonyKpS

Joined Apr 12, 2019
16
Welcome to AAC.

Thanks for a very detailed e description, but there are a couple of things that could help.

1) "Distortion" isn't one thing. Characterizing the distortion may be critical to eliminating it.

2) Drawings/Schematics/Datasheets In particular, the datasheet on the solar cell, which, it is my surmise, is simply incapable of producing an undistorted waveform at the frequencies you want. Though it could also be the transformer, or the laser diode.

Transformers are common sources of distortion at audio frequencies.
Sample Wav: https://drive.google.com/file/d/1URZNGEC9cYFaXbjuHp2-s87MPUr7YoDS/view?usp=sharing
 

Yaakov

Joined Jan 27, 2019
2,541
Can you run some simple sine waves through it?

You can use a signal generator app of website if you don't have an AWG or function generator.

The source makes working out the nature of the distortion very difficult.
 

MrChips

Joined Oct 2, 2009
23,095
Here is how I would attempt this project.

Transmitter
I would use the output of the Class-D amplifier but I would remove the output inductor and capacitor filter.
I would drive the laser from the PWM output.

Receiver

I suspect that the response time of a solar cell is too slow.
I would use a fast response photo diode into an opamp, integrator, and audio power amp.
 

Yaakov

Joined Jan 27, 2019
2,541
This is the waveform from the last sample. As you can see, the artifacts are very distinctive and repeated. Something is not linear.

upload_2019-4-14_9-11-5.png
 

Yaakov

Joined Jan 27, 2019
2,541
I'm not entirely sure on what I should be looking. Can you elaborate on your given waveform?
It should be a sine wave:

upload_2019-4-14_9-21-34.png

But as you can see, the signal plateaus after ~100μs for about 100μs, then rises again repeating that, then falls again repeating it.
Something in the system is not acting in a linear fashion. I can't tell if the timing is frequency dependent because the other samples are such low amplitude it is hard to get the scope to trigger from them.

It doesn't look like transformer saturation or anything I recognize as transformer related. Could it be something in the laser diode driver?
 

Thread Starter

AnthonyKpS

Joined Apr 12, 2019
16
It should be a sine wave:

View attachment 174961

But as you can see, the signal plateaus after ~100μs for about 100μs, then rises again repeating that, then falls again repeating it.
Something in the system is not acting in a linear fashion. I can't tell if the timing is frequency dependent because the other samples are such low amplitude it is hard to get the scope to trigger from them.

It doesn't look like transformer saturation or anything I recognize as transformer related. Could it be something in the laser diode driver?

Ahh, I see. Given the fact that the recording of the sample was taken from a headset mic, the abnormality of the waveform may be subject to the quality of the recording, am I correct? Also, in what way could the problem be related to the laser diode?
 
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