Laser microphone help

Thread Starter

masnoob

Joined Jun 21, 2018
4
Hi everyone!
I am currently working on a laser microphone project, inspired by this: Laser Microphone

ex.jpg
The diagram above is my entire setup. I am using a cheap 5mW red laser pointer (SYD1230, if that matters).

The idea is to shoot the beam towards vibrating surface (due to sound emitted by speakers)( there is a piece of CD attached to it to serve as reflective vibrating surface), and try to capture the reflected beam using the photodetector.

The beam distance to reflective surface, and the reflective surface to receptor is within 2 meters, which is short as I hope the setup is functioning before moving onto longer ranges.

In order to save the hassle of adjusting and realignment of laser beams onto the detector surface, I have chosen solar cell - which have a 1V,1W rating, with a larger surface area with the cost of longer response time, due to its larger capacitance.

I just connect the solar cell output to passive RC high pass filter, and obtain the output using 3.5mm AUX cable into my laptop to record the audio signal using Audacity.

The audio I obtained is quite noisy in general due to 50Hz harmonics, and majority low frequency noise (maybe some unknown noise source included).
However in general the raw audio is very faint and barely audible, usually got masked by possible unknown noise other than aforementioned ones.

I try to filter the audio but the raw signal is way too noisy itself, even a sharp roll-off filter also not viable. So I am currently thinking of some way to improve the raw captured signal quality using laser PWM modulation, as suggested by someone else.

I have saw many projects using lasers as means of audio transmission, which is totally possible and audio quality is acceptable in my project.
Based on the idea, I have refer to circuit schematic found online, and tried to modified some parts of it.

I going to compare DC and triangular waveform using LM311N, and using the output as a switch to turn laser on and off in a carrier frequency rate of 15kHz (this frequency suggested by someone and I think this has something to do with the ADC sampling rate on sound card)

tttt.png

(I use LED to represent laser pointer since I couldn't found the symbol in Multisim 14.0)


There are some problems arises here:

1: What happens when a PWM modulated laser shoots on the vibrating objects (windows or any possible reflective surface which vibrating at distinguishable frequency?) How does
Lets say if I shoot a 15kHz modulated laser beam to a reflective surface(glass/CD/cellophane tape) which is vibrating due to a song played by speakers behind them. How does this work? via AM? Phase Modulation? doppler frequency shift ???? D

2: If I am going for this setup, what are the possible means to demodulate the raw captured signal, i.e., to separate sound from 15kHz carrier signal?
I pretty sure there are no 100% reconstruction for such scenario.

3: As described by the article here, I would still prefer to use solar cell as my detector. How can I possibly enhance the detection efficiency and reduce the distortion rate?

4: Since this site is all about circuits :p, is there any possible improvements to the circuit above?

I was not going into those sophisticated equipment like mixers or anything else as I want to improve the raw signal to noise ratio without spending too much of my budget.

Thanks for looking at my long post
 

AlbertHall

Joined Jun 4, 2014
10,536
In your circuit, U1A generates a square wave and U1B converts the square wave into a sawtooth. Then U2 converts the sawtooth into square wave again. You could ditch U2 and the four 3k resistors and feed the MOSFET gate directly from the output of U1A. I don't think this will change the performance of the circuit significantly but it will make it smaller.
 

Thread Starter

masnoob

Joined Jun 21, 2018
4
In your circuit, U1A generates a square wave and U1B converts the square wave into a sawtooth. Then U2 converts the sawtooth into square wave again. You could ditch U2 and the four 3k resistors and feed the MOSFET gate directly from the output of U1A. I don't think this will change the performance of the circuit significantly but it will make it smaller.
I do understand that the U2 is act as comparator, but I dont understand why they want to compare triangle waveform with (originally is audio input at the pin2, but I replace it using DC) in order to achieve modulation since we can use the square wave directly from U1A. Well I will go for ur suggestion and try to reduce circuit size as much as possible. Thanks anyway
 

AlbertHall

Joined Jun 4, 2014
10,536
The original circuit results in the laser being PWM modulated with the audio fed to U2. You don't need to do that bit. In fact I am not sure you need the laser modulated at all unless it is at a much higher frequency then you could band pass at that frequency to exclude anything that didn't come from the laser. Then you could use a frequency discriminator to detect doppler changes or an amplitude discriminator to detect amplitude changes. I have no idea which of those would be useful. The doppler method would be easier to make linear (= low distortion).
 

DNA Robotics

Joined Jun 13, 2014
588
"The downside from using a solar cell is that they are extremely slow. The digital carrier is completely wiped out and it is the actual demodulated audio frequency that is coming through as signal. The advantage is that no demodulator is needed at all: just hook up the amplifier and speaker and you're in business. The downside is that since the digital carrier is not present, and therefor cannot be restored, the performance of the receiver is completely dependent of the light intensity and audio will be distorted by all stray light sources modulated in the audio frequency range such as light bulbs, televisions and computer screens."

If you put the solar cell inside a tube that is aimed at the reflective vibrating surface, you can eliminate a lot of the stray light sources.
 

Thread Starter

masnoob

Joined Jun 21, 2018
4
"The downside from using a solar cell is that they are extremely slow. The digital carrier is completely wiped out and it is the actual demodulated audio frequency that is coming through as signal. The advantage is that no demodulator is needed at all: just hook up the amplifier and speaker and you're in business. The downside is that since the digital carrier is not present, and therefor cannot be restored, the performance of the receiver is completely dependent of the light intensity and audio will be distorted by all stray light sources modulated in the audio frequency range such as light bulbs, televisions and computer screens."

If you put the solar cell inside a tube that is aimed at the reflective vibrating surface, you can eliminate a lot of the stray light sources.

Well I did isolate the solar cell from the external light source but the results are still not satisfying.
 
I also did a test for the width of maybe a football field it was in an athletic park I just forget if the field we used was soccer or football or another...

My friend was inspired by what I did and bought a higher powered laser pointer that used I think his was using 18650 cells, it carried the frequency across the field no problems it`s just that the beam was so much wider at that distance only half of the beam was covering the tiny panel so very little volume.

When we moved in closer to the panel and the whole of the laser beam was on the panel the volume was restored loud and clear...

This was all done at night in the dark so we could see the beam clearly,
 

Thread Starter

masnoob

Joined Jun 21, 2018
4
I also did a test for the width of maybe a football field it was in an athletic park I just forget if the field we used was soccer or football or another...

My friend was inspired by what I did and bought a higher powered laser pointer that used I think his was using 18650 cells, it carried the frequency across the field no problems it`s just that the beam was so much wider at that distance only half of the beam was covering the tiny panel so very little volume.

When we moved in closer to the panel and the whole of the laser beam was on the panel the volume was restored loud and clear...

This was all done at night in the dark so we could see the beam clearly,
I pretty sure transmission of modulated light is fairly easy and there are a lot sources out there.
However in my case I want to capture the light reflected off a mirror/CD which has a sound source behind them...
 
Dismantle a CD player boost laser power and bingo is your dawg ?

As long as the cd it`s being reflected off is spinning I don`t think dawgs would chase it if it`s not going in circles ?
 
Top