Hi everyone!
I am currently working on a laser microphone project, inspired by this: Laser Microphone
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)
(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 , 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
I am currently working on a laser microphone project, inspired by this: Laser Microphone
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)
(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 , 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