Laser based Conversation Detection
The purpose of this project was to create a laser-based conversation microphone. A laser microphone is a surveillance device that uses a laser beam to detect sound vibrations on a distant object. The targeted object is typically inside a room where a conversation is taking place. Such a device would
2025-06-28 16:33:58 - Adil Khan
Laser based Conversation Detection
Project Area of Specialization Electrical/Electronic EngineeringProject SummaryThe purpose of this project was to create a laser-based conversation microphone. A laser microphone is a surveillance device that uses a laser beam to detect sound vibrations on a distant object. The targeted object is typically inside a room where a conversation is taking place. Such a device would allow the user to listen to a remote conversation without ever having to infiltrate the premises. As no installation is required, you will not even need to enter a building or a room. Normal conversations within a room create minute vibrations on exterior windows which act much like the diaphragm of a microphone oscillating with sound waves. when an acoustic wave is incident on a window it causes the window to bow in and out slightly. This causes the angle of the surface of the window to change slightly, changing the angle of reflection of any reflected light. Since the reflected light from a laser on a window is not uniform, the amplitude of light measured at any given point will change if the angle of the window changes. When a laser is incident on a window, most of the light will be reflected back in the form of a beam. However, some of the light is scattered back in all directions. It is needed to detect these very small levels of light and extract the audio information from them. By determining how much the reflected laser beam was displaced, the audio signal could then be recreated.
Project ObjectivesThe objective of developing a laser-based conversation microphone is to design, fabricate and test a device to listen to conversations produced by people talking at normal levels in a room.
The laser will be emitted against the target window while the detector will capture the diffused scattering of the reflection in order to be processed into an audio signal.
This objective will be achieved by the use of a unit that will emit, detect, and analyze a laser light source. The light source that is emitted will be aimed at a glass window in order to detect the vibrations caused by a standard conversation. The method that will interpolate the signal will consist of a hardware and software system. This will convert the feedback from the light source into the audio signal being detected. This project is expected to be operable by a single trained person, be reasonably portable, and safe to the target. It is also desired that the device reaches a certain degree of accuracy with the received signal and be able to interface with standard audio equipment.
Project Implementation MethodThe design layout required extensive planning in order to meet the objectives, proof of concept, development, programming, and field testing. First, it is investigated about possible implementation methods. Then programming is started as it does not require any physical material. I will use MATLAB for audio filtering by designing a notch filter and bandpass filter. Also Pspice is used to view waveforms and input/output rating of various designed circuits. Then the required components were ordered to create the final product after studying background and literature review. The system collimates and modulated laser beam and projects it against a window, uses optics to focus that beam onto a receiver circuit, does some early signal processing, then correlates the signal to remove noise, digital signal processing, and reconstructs the analog audio signal. Then I drive a headphone speaker with the audio signal being generated behind the window. The driver circuit will modulate a laser diode at the required frequency. Optics will be used to collimate light from the laser diode and receive light being reflected off the window. A very sensitive and noise-free detector circuit is needed to sense the weak modulated light delivered by the optics. A gain stage is required to boost the signal up to a more manageable amplitude. The signal next moves into the digital domain. The digital portion of this project performs real-time digital signal processing as well as provides several control signals used by the rest of the circuit. The output of the digital signal processing is pulse width modulated and needs a reconstruction stage which consists of an analog low pass filter to return it to an analog signal. Then the signal can finally be used to drive an audio amplifier IC designed to power headphones.
Benefits of the ProjectA voice monitoring System using a laser beam for monitoring voice within a room from and outside of the room will be used for spying purposes for government agencies. This will allow hearing someone without entering the room in a secret manner. Another benefit is that I will use an infrared laser beam which is invisible to the naked eye and will not be detected with typical radio-frequency finders. Also, this system can be turned on/off avoiding further detections. This project is expected to be operable by a single trained person, be reasonably portable, and safe to the target.
Technical Details of Final DeliverableThe final hardware system will be a laser transmitter, receiver, detector, data processing system and amplifier where the audible inforamtion is filtered, heard and can be stored for further use. The product will be operated by a single person by reasonably portable and easily adjustable in a short time at a safe distance from the targeted window. The device will have a certain degree of accuracy with the received signal and be able to interfere with the standard audio equipment. The final product is capable of detecting and recording the conversation behind the window and can operate covertly without any visible signature to any targets behind the window.
Final Deliverable of the Project Hardware SystemCore Industry SecurityOther Industries Media , Telecommunication Core Technology OthersOther TechnologiesSustainable Development Goals Peace and Justice Strong InstitutionsRequired Resources| Item Name | Type | No. of Units | Per Unit Cost (in Rs) | Total (in Rs) |
|---|---|---|---|---|
| Total in (Rs) | 72502 | |||
| laser diode system | Equipment | 1 | 20000 | 20000 |
| photo transistor | Equipment | 2 | 1000 | 2000 |
| Fresnel lens | Equipment | 1 | 16000 | 16000 |
| microcontroller | Equipment | 1 | 1000 | 1000 |
| Op-Amp LM741 | Equipment | 1 | 100 | 100 |
| Op-Amp AD818 | Equipment | 1 | 100 | 100 |
| battery | Equipment | 2 | 1200 | 2400 |
| regulator | Equipment | 1 | 1000 | 1000 |
| zener diode | Equipment | 3 | 60 | 180 |
| potentiometer | Equipment | 1 | 150 | 150 |
| resistor | Equipment | 20 | 11 | 220 |
| capacitor | Equipment | 15 | 100 | 1500 |
| mini speaker | Equipment | 2 | 2000 | 4000 |
| PCB | Equipment | 3 | 500 | 1500 |
| filter | Equipment | 2 | 3000 | 6000 |
| aluminimun rails | Equipment | 5 | 300 | 1500 |
| connecting wires | Equipment | 50 | 10 | 500 |
| NPN Transistor | Equipment | 4 | 50 | 200 |
| diode | Equipment | 4 | 38 | 152 |
| stationery | Miscellaneous | 1 | 2000 | 2000 |
| printing | Miscellaneous | 1 | 3000 | 3000 |
| overhead | Miscellaneous | 1 | 5000 | 5000 |
| mirrors | Equipment | 2 | 2000 | 4000 |