Introduction: Laser Beam Microphone

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The Laser Spy System is considered by many to be the Holy Grail of high tech spy devices because it can give the user the ability to listen in on conversations that take place in a distant building without having to install a bug or transmitter at the location. The Laser Spy System was said to be invented in the Soviet Union by Leon Theremin in the late 1940s. Using a non-laser based infrared light source, Theremin's system could detect sound from a nearby window by picking up the faint vibrations on the glass surface. The KGB later used this device to spy on the British, French and US embassies in Moscow. It is also interesting to note that Leon Theremin invented the world's first electronic instrument, a wand operated synthesizer named "The Theremin" after him. The Laser Spy System goes by several names such as the Laser Microphone, Laser Listener, Laser Bug, Window Bounce Listener and a few similar names. The Laser Spy certainly works well under ideal conditions, but it has many strengths and weaknesses that will be discussed in this plan. Building your own Laser Spy is by far the best way to experiment with this technology as you can adjust the design to suit your needs, rather than forking over hundreds or thousands of dollars for an assembled kit that will likely be far inferior to one that you can build yourself. Many of the kits I have seen for sale over the Internet not only use dated technology, but they incorrectly state that the system uses a modulated laser beam to convert window vibrations into sound, which is simply not the case. Let's put the mysteries to rest once and for all and build a working Laser Spy System from the ground up and explore the functionality of each subsystem that makes a working unit. We will be starting with an ultra basic proof of concept test system that will show you how the Laser Spy converts vibration into sound and how careful alignment of both the laser and receiver are required for optimal performance. Ironically, the most basic configuration may prove to be the most useful, and the $20 you spend in parts could create a system that works as well (or better) than some of the ones that are for sale on the internet for thousands of dollars. As you will find out, the key to spying with a laser beam is in the alignment and reception of the beam, not some magical black box full of fancy filters and optical components.

Step 1: You Need:

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This is what you need!!!

Step 4: Set Your Speaker

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Any small piece of a highly reflective surface such as a mirror can be used to deflect the laser beam during these tests. A mirror works best, and a piece can be snapped from an old mirror using pliers or a small dental mirror can be taken apart for to remove the small round mirror from the plastic housing. A hot glue gun or even some double sided tape can be used to glue the small mirror section to the center of the speaker. The size of the mirror is not important since the laser beam will only be a few millimeters across when it strikes the surface. If you intend to snap a bit off a larger mirror, use a cloth or paper towel to wrap the corner so that small slivers of glass do not fly from the mirror as you break it. A highly reflective plastic or metal surface will also work for this experiment, and even a shiny dime will do the job in a pinch.

The speaker needs to be driven by some audio source, but the level should be so low that you can only hear it when your ear is right next to the speaker. The goal is to recreate the same conditions that you will be dealing with during your covert spying operations, so the reflective surface should just barely be vibrating. A portable audio player is perfect for this test because it has a low power amplifier and will run for hours at a time. Set your player to loop indefinitely and then adjust the volume as low as it will go until you can just barely hear the output from the speaker.

The alignment of the laser from the source to the target and back is not a trivial task, and as you increase the distance from the laser to the target, you also increase the error level. At 500 feet, the beam will become so sensitive to displacement that you will have to be careful when moving around the laser because the deflection of the floor inside your home will be enough to throw the beam out an inch or more. When I finally managed to setup a successful long range configuration, I found that the alignment was so sensitive that even a passing car would create waves in the system due to the vibrations between the house and the road. The laser Spy Device is certainly capable of working for many hundreds of feet away (even across a few city blocks), but vibration would become so critical that you would probably need to have everything mounted on an extremely heavy concrete or metal base secured to the ground. These are things you will find out as you experiment with this project. To make your life easy when experimenting with the basic indoor setup, create some kind of easily adjustable speaker stand like mine using an old webcam base or adjustable bench vice. The ability to move the speaker to any angle and secure it will be key to testing the operation of your Laser Spy device as you build it. The targeting laser will also need some kind of adjustable base as well.

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Step 10: Photoresistor

The purpose of this initial experiment is to verify that you can indeed listen to sound that is vibrating a nearby reflective surface. In our case, it will be the small mirror glued to the speaker cone that will vibrate due to either sound from the radio feeding it, or by having a helper talk directly into the speaker as you listen for their voice at the receiver. This test receiver will be the most basic system possible, consisting of only a CDS cell "cadmium sulfide photocell" feeding a simple transistor amplifier that will convert the tiny changes in the laser beam position into changes in voltage that will be sent to your headphones as an audio signal. Think of the laser beam as being the "needle" on a record player and the vibrating window surface as the bumps in the record groove. A CDS cell is basically a resistor that will change its impedance depending on how much light strikes the surface. By feeding it to a battery connected in series and then into an audio amplifier, the result is a light sensitive audio system that will allow you to "listen" to light. Because this system has very little gain and no filtering, it will be a very minimal system, but will certainly let you hear the secret audio signal being sent along your test laser beam. This basic system would even work from hundreds of feet away if you could align the beam properly, which is part of the difficulty in using a Laser Spy System. A CDS cell can be purchased for a few dollars at most electronic suppliers, but if you don't want to wait for delivery or can't find one in stock, just purchase a night light and rip it apart to extract the CDS cell. The CDS cell will be the small disc with two leads and a will have a wavy line on its surface. Just unsolder the two leads or bend the part back and forth until it frees itself from the tiny circuit board.

The laser will need to be installed on some type of adjustable stand or held in place at an approximate horizontal position so you can target the speaker and then catch the reflected beam at your breadboard. I clipped my red laser module into an adjustable bench vice so that it could be aimed at the speaker and then locked into position. When setting up the laser and speaker, remember that according to the law of reflection, "the angle of incidence equals the angle of reflection". In other words, if your beam is lower than the speaker mirror, it will be bouncing back to you at a higher angle. As the distance from the target increases, this deflection becomes much greater.

The initial experiment using the photoresistor will only require a single NPN transistor, resistor and a battery in order to prove that the laser beam is definitely capable of picking up faint vibrations and changing them into audio. Any generic NPN transistor such as a 2N3904 or 2N2222 will work in this circuit. The battery voltage can be anywhere from 3 volts to 9 volts. If you really want to go basic, then just run a 9 volt battery directly into the photoresistor and out to the headphones, although without the transistor to amplify the signal, the audio will be very faint.

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Step 12: Target Your Laser

This basic light to sound converter works because any change at the base of the transistor will amplify the current to the headphones. Since the return laser beam will be bouncing around due to the vibration at the speaker, this will cause the headphones to respond to the beam as if it was an audio signal. Although the laser beam is moving, not modulated, the principals at work are exactly the same with the exception that in this configuration, the beam needs to be slightly offset from the center of the photoresistor so that when it moves across the surface, there is a corresponding change in voltage. If the principal at work here was in fact modulation, then a direct hit onto the surface of the photoresistor would be optimal.

Turn on your audio source and adjust the volume so you can barely hear the output on your speaker and then set up the laser and receiver so that the beam strikes the front face of the photoresistor. As soon as the beam hits the photoresistor, the impedance will vary significantly and you will hear a pop and probably a bunch of noise. Play around with the position of the beam to see how the position of the beam on the face of the photoresistor alters the reception of the audio signal. You will see that the optimal position for the beam is just touching the surface of the photocell so that any changes in the beam position from the vibrations will result in the most significant swing in voltage at the output of your headphones. If all you hear is a loud hum, then you probably have too much ambient light in your room. Incandescent light bulbs actually vibrate at 50 or 60 Hertz, and you will hear that in your system as a loud constant hum. As you have probably guessed, the Laser Spy system will not perform very well in the daytime due to ambient light sources competing with your laser beam, but this is fine since real spies usually operate in the darkness!

If you have your speaker and laser setup on the same workbench, then the process of targeting your photoresistor was probably only a 10 second job. Now, try to place your speaker at the other side of your room and see how long it takes to get the beam back to the target! I found that the distance across a room made the alignment significantly more difficult and even the deflection on the floor as I walked around made huge changes in the position of the beam. You will also have noticed that any slight vibrations of your desk or speaker stand resulted in all sorts of wild and wacky sound effects coming through your headphones. At one point, I was able to hear my own voice due to the thin surface of one of my tables vibrating the speaker stand.

Step 13: Example

Have fun with this instructables!!

#This was made with my efforts and money, so i kindly request every viewers of this instructables to vote me in First-Time Author contest. Thank you very much!!!

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