Figure 0 - Build the long range Laser Spy system
Build the long range Laser Spy system

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.

Figure 1 - You will need some type of laser to bounce off of the target window
Figure 1 - You will need some type of laser to bounce off of the target window

The obvious first component in the Laser Spy system is the laser, which will target a distant reflective object and send the beam back to your receiver for decoding. Before digging deep into this project, let me explain how this system works, and dispel some of the myths that are circulating on the internet regarding the operation of this device. First of all, these laser spying devices do NOT work on modulation of the laser beam like some laser communication devices. Modulation of the laser beam is impossible because it would take some type of circuitry installed in the laser driver to actually modulate the intensity of the beam, and the laser is going to be installed at your location, not the target location. The principal that is at work here is not modulation, but movement! As the laser reflects from the target window, the slight vibrations from conversations or noise that vibrate the windows cause a very slight change in position of the returning laser beam. This change in position is converted into voltage as the sensor in the receiver catches the returning laser beam. This is why the optimal operation of the receiver requires the laser beam to be slightly offset from the phototransistor as will be shown. So movement, not modulation the principal on which this system operates.

You can use any laser you like for this project, and there will be no quality difference whatsoever between a state-of-the art lab laser and a $2 pointer. The only disadvantage to using a cheap laser pointer is that you will have to modify it for an external battery pack if you plan to have it on for more than a few minutes at a time, but that is an easy task. Also, a visible red or green laser will not allow for covert operation due to the fact that it is very easy to pinpoint the source of a laser beam, especially at night. For this reason, a visible laser is used for initial daytime targeting and then an invisible infrared laser is switched for optimal long duration and night time operation. Of course, this all depends on your intended use, and for demonstration purposes a visible red laser is best.

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