Whether you're experimenting with the physics of light and sound or equipping yourself as the next James Bond, long range listening devices are a fun and exciting addition to any sleuth's collection. An optical listening device is the perfect project for any do-it-yourself espionage enthusiast, and can be cheaply constructed without a lot of technical know-how.
Laser Microphone Basics
The optical listening device, sometimes called a laser microphone, uses a concentrated beam of light to listen in on sounds hundreds of feet away. By reflecting a laser beam off of a suitable reflective surface, the sound vibrations in a different building can be listened to and recorded.
While older technology made the use of these devices prohibitively expensive, the components can now be purchased for very little. A laser microphone system consists of four main parts: the laser, the reflector, receiver and recorder.
The laser microphone utilises a concentrated beam of light, known as a laser. In past decades, a portable laser unit would cost hundreds, if not thousands, of dollars. Since the introduction of CD and DVD players, however, lasers have become extremely cheap. Keychain laser pointers are now commonly sold at most major retailers such.
While a professional grade surveillance system might use an invisible infrared beam, any laser source will work. Visible lasers, in fact, are far easier to use when aligning the reflected beam and the receiver. For this project, a simple keychain laser pointer will work quite well.
The beam produced by the laser is reflected off of a flat, reflective surface, such as a window. As sound waves vibrate the glass, these same vibrations are transferred to the reflected beam. When the beam is captured with a light sensitive receiver, these vibrations are translated back into sound.
The Receiver and Recorder
Receiving the beam as it is reflected off of the window requires one special piece of equipment called a photoresistor. A photoresistor is an electrical component that reacts to light, converting the light's energy into electrical current. When the light of a laser is bounced off a window, the sound vibrations are transferred to the reflected beam. These vibrations, when picked up by the photoresistor, cause variations in the electrical output of the sensor.
By wiring the photoresistor to a standard 1/8" stereo jack, it can be plugged directly into a digital voice recorder or laptop microphone jack. The variations in current are treated like input from a microphone, and recorded as sound.
If used in an outdoor set-up, it is also a good idea to protect the photoresistor from weather and possible interference from sunlight. This is easily achieved by placing the sensor into a tubular housing, such as an empty Pringles can. The can shades the sensor from external light sources, and protects it from the elements.
Keep the laser stable and easy to aim by mounting the laser pointer onto a camera tripod. While helpful, this is not essential, as any stable base will do.
Place the laser so that it is pointing at the reflective surface at a 45 degree angle. The reflected beam will then bounce off of the window, also at a 45 degree angle from the reflective plane. Find the reflected beam. This will be far easier if you have used a visible laser instead of an infrared laser, which is invisible to the naked eye.
Once the beam is located, place the receiver so that the beam strikes the surface of the photoresistor. As with the laser, it is important to keep the receiver very still, as any motion will disrupt recording. Once you have acquired the reflected beam, plug your receiver into your recording device. You are now set to listen and record.