Long-range home hearing device

Whether you’re experimenting with the physics of light and sound or equipping yourself like the next James Bond, long-range devices are a fun and exciting addition to any detective collection. An optical hearing device is the ideal project for any spy enthusiast to do it yourself and can be built economically without requiring much technical knowledge.

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The laser’s microphone uses a beam of concentrated light known as a laser. In past decades, a portable laser unit would cost hundreds if not thousands of dollars. However, since the introduction of CD and DVD players, lasers have become extremely cheap. Keychain laser pointers are now normally sold in most large wholesalers. While a professional surveillance system could use an invisible infrared beam, any laser source will work. Visible lasers, in fact, are much easier to use when aligning the reflected beam and the receiver. For this project, a simple laser pointer on a keychain will work quite well.

The reflector

The beam produced by the laser is reflected off of a flat reflective surface like a window. Since the sound waves vibrate the glass, these same vibrations are transferred to the reflected beam. When lightning is captured with a light-sensitive receiver, these vibrations are translated into sound.

The receiver and the recorder

Receiving the beam as it is reflected outside the window requires a special piece of equipment called a photoresistor. This is an electrical component that reacts to light, converting the energy of light into electrical current. When the laser light bounces out of 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 connecting the photoresistor to a standard 1/8 “stereo jack, it can be plugged directly into a digital voice recorder or a portable computer socket.The variations in the current are treated as input from a microphone and recorded as sound, if used in an outdoor context, It is also a good idea to protect the photoresistor from the weather and possible interference from sunlight. This is easily accomplished by placing the sensor in a tubular protective box like an empty Pringles can. The can protects the sensor from external sources of light and from the elements.


Keep the laser stable and easy to aim when mounting the laser pointer on a camera tripod. Although it is useful it is not essential since any stable base will work. Position the laser so that it is pointing towards a reflective surface at a 45º angle. The reflected beam will then bounce off the window, also at an angle of 45 ° from the reflecting plane. Find the reflected ray. This will be much easier if you 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 hits the surface of the photoresistor. As with the laser, it is important to keep the receiver very still as any movement will affect the recording. Once you have obtained the reflected beam,