Laser microphones are often seen as the pinnacle of spy technology as you don’t have to install a transmitter to hear what’s going in a building. Instead a laser beam is projected onto a window and bounced back — sound waves from the voices in the target room are picked up with the laser beam and bounced back to your receiver where the audio is played through a speaker. You can build a basic laser microphone using a laser pointer, an NPN PhotoTransistor, a headphone amp and a handful of miscellaneous electronics parts.
Fringe electronics weblog Lucid Science created two laser microphones; a test system using a visible red laser and a final stealthy version incorporating an infrared laser. The full parts list, instructions, and schematics can be found at the source link below. The difficulty with both the test and final versions was that to get a clear audio signal it is imperative that the angle of the laser beam be close to 90 degrees to the surface of the window.
Build the long range Laser Spy system [Lucid Science]