This $30 Laser Mic Turns Glass Into Gossip
With just a laser, photodiode, and amplifier, you can make your own laser microphone to eavesdrop on secret conversations like James Bond.
Movies like Mission: Impossible and James Bond may heavily dramatize the life of a spy, but in reality, espionage is often a more subtle and strategic game of cat and mouse. In the course of playing that game, intelligence agencies have developed some very sneaky technologies to try and give themselves the upper hand. One such technology is the laser microphone.
The laser microphone is by no means a new technology. The exact origins of these spy devices are, as you might expect, a bit murky, but they are generally believed to have first appeared in the latter half of the 1940s, as the United States and Soviet Union went to great lengths to gather intelligence on one another. A laser microphone helps in this area by using an invisible infrared laser beam to remotely detect the vibrations that sound waves transfer to the glass of a window. These vibrations can then be used to reconstruct the sounds that caused the vibrations.
As high-tech as these devices sound, YouTuber SomethingAboutScience showed that they are actually pretty easy to build these days. With just a few common parts costing around $30, you can make your own James Bond-worthy laser microphone.
Typically, an infrared laser would be included in such a build, so as to remain undetectable to the naked eye. But for demonstration purposes, SomethingAboutScience went with a red laser diode. After reflecting off a piece of glass, the light is measured by a photodiode, similar to the sort that might be found in a television with an infrared remote control. The photodiode converts light into electrical signals, but those signals are pretty weak, so an amplifier circuit was used to boost them.
The amplifier output was wired to a 3.5mm headphone connector and plugged into a computer. Next, a box with a piece of glass on one side was made, and a phone playing loud music was placed inside of it. The laser was then trained on the glass, and the reflected light was positioned just off of the photodiode. As the glass vibrated, it caused the laser light to oscillate over the top of the sensor.
The audio that this setup produced was surprisingly good. It did have some static, but the audio still could be heard clearly enough. SomethingAboutScience made a 3D-printed case for the laser/photodiode pair to position it on a tripod for stability, and also put a polarizing filter over the laser beam in an effort to eliminate ambient light and vibrations.
That did not make much difference in the audio quality, but it was noted that one could, in theory, use two lasers and photodiodes. Anything that did not match between the two sources could be removed computationally. That should clean up the audio further, but was not tested in this project.
If you have graduated beyond the toy-quality listening devices with big plastic dishes meant to collect and amplify distant sounds, then be sure to give this video a watch. You can make your own laser microphone in a few hours max.