Cheap DIY build lets users eavesdrop with the speed of light

With all the electrical components available to consumers today, it is no surprise that the parts needed to build a working laser microphone from scratch are within reach, not just from online retailers but maybe even from e-waste around the house. Using three components, with an optional fourth, users can listen in on distant audio with their own laser microphone. The build is pretty straightforward as long as a user has a device that can accept and record mono audio input.

As SomethingAboutScience explains, a laser microphone works by shining a laser beam at a window or picture frame inside a room and capturing the reflection of the beam with a photodiode. Any audio generated in the room will send vibrations through the glass, causing it to modulate the laser reflection, which the diode can then read and help transform into audio. As a bonus, SomethingAboutScience explains that a red light laser isn't necessary, and a covert infrared laser can be used instead.

The photodiode is among the easiest components to source, as it is commonly used as an infrared receiver in TVs and some smoke alarms. However, the diode alone can't function as the sole processor of light, as the signal coming from the laser isn't strong enough. This is where an amplifier circuit can help boost the signal, provide power, and push mono sound output. A modified amplifier circuit, which looks like the MAX9814, was used in this case. To assemble the laser mic, the built-in microphone is removed and replaced with the photodiode, a USB cable is sacrificed and soldered onto the board to provide power, and a mono audio wire with a jack is added to the board to provide audio output. With this assembled, all left is to position a laser so that the beam hits the photodiode set-up at an angle to provide ample room to detect modulation of the laser's light and record the output.

While this setup is easy, it comes at the expense of effectiveness. Firstly, the laser and receiver are prone to environmental vibrations, and any physical obstruction between the two causes a break in the audio feed. To partially remedy this, SomethingAboutScience 3D printed a case and added a filter to the receiver for slightly improved results. Overall, this type of build is primed for enhancements and showcases how a little DIY ingenuity can go a long way. Those interested can find more information on the build and its results in the video below.