An El Cheapo Laser Listener (<$30.)

With the availability of electrical components, building a laser microphone from scratch is possible. Using three components, with an optional fourth, users can listen to distant audio [sort of]. The build is straightforward if a device accepts and records mono audio input.

As SomethingAboutScience explains, a laser microphone works by shining a laser beam at a window or picture frame in a room. The beam’s reflection is captured by a photodiode, which converts the vibrations through the glass into audio. A red light laser isn’t necessary; a covert infrared laser can be used instead.

For a more in depth look at laser eavesdropping, click here.