Here’s a super simple laser microphone project which is fairly easy to construct, yet is still impressive. This laser microphone is a covert surveillance device which uses lasers to capture vibrations from a smooth surface. The bounced signal is then converted into an audio signal by a receiver. Check out the build instructions to make one or yourself for under $20.
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An invisible infrared laser is used to detect the vibrations of the window. A phototransistor picks up the reflected infrared laser light, and an LM386 audio amplifier is used to convert the signal into audio. While this laser powered spy device is not a new invention by any means (the ideas has existed since the 1940s), it’s still a fun and rewarding DIY security weekend project.