Nowadays, on the Net you can find instructions for everything, ranging from the recipe for a pasta dish all the way to details for making a homemade bomb.
Therefore, it does not come as a surprise if you happen to find the instructions for making a amateur spy microphone.
At a cost of about $ 20, virtually anyone can be able to build a small laser light spy microphone. This microphone is able, through an invisible infrared laser beam, to detect the slightest vibrations of a door or window, caused by sounds produced by the human voice beyond the wood, metal or glass.
The laser beam is collected by a transistor, and thanks to a small amplifier it is then converted into an audio signal. The process of building up the whole thing is relatively easy and would not take too long.
Obviously though, this is amateur equipment, which at most can be used to spy on your neighbor (possibly with harmless intentions and just for fun!). For professional use, it is definitely advisable to use a completely different kind of equipment, such as a directional microphone.
Based on the same concept, laser directional microphones are capable, if their invisible beam is pointed at the glass of a window, to read vibrations and pick up conversations behind it, up to a distance of over a mile. This kind of equipment for remote listening is recommended for use in completely different situations, for example for police forces gathering evidence, or for private detectives during an investigation.
Obviously, compared to this amateurish spy microphone, these devices have a much higher cost, but the result you can achieve are in a class of their own, and can also be used as evidence during trials.