Long Range Spying Using Laser Microphone

This easy-to-construct laser microphone is capable of converting surveillance device which uses lasers to capture vibrations from a smooth surface. Also illustrated is the careful alignment of both the laser and receiver for optical performance. A receiver converts the bounced signal into an audio signal. The completed laser spy system is shown in the image below.

To bounce off of the target window, some type of laser will be needed as shown below since the laser is the obvious first component in the Laser Spy system. This will target a distant reflective object and send the beam back to the receiver for decoding. The intensity of the beam will be modulated by some type of circuitry installed in the laser driver. For initial daytime targeting, a visible laser is used and for optimal long duration and night time operation, an invisible infrared laser is utilized.

The image below shows a small speaker that will be used to stimulate a vibrating window. It connects to some audio source such as a radio or computer headphone port in order to create a test window to allow the deflection of the laser beam. The laser beam can be deflected using any small piece of a highly reflective surface such as a mirror. Since the laser beam will only be a few millimeters across when it strikes the surface, the size of the mirror is not important.

Some audio source needs to feed the speaker in order to recreate the same conditions that will be dealt during covert spying operations. A portable player with a low power amplifier and runs for hours at a time is a perfect choice. The player is set to loop independently and the volume is adjusted as low as possible.

The base of the speaker should be easy to align since the key to testing the operation of the Laser Spy device is the ability to move the speaker to any angle and secure it. It should also be adjustable for basic indoor setup as well as the targeting laser. When creating and debugging this design, the adjustable window pane simulator will help as shown below.

To respond to a change in light, a photoresistor is used which contains CDS cell that hides behind the plastic lens. It is the small disc with two leads and a wavy line on its surface as shown below. Its internal circuitry converts the light into sound where the change in resistance from the photoresistor is fed into the base of the transistor or switch that controls the current to the headphones.

On a test bench, the speaker is targeted by installing the laser on some type of adjustable stand or held in place at an approximate horizontal position as the reflected beam is caught at the breadboard. The law of reflection applies when setting up the laser and speaker. The deflection becomes much greater as the distance from the target increases. So the beam will bounce back at a higher angle if the beam is lower than the speaker mirror.

Since any change at the base of the transistor will amplify the current to the headphones, this basic light to sound converter works. The headphones will respond to the beam as if it was an audio signal because the return laser beam will be bouncing around due to the vibration at the speaker. The fully functional test system is shown below.

A photoresistor is not the most optimal part for the job due to being slow although it is able to convert the movement of the laser into a voltage change. A phototransistor acts like an amplifier, has a much faster response time, and has a smaller reception area which makes it much better light receiver.

For the light receiver, a suitable box is needed in order to mount it on a tripod. The placing the lens of the phototransistor on the front of the cabinet as well as placing the headphone jack and volume control on the rear is the optimal installation. The box shown below will provide enough room for the battery, controls, and the small circuit board. The power switch, volume control, headphone jack, and phototransistor are installed onto the box once the small circuit has been completed.

To create traces, wires are used on the underside of the perfboard or bend the leads of the semiconductors. Also added were the 9V battery connector and all of the wires that connect to the switch, headphone jack, and volume control. All of the parts can be held from bouncing around inside the box by a double sided tape, Velcro or hot glue. The battery should be placed in a position that makes it easy to remove since it will eventually be changed.

The completed receiver with a tripod mount for the laser and the receiver is shown below. A hole is drilled and then threaded to affix the box to any tripod. All parts are ensured securely mounted since any slight change in position can easily throw the beam out of alignment as the devices are mounted to a tripod with a bit of Velcro. Also shown is the tripod mounted light to sound receiver and the completed Laser Spy components.