Using a basic laser pointer and a sensitive amplifier it is possible to listen in on conversations through exterior windows! The price of $20 is simply an estimate, in my case I did not need to buy anything.
The system described in this instructable works on the same principle as commercial projects like this:
NOTE: For a similar projects that may be of interest, check out my blogcheck out my blog. You may find of particular interest the Polarizing Laser Music Mixer!Polarizing Laser Music Mixer!
(Photo of me in the lab, with my own bad-ass glasses, replacing the random military dude who was there before.)
Step 1: Gather Materials
a building with a window to be listened-in on
a laser pointer
a magnifying glass
a phototransistor (aka IR detector, can be gotten from remote control recievers)
microphone preamp and amplifier (this can be replaced by a laptop with a MIC port)
an extention chord or batteries for the preamp/amp
someone to listen in on
a potentiometer (to control the laser brightness)
an infra-red laser (so the light can not be seen)
a digital camera (to see the infra-red laser during calibration)
extra bateries for the laser (upgrade to D cell or something)
(some of the needed equipment is shown below)
Step 2: Attach the Laser to the Tripod
In the photo below I also attached wires that run to an external battery pack for extended life.
Step 3: Rig the Reciever
If you are using a preamp, you should be able to screw the leads of the phototransistor right onto the back of the device, but if you are using a laptop, you will need to tape or solder togeather the pins of the phototransistor to the wires of a stripped headphones chord. Remember the ground (-) lead of the phototransistor is the one that has a little flat spot on the plasitc. Look to the photos for help.
Step 4: Position your Spy Gear
Look at the drawings below to help with the positioning of your gear.
(this only works if you are at the same elevation!)
Step 5: Aim the System
Once the laser is aimed, look for the reflected beam/dot near the recieving station. White paper or cardboard (pizza boxes) can come in handy. Position the phototransistor in the beam.
Step 6: Magnify
At this point you may need to adjust the volume on the amp and preamp. Turn it up until you hear feedback, then turn it down until it goes away.
Step 7: Give it a Try - Taking it further
If you get this project to work, the worst thing you could do is stop there! Try to build on the project. Try more than one laser, or getting more distance, or whatever comes to mind. One thing I have done to extend the project is to send music over the laser beam, and even sending two channels of music using two lasers polarized at right-angles to one another.
Feel free to leave some comments.