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:
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(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.