Instructables
Picture of Long Distance Laser listening device
the following project will show how you can use a laser pointer to hear noises from hundreds of feet away (the distance will be relative to the strength of the laser).


 
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Step 1: Materials

Picture of Materials
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We need
1. a laser pointer
2. a old pair of headphones (mic cable with stripped ends)
3. cadmium sulfide photocell (avaible at radioshacks)
4. 2 Tripods
5. A computer with audio recording software 

          

Step 2: Connect the Photocell to the Mic. cable

Picture of Connect the Photocell to the Mic. cable
twist the photocell wires around the stripped ends of the mic cables

Step 3: Testing the photocell

Picture of Testing the photocell
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1.first plug in the mic cable into the laptop.
2.notice the audio meter when the light hits the cell.

Step 4: Setting the position of the photocell and The Laser.

Picture of Setting the position of the photocell and The Laser.
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1.Attach The Photocell To Tripod #1
2.Set Laser To Tripod #2

Step 5: The Important Part.

Picture of The Important Part.
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1.Adjust Laser Until Reflected Beam Hits Photocell.
2.let the rays of laser reflect on a mirror or a glass to hit the photocell.
3.Now just hit record and then u can spy on them

Step 6: Instructions on the listening device

Picture of Instructions on the listening device
1.Find a room with a window you'd like to spy on, and aim the laser at the window at an angle.
2.Determine where the laser reflects, and situate your photocell so the reflected beam hits the front of the cell.
3.Hit record and listen!
4.Any noise from inside the room will cause the window to vibrate, which will get picked up by the laser and photocell. You might have to process the raw input to remove unwanted noise and isolate the voices. 

now here is a picture how does it work.