Step 1: Materials
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
Step 3: Testing the photocell
2.notice the audio meter when the light hits the cell.
Step 4: Setting the position of the photocell and The Laser.
2.Set Laser To Tripod #2
Step 5: The Important Part.
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
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.