Thursday, November 24, 2011

Laser microphone for audio surveillance via window panes





I bounced a laser beam off of a window in my house and recovered the audio from inside the room via the beam deflection. I used a Hamamatsu S7815 amplified photodiode and connected it with a 9V battery to my stereo's microphone input jack. The audio quality was very low -- probably due to the double-pane windows in my house. Speech was just barely intelligible.

I also tested the procedure of bouncing a laser beam off of a framed picture that is hanging on the wall inside the room to be monitored. The reflected beam will hit a wall somewhere else in the room, and the dot can be monitored by a telescope from remote. The goal would be to measure the beam wobble via the telescope and recover the audio without needing a stringent geometric relation to the target room. This didn't work at all, but I think with a sensitive detector, it has potential.

More about laser microphones:
http://www.williamson-labs.com/laser-mic.htm
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4 comments:

  1. ptxNovember 24, 2011 at 11:30 AM

    Lucid Science have a nice description of a similar project: http://lucidscience.com/pro-laser%20spy%20device-1.aspx

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  2. KeithNovember 26, 2011 at 8:38 AM

    Looks like a good one for Mythbusters as well ... only they will never show us the interesting details that folks like us need. That's why we have to do it ourselves I guess. Funny how I knew it was Tull before you said it.

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  3. KC9IQWDecember 4, 2011 at 8:00 AM

    I love this web page Mr. Ben. When I was a boy my friend Don and I taped a mirror to the back of a pie plate and talked into it. We listened to the suns reflection with a photo transistor put across the input of a radio shack amplifier that looked just like a transistor radio. Seemed to work pretty good, and was a big thrill for a couple of sixth graders. Good times back in the 1970's. Very best regards, Tom, AB9NZ, http://radiotelegrapher.posterous.com/

    ReplyDelete
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