DIY laser long-distance listening device
- by Dan Chilton (RSS feed) on Aug 22nd 2007 5:00PM
- Filed under backyard science, household hacks, recreation, toys, weekend projects, geek it yourself, electronics
No DIY espionage kit is complete without a long-distance listening device, and no DIY long-distance listening device is complete unless it uses a sweet laser in some form or fashion. If you agree with that statement, then you're in luck, because 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).
As with any tutorial or project that has the potential to be used illegally, we should warn you only to use this new-found knowledge for good, not bad. We won't be held responsible if you get caught eavesdropping on your crazy old neighbor, and wind up being incarcerated for the rest of your life. There's a fine line between good spying (i.e. bird watching, neighborhood watch, etc.) and bad spying (*cough* patriot act *cough*), so please err on the side of caution here.
Materials
- Laser pointer
- Old pair of headphones
- Cadmium Sulfide Photocell (available at most Radio Shacks or electronics store)
- Tripod
- Binder clips or duct tape
- Laptop or other recording device
- Soldering iron with solder
- Wire cutters / stripper
Between one to two hours for construction, set-up, and alignment.
Steps
- Clip the earphones off your old headphones, and strip back the protective conduit to expose three wires (red, white, and black).
- There are two contacts on the photocell. Solder both the red and white wires to one contact, and the black wire to the other.
- Plug the headphone jack into your laptop or other recording device.
- Using binder clips or duct tape, secure the photocell to something sturdy so that it can be easily moved and aligned.
- Again using your clips or tape, attach the laser to your tripod and secure the power button so the laser remains on.
- Find a room with a window you'd like to spy on, and aim the laser at the window at an angle.
- Determine where the laser reflects, and situate your photocell so the reflected beam hits the front of the cell.
- Hit record and listen!
[via geeks are sexy]
Comments [13]
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Reader Comments (Page 1 of 1)
8-22-2007 @ 7:04PM
Nick said...
Hmm, it looks like if your target is 200 ft away and your beam is even one degree off perpendicular to the surface of the glass, your laser and your detector will have to be roughly 7 feet apart. So if you want to use this from "hundreds of feet away", it'll be really inconvenient unless you're damn close to perpendicular.
And now it occurs to me that there's kind of a rock and a hard place thing going on here: for this to be convenient, you want to be very close to perpendicular but the reflection will be stronger the farther from perpendicular you are. Hmm...
Reply
8-23-2007 @ 2:08AM
Jason said...
Very cool. Thankyou for fulfilling my nerdiness today
Reply
8-23-2007 @ 3:07AM
Jason said...
Hmm, thats cool but will it be easy to find the reflection from a range more than 100 metres, and detecting it? Any ideas on best way guys?
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8-23-2007 @ 6:01PM
Jason said...
Cool, though i imagine it be very difficult to detect beam from a range more than 100 metres, what would be best way?
Reply
8-23-2007 @ 3:27AM
alex said...
This device wont work as effectively on double pained glass for two reasons.
First-the glass is thicker with and there is an air buffer zone in between the two panes.
Second- the laser will have two reflections, one off of the outside pane and the other off of the inside pane. so setting up the photo cell would be difficult.
Reply
8-23-2007 @ 6:45AM
Eugene said...
The cia used this successfully but also ran into the perpendicular problem.
their solution was to embed micro prisms into glass and impersonate building maintenance personel and install this new glass in buildings they wanted to bug. this allowed the laser to work at an angle allowing the bug to work from ground level on tall buildings
Reply
8-23-2007 @ 7:44AM
G said...
Actually I made one of these in college. Using an ultra sensitive photo transistor, cascaded amplifiers and voice-band filter. I am skeptical of the results in this particular set up. The linear region of the photocell is very small, and to detected amplitude change, the photocell needs to operate at the linear region. With a laser, the photocell will either saturate when the beam hits it directly, or it will not detect light at all. In addition, there was no pre-amp for the photocell, with means the change in the induction of current into microphone jack would be in the millivolt range (and that's on a good day, probably in the sub-millivolt range). Add in the noise from a 2 feet unshielded audio cable and you won’t hear anything. Not to mention, the natural harmonics of the window itself would be unfiltered (more noise), as well as vibrations to the laser and the receiver. These factors would be more than ample to flood out the vibration from the voice.
Reply
8-23-2007 @ 10:10AM
Bubba said...
"And now it occurs to me that there's kind of a rock and a hard place thing going on here: for this to be convenient, you want to be very close to perpendicular but the reflection will be stronger the farther from perpendicular you are. Hmm..."
Hey Nick - nothing says the laser pointer and pickup need to be near each other.
Reply
8-23-2007 @ 12:28PM
Tyrone Shoelaces said...
I worked in a Navy research lab around 1973. We developed this very tool and built it into a pair of binoculars. You put on the headphones, and just looked at the window in question. The laser and photocell were built in and it worked. I don't know if it ever worked well enough. It wasn't my project.
Reply
8-23-2007 @ 12:57PM
sygyzy said...
Wow, I have never seen an entry with so many legitimate (and helpful) comments. [I know, sorry for breaking the trend, but I had to applaud these folks!]
Reply
1-08-2009 @ 10:05AM
ASLAM said...
really cool man maybe with more power laser u can increase the range
Reply
10-10-2007 @ 6:26PM
gotpls said...
Couldn't the subject see the laser beam if they looked out the window? Is there an invisible laser beam available and where might one purchase that?
Reply
11-21-2007 @ 3:57PM
jim said...
hi
any idea if this will work listening through concrete?
is there anything out there on the market that really works listening through concrete?
thx
Reply