Laser Mic Makes Eavesdropping Remarkably Simple

Here’s a surprisly simple way to build yourself a laser-based listening device. It consists of two modules, a transmitter and a receiver. The transmitter is a set of lasers, one is visible red for aiming, and the other is infrared for measuring the vibration of a surface. Point the transmitter at the window of the room you want to listen in on and the laser can be reflected back to the receiver. The receiver module has a phototransistor to pick up the infrared laser light, and an LM386 audio amplifier to generate the audio signal sent to a pair of headphone. The need to be well-aligned which is easy enough using a pair of tripods. Check out the demo after the break.

Looking for something to do with the leftover laser diodes from this project? Try making yourself a laser microscope.


55 thoughts on “Laser Mic Makes Eavesdropping Remarkably Simple

  1. This is straight out of spy gadgets for the evil genius… and I’m really happy that they proved the concept.

    Alot of FUD and disbelief surrounding the phototransisor theory.

    Green laser+developed 35mm film=infrared laser btw.

    1. There was an article on building one of these in, I believe, Popular Electronics, several decades ago. I have the photocopy in my files. Of course it isn’t as small and simple as the above, but the principal was the same.

  2. Beautiful work!

    I swear HAD and the supply of junk parts at work have REALLY got me energized towards a couple of different tinkery hacks.

    I really have to document and share.
    I’m sorry for slacking. :(

  3. Old. But new for anyone not into electronics circa 1969, or 1971, or 1978, or 1984, or 1992 or any of the other times this was all over the place. Listen to the bitter old guy, kids… You can do this with any light source you can bounce off a window. You don’t necessarily need a light source, but I guess that’s still spy stuff.

    Wanna defeat this? Close the curtains/shades and run a couple fans or white noise generators – like a radio. S/N ratio vanishes and unless your spy is a DSP demi-god, you’re safe.

    Let me guess – the next big breakthough will be putting microphones in pipes of varying diameters to get a high gain directional mic, or someone will realize that you can put a mini version of this built with soda straws on a directv dish and listen to people a block away…

    Or wait, wait – the 1960’s vintage russian passive modulation microphone bug triggered by microwave energy and found by the 1970’s in the big eagle at the state department.

    and… and… wait! wifi IS microwave energy…
    yawn! Now someone bring me a 1922 copy of popular science so I can read about flying cars again.

    1. Well, we have flying cars now, just trying to get them commercialized and FAA approved.

      Go takes your meds and stop yelling at the kids on your lawn, you don’t sound anywhere near as insightful as you think

  4. My father was an oilfield tool company executive who traveled to the former Soviet Union on business. He and his fellow execs were sitting in their Moscow hotel playing cards and drinking vodka when he noticed a red laser dot on the curtains.
    “What’s that?”
    “Oh, don’t worry about that,” said one of the old Russia hands, “That’s just the KGB bugging us with a laser microphone.” And the game continued uninterrupted.
    This was about 1981.

  5. this is well know but absolutely unpractical way to spy (that the reason it is known to public). It hard to find reflection (almost impossible for IR) The air is not constant pressure and temperature so it bent and twist laser enough to make it useless for sound carrier. You will be detected soon with viable laser and for Infrared Sun is giant IR projector carrying white noise, as for night fluorescent light emit too much IR to dim any useful signal. Last but not least when was the last time you seen single layer glass window ?

  6. No one noticed they are bouncing off a mirror attached to the speaker? There’s no way a window would give that sort source signal quality. Good for a POC, but would like to see it working on a window.

    I’m guessing for it to work on a window, you’d need to amplify the received signal another 1000 times?!?!

    1. I think you’re correct, the window is not going to vibrate as much, not even close. This detector is based on that the laser is aimed at the reflector at an angle and the reflected beam will then move and the movement can be detected at the edge of its spot.

  7. @therian if you recess the detector and add a small telescope attachment then you can get only the laser to arrive at the sensor and lasers come in a very small bandwidth so with the right filter at the receiver only the laser signal will be evaluated, as for double windows, you can just reflect off the inner one and that actually would be an advantage since external noise would be filtered out.
    As for your air argument, didn’t the guy just demonstrate it works? And didn’t they already use this in real operations? Not that there isn’t a distance issue, but you are too optimistic if you think you are safe from this.

    And as for it being known to the public, I think an amateur actually thought this up originally.

  8. Pretty low tech for 2010, btw.

    The hip kids simply focus the light from any internal light source (in the room) and record the modulated signal of the flourescents [and they ALL use florescents]… I’m leaving an important step out, said the null, but anybody with any physics knowledge can fill it in.

    Then you run the signal back through signal processing and get voices back out. Well, voices plus A/C plus random electrical noise plus thermal effects plus so on and so on, which is why real bugs are preferred 12 to 1 by most dentists.

    Just about the only thing this crappy tech gets used for these days is listening to in-vehicle conversations, which is why cracking a window… well, who needs that information anyway. You all carry microphones with you now.

    However, if you live somewhere people still get it on in cars, you can assemble a lot of spectacularly low quality audio porn. But really, listening to “wait, go slower”, “Ow, that really hurts” and “Did you bring condoms” and my personal favorite “Shhh, did you hear something” was interesting for about a week in 1971.

    Then I discovered that it’s way more fun to have a remote controlled air horn (like boaters use) and simply trigger it when the car “steams up and starts rocking”. The first IEOD (instant erectile dysfunction on demand) the world has ever seen* circa 1971 using a filter tuned to CB radios.

    Ah, america, how I miss you and your radio shack battery cards.

    * barring: crying babies, parents arriving home unexpectedly, olfactory assaults and hearing your mistress say “I got a call from your wife today, Bob.”

  9. bob don’t forget our rigged transcontinental communication boxes that could melt if they were sent the right frequency-preferably in a wartime. Pre-satellite it could cut a country off from the rest of the world lol.
    And yeah this has been in almost every spy book that Delta Press has sold. Geologic survey lasers to monitor avalanche conditions ;)
    The super embassy bug was built by the man himself, Theremin. Etherwaves and Espionage. Decent read. I always wanted Tesla and Theremin to team up and set the moon on fire :)
    Thanks for the post :) Good times…

  10. Heh. Vibrating didlo/toothbrush works well i hear. <3 burn notice.

    remember to watch out when working with high powered IR lasers. They can do some serious eye-damage if you aren't careful

  11. I know this works.
    But there is a better, not easier, way to decode.
    It depends on interference. You have to overlay the original and the reflective beam. Since the movement of the window changes the runtime of the reflective beam, you can decode the sound from this.
    I am searchin for plans for this kind of laser mic, but never found them again.

  12. Modern surveillance devices of this type use microwaves to penetrate walls. They are also immune to curtains and noise maskers as you can target the individual directly, if your beamwidth is narrow enough. You need to modulate the RF carrier with an ultrasonic modulation and extract the vibrations using lock-in amplifier. Google William McGrath.

  13. @Allen dulles = Excellent troll, It’s a website, but the website links do point to the original magazine articles with construction info.

    The last link is an amateur’s wet dream, and not going to yield pay dirt with the junk he’s using… pretty block diagrams, wrong approach.

    Let me say this: There is nothing you will think of in surveillance that hasn’t already been thought of, proposed, rejected, reqqritten, funded, hastily prototyped just as the money was about to run out and discarded as useless upon project cancellation – at least three times.

    If you can’t het the signals with two or more tunable frequencies in basically real-time to cope with breathing, gas, head shifting and other muscle distractions, it won’t work.

    It’s like decoding FSK data using your ears – not a chance of valid data at the speeds required.

    The catch with direct interrogation of the voice boxes of biologicals is that it’s useless if they’re not sitting still, you have to basically cook them to do it and the shear amount of dsp that needs to be done to try and pull text out of formants. You’re more likely to interrogate the inner ear of a participant than pull useful speech from the throat of someone. Ever hear a throat mic?

    High resolution strobe studies have been done with no results, and the two or three membranes that you can monitor to reproduce sound from are too small to be useful. But let’s say your method was “adequate” and you could chet the membrane and pull sound back out of it…

    It’s much worse than the problem of pulling text from muddy speech recordings, as we still need to “point the mic” except under lab conditions. In theory computation can do anything, but in practice it isn’t that good.

    Now, imagine that the user is moving in a random hotel room, and you want to do speech capture. Good luck with that, even with a decent scanning device and a static polar map of the room.

    Again, cellphones to the rescue here… Cellphones [and a company that shares initials with National Semiconductor who are “the keeper of the keys”] have single-handedly delivered the holy grail of surveillance to anyone with the right connections, or failing that, adequate funding.

    BTW, the amount of gear required to correctly sweep and monitor a room using complex heterodyned microwave signals is substantial, and isn’t gonna be available in the wild any time soon.

    And – MD – even microwave penetration of walls STILL relies on cell phones or a cavity bug.

  14. Probably time to stop posting, I’d guess, before I go full PKD on everyone.

    Anyway, this kind of stuff is used mostly for following breathing rates and heart-beats from a distance. You can track pulse rates pretty well with an ordinary laser. Ah, 7th grade, how I miss it.

  15. I invented a multi-frequ adjustable audio osc.
    that(1)amped and applied to a speaker on a window
    would give serious competition to those reflected
    laser beams, & (2) run through phase shifters can
    produce a rotating, head twisting effect on in-
    truders silly enough to hang around more than a few seconds. Some of the older projection TVs had an aluminized mylar mirror, not glass, that could have interesting uses for hackers.

  16. hi , can any buddy plz help me tht hw IR beam is received at the receiver end ? CZ IR is invisible and expanding in all directions so how are we gona converge it at receiver end

    1. @mikal you maybe a little confused. normal IR bulbs spread out as they are incoherent light. but this is an IR Laser. laser light is a coherent, essentially a beam of photons at a specific frequency & wavelength, therefore is easy to converge. the pro kits use the red laser to aim and IR for the audio, you can then see where the beam is reflected so you can align with the receiver, then switch the red off. alternatively you can use a video camera (removing the ir filter from the sensor) to view the IR beam without needing the red laser to aim.

  17. Truth be told that USG knew of a secret method to intercept audio from old style 500 set AT&T phones for decades. After WW2 many federal agencies where unaware of it. It involved “illuminating” the carbon microphone with a ultrasonic transmitter on the wire from a few hundred feet away. The properly focused reception device would do the rest. Some of these old phones are still being used in 2015. They are banned at FBI et al.

    The LASER BUG was in fact invented by THEREMIN and his ex-NAZI scientists after WW2. The solid-state LASER was actually invented by NAZIS not Dr. Ted Maiman at Hughes Aircraft.Laser bugs as history would have you believe, they where a Soviet thing. They all suffer from angle of aiming is proportional to opposite angle of return (bouncing away from you). So straight-on aiming is required. But now there are optical metamaterials that allow special return angles and is totally unnoticed on the target window. The window glass prism method is too easily detected as it is too big.

    Ideally you can aim at interior reflective objects like wall mirror and not depend on a window. By modulating the laser beam with a ultrasonic or rf signal much interference from other sources can be avoided. By using DSP (digital signal processing) you can avoid “buzzers”. White noise is a lot harder to avoid though, but not impossible.Curtains only slightly mute the room audio but not negate it. Just need more audio amplification to overcome that.

    You don’t need a visible laser for aiming. There are special eyeglasses or cameras that allow this.An IR laser does NOT cause eye damage.You can detect laser bugs with cheap or toy night-vision goggles or a simple modified toy digicam. However, lasers not needed when you can use ultrasonics or microwave interrogators, they bounce off of metal objects like old fashioned burglar alarm window tape.

    There are other devices that can bounce off of target’s skeleton, skull, teeth, that can return spoken audio, not room audio. Not very good audio but audio nonetheless. All of these types of bugs allow audio interception without actually entering the premises under guise or breaking and entry.You can also use lower-tech shotgun mics or parabolic mics.

    The best bug is a store-forward type. It never transmits a signal until remotely interrogated by the bugger to download it’s stored MPEG storage. Also scheduled Wi-Fi connection/upload could be used too, It is not impervious to the NLJD detectors (microphone detectors) – just use old fashioned carbon, dynamic, or crystal mics not electrolytic. Also Faraday shield of the chassis to ground (need to block 900 Mhz at 25 watts to defeat). It requires covert placement though.

    If someone could invent a multi-target lip-reading robot with A.I. would be a real coup. All needed is wide-angle telescope and video camera for the robot to see target’s lips moving. If a human can do it then a robot (i.e. computer) should be able to duplicate it.

  18. I suspect a neighbor is listening to use via our windows. We have white noise generators on almost all windows along with white noise playing through a stereo system through out the yard. How can I stop them from listening ?

    1. Get a toy night vision scope from JAAKS. They are cheap. Sometimes on EBAY or AMAZON. Walmart used to sell them. They sell real NV goggles in Sporting Goods but are expensive. You can see the infrared laser being aimed at your windows. Also a SONY HandyCam on night vision mode will work too. Try putting your electric razor on the glass. Really makes the bugger’s day. Also double hung windows thwart laser bugs too. There is an air gap between the panes.

      Real fun would be to do NOTHING at all and just feed him disinformation. Set up an amazing false story and watch what he does to investigate further what you said. Like you buried $5,000 cash in a coffee can in backyard near his fence out of sight. You marked it with a flag or something. Then put a web cam on it and watch what happens. Or some variation on that theme in where he would have to reveal himself in an uncomfortable way. Think of something really creative…


  19. I seen these before and thought about building one for fun,
    However, as someone in the comments said,
    why do you need a laser at all,
    If the room is illuminated inside, the image of the window could be brought to a focal point, on to a photo transistor + Amp.
    Also, the aiming Red Led could be reduced in power, so it’s not even visible and even turned off once you have allignment.
    Great article.

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