Laser Surveillance System for Under $20




About: I am a student and hobbiest. I'm into physics, linguistics, sailing, electronics, hacking (not cracking), music, and the like. I post most of my project stuff on my blog: ...

WARNING: this project involves the use and modification of laser devices. While the lasers I suggest using (store-bought red pointers) are relatively safe to handle, NEVER LOOK DIRECTLY INTO A LASER BEAM, BEWARE OF REFLECTIONS, and be EXTREMELY CAREFUL when MODIFYING a laser product. Also, I am not liable for anything stupid you do.

Using a basic laser pointer and a sensitive amplifier it is possible to listen in on conversations through exterior windows! The price of $20 is simply an estimate, in my case I did not need to buy anything.

The system described in this instructable works on the same principle as commercial projects like this:

NOTE: For a similar projects that may be of interest, check out my blogcheck out my blog. You may find of particular interest the Polarizing Laser Music Mixer!Polarizing Laser Music Mixer!

(Photo of me in the lab, with my own bad-ass glasses, replacing the random military dude who was there before.)

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Step 1: Gather Materials

All you need for this project is:

a building with a window to be listened-in on
a laser pointer
a tripod
electrical tape
a magnifying glass
a phototransistor (aka IR detector, can be gotten from remote control recievers)
microphone preamp and amplifier (this can be replaced by a laptop with a MIC port)
an extention chord or batteries for the preamp/amp
someone to listen in on

optional stuff:
a potentiometer (to control the laser brightness)
an infra-red laser (so the light can not be seen)
a digital camera (to see the infra-red laser during calibration)
extra bateries for the laser (upgrade to D cell or something)

(some of the needed equipment is shown below)

Step 2: Attach the Laser to the Tripod

Tightly attach the laser to the tripod with tape. Also tape or rubberband down the power button so that the laser stays on.

In the photo below I also attached wires that run to an external battery pack for extended life.

Step 3: Rig the Reciever

Phototransistors work very similarly to microphones in that they vary the current they allow to pass through them when exposed to chaning light levels. So, treat the phototransistor like a mic, and attach it to the MIC port on a laptop, or to the MIC terminals of a preamp, then hook the output of the preamp to the input of an amplifier.

If you are using a preamp, you should be able to screw the leads of the phototransistor right onto the back of the device, but if you are using a laptop, you will need to tape or solder togeather the pins of the phototransistor to the wires of a stripped headphones chord. Remember the ground (-) lead of the phototransistor is the one that has a little flat spot on the plasitc. Look to the photos for help.

Step 4: Position Your Spy Gear

Find a target building and window.

Look at the drawings below to help with the positioning of your gear.
(this only works if you are at the same elevation!)

Step 5: Aim the System

Stand by the laser tripod. Aim the laser at the window. You should be able to see the reflection of the recieving equipment in the window. If not, reposition the laser or the reciever until you can. If it is dark out, turn a flashlight on near the reciever pointed at the window, that way you can see the reflection in the window and locate the laser appropriately.

Once the laser is aimed, look for the reflected beam/dot near the recieving station. White paper or cardboard (pizza boxes) can come in handy. Position the phototransistor in the beam.

Step 6: Magnify

Position a magnifying glass in front of the phototransistor, in an attempt to focus the beam. You should hear the most noise from the amp when it is positioned correctly.

At this point you may need to adjust the volume on the amp and preamp. Turn it up until you hear feedback, then turn it down until it goes away.

Step 7: Give It a Try - Taking It Further

Listen for voices. You should be able to hear the low frequencies of any conversation inside the room of the targeted window. If you cannot hear anything, try banging on the window! That should produce a VERY loud sound from the amp.

If you get this project to work, the worst thing you could do is stop there! Try to build on the project. Try more than one laser, or getting more distance, or whatever comes to mind. One thing I have done to extend the project is to send music over the laser beam, and even sending two channels of music using two lasers polarized at right-angles to one another.

Feel free to leave some comments.
Happy experimenting!

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    187 Discussions


    7 months ago

    Cool project! I was wondering if I am supposed to attach to mic to the laser pointer and then plug it into the laptop?


    Wow, super cool build! Will this work on double-paned windows (like ones for cold climates)? Also, will the subjects of your surveillance be able to see a laser dot? Thank you.


    6 years ago on Introduction

    hi , dear ,i am thankful to you for your this work done on this area , i need your help in this project as i am also doing my project in my university but i am doing on IR beam of 980nm and 50mW
    i dont know how to detect this IR beam and then after striking on window i.e target how to demodulate this IR beam into audible sound. i seriously need help please

    1 reply

    Reply 6 years ago on Introduction

    First of all, BE EXTREMELY CAREFUL with IR lasers. 50mW is a LOT of power. That laser can cause serious eye damage, and you won't even know because you can't see it. I strongly recommend that you try this project first with a visible light laser, and only move on to the IR laser if you or someone helping you is experienced with IR radiation and you have taken all the necessary precautions. -- Furthermore, if you are shining this into a building, the people inside will need to know about it to avoid getting eye damage from the laser. It might work well for government spies, but it would really suck to be on the receiving end of a blinding laser. -- I have had some temporary vision loss due to IR in the past, it's no joke.

    That said, it is done almost exactly the same way. You just need to adjust the power supply to fit the laser you are using.


    7 years ago on Introduction

    I have not tried this but if you are on the same level as the target room both rooms are equipt with crank out casement windows. The entire apt bldg's glass has IR flim on them for heat reflection. The target apt's left window's left halfside is cranked open to 45 degree angle and the right window (observation point next room down) it's right halfside is cranked open to 45 degrees. Now both windows are in alignment facing each other. Sound from one room will reflect into the other via the positioned windows. Hint: the observer must be quiet. If an infrared illuminator coupled with a digital camcorder with Congo Blue filter on the lense is setup. I think you would be able to see into the other apartment's room.


    12 years ago on Step 7

    Would one be able to do the same, but in reverse? Could a signal or vibration be sent to the window by the lazer? Could a signal be strong enough to vibrate the window?

    6 replies

    Reply 11 years ago on Introduction

    No chance. Thermal shock from that much laser would shatter the glass long before any noticeable vibration was set up.


    Reply 9 years ago on Introduction

    Which is cooler...

    stick C4 in any object, light it and it will instantly be cooler.


    Reply 7 years ago on Introduction

    You can light c4 on fire and heat your coffee or other trail food. It won't explode from just lighting it on fire.


    Reply 10 years ago on Step 7

    This link isn't direct laser communication but it is a way to transmit data through laser beams (especially sound)

    It has only been tested by transmitting sound but other encoded data should work to.
    Also this actually is direct communication except it is transmitting data in area of affected particles that cannot be heard (the electromagnetic spectrum)


    Reply 12 years ago on Step 7

    whilst you cannot rely on the photon momentum to generate vibrations, with a powerful enough laser, one could generate localized heating in certain areas to generate thermal differentials that vary with time. These differentials could generate enough stresses to cause the glass to vibrate. It does however rely on the thermal properties of the medium you want to vibrate and the power of the laser. and you have to have some nicely coordinated movement of the laser to create this thermal wave.


    Reply 12 years ago on Step 7

    in theory, yes. Photons have non-zero momentum. So by relecting light the window does diflect a slight bit. However, you would need a laser so powerful that the window would melt before you were able to move it.


    7 years ago on Step 7

    I've made basically the same device, only with a crude amplifier (transistor circuit) and headphones instead of the pre-amp; but I keep on hearing feedback although I can clearly hear when the laser shines on the phototransistor. Even while someone is knocking on the glass I still only hear feedback, could it be that I'm saturating the transistor (I've only done this at a range of about 10m) ?


    These laser bugs are fun! Here is another nice one...

    Now the neighbors are no longer safe!


    9 years ago on Step 1

    Where can I get a microphone preamp?


    9 years ago on Step 7

    sending 2ch stereo over laser using polarized beams is interesting, the cheap plastic "REAL-D" 3d glasses you get from cinemas for about 75p ($1) would be a start.
     what about the receiver? a kinda 2way mirror at 45D to split the beam in two then use two phototransistors?

    1 reply

    Reply 9 years ago on Step 7

    You could use the linear polarization filters from traditional polarized 3D glasses. The Real-3D glasses however use a combination of quarter wave-plates and linear polarization filters to enable decoding of circularly polarized light. So, you are better off using the old (and cheeper) glasses, or ordering a larger sheet of filter from a supplier.

    If you want to see a working 2ch system in action, check out my blog post where a few guys and I tried it out. Rough schematics and video included:

    Happy Hacking,