Laser Surveillance System for under $20

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!

Comments

[HevnsDevl]

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) ?

[Djdavies83]

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?

[navaburo]

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:

http://hotwigati.wordpress.com/2008/11/15/polarizing-optoacustic-mixer/

Happy Hacking,
 Chris

[munzer]

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?

[curecreator]

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

http://www.metacafe.com/watch/2225704/make_a_simple_laser_communicator/

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)

[_soapy_]

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

[beehard44]

Which is cooler...

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

[Capricorn.san]

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.

[navaburo]

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.

[HuggyBear]

My only concern would be the light that is not reflected by the window. Like others have said, there would be a giant dot where ever the laser was pointed. An IR laser would probably work but setting up the system would be extremely difficult (you have to aim it correctly even though you cant see it). Other than these dilemmas which I dont blame you for, your construction is awesome. Something I most definitely have to try when I come across a laser pointer.

[freewheeler]

cool,the only disadvantage is that it will not work on frosted glass,as the relected beam will be too weak to reach you.

[_soapy_]

That makes no sense. You are getting the reflection from the outside face of the glass. What is more likely is that your target will notice a huge red splodge on his frosted white window, and guess something is up!

[freewheeler]

yeah,i suppose you are right... ;-(......................unless the frosted side faces you,as it will get reflected in all sorts of directions

[James (pseudo-geek)]

um......DUH use an IR laser!

[freewheeler]

ferfect,as my math teacher used to say.

[James (pseudo-geek)]

oops.....it would appear that htis topic has already been discussed....IR disperses too fast. maybe UV??

[gallamine]

Where'd you get this info? IR doesn't disperse in air, in fact, almost all freespace laser comm. is done using IR lasers. You can get UV lasers BTW - well, not in any accessible form.

[gallamine]

Yeah, i was trying to find a good source, but most are technical papers. The Wikipedia page on FSO is pretty good - http://en.wikipedia.org/wiki/Free_Space_Optics

Also, think about IrDA and your tv remote - those are "near infrared" though.

I think a lot of FSO is in "far infrared", like 1500 - 1800 nm.

[James (pseudo-geek)]

oh. I jsut heard somewhere else on instructables that it does. probably wrong

[trackzero]

Oh, forget the surveillance system, then...where's the instructable for this window melting laser you mentioned? :)

[nrdesign]

well done instructable! i'll try this out!