Laser Surveillance System for under $20

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)

Comments

[frogjake9]

Where can I get a microphone preamp?

[Gilly001]

with the mic port on a laptop, could someone please give me the details to get it into the port. Like, connect the wires and all. thanks.

[-ddrblank-]

I'm in the process of creating this into a wrist watch. Right now its going OK, I'm looking for a laser at the moment. But I don't understand how the audio transformer and phototransistor fit into it...

[legionlabs]

A phototransistor is a glorified photodiode that applies voltage to the base of a transistor.

A photodiode is (basically) any diode where the semiconductors are exposed to light, so that it demonstrates a photoelectric effect like a solar panel.

So, the role of the phototransistor is to transform received light that bounces off the window into a voltage, and amplify that voltage. A perfectly flat window is optically a semitransparent mirror, as it vibrates due to sound it becomes alternatively a convex or concave reflective surface, which changes the intensity of the received radiation.

A second way to accomplish this is to pulse a laser at the window at a high frequency. The reflected light will be *both* amplitude and frequency modulated. The frequency modulation would allow for good resolution if you can make the pulses fast enough.

[Wolf Seril]

Hey question. you said "remote control reciever". Do you mean as in like a tv? Because I have an old tv that we lost the remote for, so could I find a phototransistor in there?

[navaburo]

sure thing however, many times you will find that the phototransistor is part of a sealed digital receiver package, making it hard or impossible to use. you might save yourself the trouble and get a discrete part from radio shack or the like.

[YarenZero]

when you say a "MIC port", do you mean a port that you plug a microphone into, like on any computer or anything else you speak into that can have a microphone plugged into it? If so, I should be able to make a portable version withanything that has a Microphone port on it, right?

[navaburo]

thats right.

[thermitemaster]

I think you can buy those toy laser things at toys r us for about the same price. No shit. that stuff works.

[Neodudeman]

lol.. "All you need"

[navaburo]

well... you dont realy need the meter, or the batteries, or the magnifying glass, or the iPod, or the potentiometer. But they can help!

[ceridan]

Considering what this device does it's not that much stuff :) and fairly easy to get too :)

[navaburo]

yes. that is my favorate part about projects like this. even if you need lots of stuff, most of it can be found in a household, or found in the trash/garagesales/for cheap.