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OPTOELECTRONIC EAVESDROPPING TECHNIQUES "Practical Guide to Constructing" "Lightwave Transmitters & Laser Listening Systems" Written for P.I.M.P. electronic-magazines on 15 April 1997 by Alan Hoffman (a.k.a. "Q").

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"Practical Guide to Constructing"
"Lightwave Transmitters & Laser Listening Systems"

Written for P.I.M.P. electronic-magazines on 15 April 1997
by Alan Hoffman (a.k.a. "Q").


The following article deals with the use of optoelectronics for purposes
of communications and/or eavesdropping. The term optoelectronics itself
is derived from the fact that two technologies are used in conjunction
with each other in order to form a complete system. Those two technologies
being: (1) Optical components including electromagnetic transmission
sources as well as any associated lenses, optical filters, splitters,
mirrors, etc.. and combined with (2) electronic components which serve the
purpose of receiving, demodulating, amplifying, filtering and processing
the transmitted and/or received optical signals.

The article herein will consist of only practical applications to aid the
reader in the construction of a completed system. The author will not
make any attempt to delve into surveillance technology, as it is assumed
that the reader knows about such topics already. If one is not familiar
with this topic, then some cursory research can be done by reading
a few books or even computer text files on the subject.

I have selected two items in particular from the thousands of "gadgets and
gizmos" which those in the surveillance field get to "play with". These
selected items are easily constructed, are mildly inexpensive, and most
importantly should appeal to anyone of the "hacker persuasion". It is
just a darned fun project to build, and experiment with at home or
even against unwitting victims. Of course, their are those pesky federal
laws which state that it is a felony to use these devices for surreptitious
purposes; so bear that fact in mind.

Project 1: Lightwave Transmitter

Lightwave communications systems are without question the oldest devices
around; and was invented by Bell, a year before he even developed
the telephone. His original device was named the "photophone", and used
the sun and mirrors as a transmission source.

THE CONCEPT: Put a "voice" (music, someone talking, morse code, or even
computer data) onto a lightwave. That lightwave could be "visible white
light" (ie: from a flashlight), it could be RED colored light (632nanometers)
from a Helium-Neon (HeNe Laser), 635, 640, 670, and even 720nm light
from Laser Diodes. Then you could move down into the infrared spectrum
which is a lower frequency and you have 800 - 1500nm laser diodes in
the near-IR and mid-IR range, above 1600 would be the far-IR range which is
occasionally used in fiber optic transmissions. Conversely you could
move up in frequency and use orange light, green light, violet light,
and up even further than that, is ultraviolet light.

Their are 3 main methods to place intelligence (audio, video or data) onto
a lightwave. The methods are (1) AMPLITUTDE MODULATION, (2) FREQUENCY
MODULATION and (3) PULSE CODE MODULATION. Their are many variations
especially with respect to digital transmissions which use ultra-exotic
modulation techniques in order to compress the maximum amount of
data onto the lightwave. However for the purposes of our project we
will be utilizing amplitude modulation. At this point, I will interject
that an AM transmitter is not the best method for surveillance purposes;
it is merely the simplest to build. In actuality, PCM provides by far the
best results as it has an inherent sort of AGC (Automatic Gain Control)
(or "compression") effect whereas the audio signal remains at a fairly
constant level and does not fade away when a person gets too far away from
the microphone, nor does the circuit get saturated causing nmassive
distortion when the intercepted audio gets too loud. Uniform volume at the
receiver can also be achieved through an almost identical method referred
to as PFM (Pulsed Frequency Modulation).

The process of putting intelligence onto a lightwave or radio wave is
referred to as "modulating". Another term which one might encounter is
the word "carrier", which simply refers to lightwave or radio wave that
gets modulated. An unmodulated carrier would look like a pure sine-wave
if viewed on a spectrum analyzer or oscilliscope. However, once the carrier
gets modulated with intelligence, the carrier takes on a unique and very
complex pattern of shapes.

The next task is to send that lightwave either in free-space (through the
atmosphere) or down a fiber optic waveguide. In this project our transmitter
will send the modulated signal through free-space. If a laser system is
used, the signal can be sent upwards of a mile away with even a relatively
low powered laser (10 - 25mW) provided it has low divergence characteristics.
The use of a LASER is not a necessity. Once could just as well use a
incadescent bulb (ie: a flashlight bulb), an LED (Light Emitting Diode) or
a series of LEDs. However, since LED's and flashlights do not produce a
coherent beam (ie: it is not polarized and has an extremely large divergence
which is measured in degrees rather than milliradians), the use of a
collimator or lens assembly is usually a necessity and can extend the
range upwards of several hundred feet. Also by placing the transmitter
and receiver elements into the ends of a sufficiently long piece of
PVC pipe (1 inch wide by approx 1 meter long (3 feet) the range can
be extended even further with the added advantage of having a potentially
lower noise-floor caused by external light entering the receiver.

The last task is to receive the lightwave signal with an optoelectric
element which will convert the lightwave into an electrical impulse.
It is important that this element be matched specifically for the transmitted
wavelength in order to get maximum efficiency from the system as a whole.
If your system uses an infrared LED or IR Laser Diode, then one should
use a phototransistor that is specifically designed for the IR region.
If one is using a Red Laser or Red LED then phototransistor should be
specifically matched for the Red to near infrared spectrum. And if one is
using a mid to far-infrared laser diode (800 - 1,500nanometers) then it
is EXTREMELY important to have the matching phototransistor. Ordinary
infrared detectors such as those which are sold at electronic supply
companies for $1.35 apiece are NOT the correct device, and you will achieve
crappy results; if it even works at all. For far-infrared you need a
special phototransistor which is usually pretty damned expensive ($5 for
a cheap one to $195 for a massive supersensitive array about 2 inches
square) The latter two must be purchased from laser supply companies.

After the signal is received and converted into a minute electrical
impulse; it must be amplified with a small audio pre-amplifier with sufficent
gain to provide clear audio. An amplifier with less than 1 Watt is totally
sufficent for this task, and one can even get away with using a 1/4 Watt
(250mW) audio amplifier.


The following transmitter project is an EXTREMELY simplified version which
can be built for under $20. By no means is this the most efficient system and
the resultant audio is not particularly clear. Nevertheless, for surveillance
purposes, it ceartainly suffices in its task. One does not need crystal clear
"CD-Quality" audio for eavesdropping. The level of "intelligibility" need
only be to the point where the eavesdropper can understand the audio which
is being intercepted.

------------------------------------------- 1/8th inch jack
| | Headphone Output
|1/8th 1/4 Watt Radio Shack | of Amplifier
ÛÛÛ=============|inch Pre-Amplifier. $12 |--------------
|audio or use any other pre-amp | |
Dynamic or |jack or build your own. |---- |
"phantom power" | See Radio Shack book on | | |
FET microphone | Op Amp IC Circuits. | | |
------------------------------------------- | |
| 9.1V |
| Zener Diode
| |
| |
IR or Red L.E.D.
Laser Diode
Preferable for
ranges over
100 feet.

About the transmitter unit. The reader needs to understand that a bit
of imagination is required in constructing this unit. If you dont understand
this simple diagram then obviously you need to brush up on electronics
skills because it just doest not get any easier.

The diagram I showed is simply for illustration purposes and it serves as
an excellent "prototype" or first project. After you build it and understand
how it works and what its capabilities are, then you'll want to make the
device smaller. This is mainly achieved by building your own amplifier.
This task is VERY VERY easy. Get the Radio Shack book called OP AMP IC CIRCUITS
for an example of a very simple amplifier. The unit above is very large and
what I recommend doing, is to put the LED or Laser Diode and the zener diode
(you NEED the diode on their or it wont work right, you can also use a
full wave bridge rectifier in its place if you know how and that provides
slightly better transmitted audio quality.) into a seperate small box about
2 inches long by an inch wide. (the box can also be bought at Radio Shack
and you should add a "beam spreader" followed by a "collimating lens
assembly", to give you a much greater range. This is particularly important
if your going to use an "Red" or "Infrared" LED. With a laser diode, the
beam is coherent enough and it will go a 1/2 mile no problem, providing no
interference is encountered (which it usually is due to stray light
and IR signals which are present all around us.)

If you are going to use a LASER DIODE (I will tell you where to get them
at the end of the article) it should be a minimum of 5mW (milliwatts)
in power. The frequency does not particularly matter. You can use 635 nm
(nanometer) diodes, which is essentially pure RED light. You can also use
the 670nm or even 720nm diodes which are on the border of RED and near-IR.
If you really wanted to go all out, and make a damned good system that will
have little interference, you will need 800nm laser diodes which is
mid infrared; but the latter is about twice to three times the price of
the Red or near-IR laser diodes. No matter what kind you choose, you should
expect to pay out between $20 - 50 on average, or 60 - $120 US Dollars
for some of the more expensive units, particularly the 800nm ones.
The prices for these diode lasers are very erratic and essentially
you just have to have a familiarity with LASER products and where to
get the best values. Shop around and compare. Their are units with identical
specs; yet one may cost 10 times more for no apparent reason, perhaps
other than the manufacturers brand name.

Just to give an idea of the capabilities of such systems, the author
has constructed a transmitter which measures less than 1 cubic inch,
yet can easily transmit upwards of a half mile away to a listening post.
I built my own amplifier using SMT [Surface Mount component Technology] and
utilized special amplifier chips which are very small, and I coupled
that with the smallest yet best quality 800nm laser diode I could find.
The device uses an external microphone, I prefer to use a "phantom
powered" FET (Field Effect Transistor) microphone which is EXTREMELY
sensitive and can pick up any sounds, even faint ones within a distance
of 50 feet. You could just as well use an ordinary DYNAMIC microphone or
ELECTRET microphone such as those used for singing. The microphone can be
hardwired to the eavesdropping area and the wire can be run up to an attick
and the transmitter can be aimed out of the eaves of the house to the
listening post a great distance away.
(BTW: This is a hell of alot easier said than done. Aligning the receiver
with the transmitter is a real pain in the ass. An infrared viewer,
night vision device or Black and White CCD camera comes in handy in such
scenarios as all of the aforementione have the ability to see infrared light
beams wheras the human eye cannot.) Another more realistic scenario if
one simply wants to play a prank one ones friend or relatives, is to
simply aim the device out the window and hide the unit away, perhaps behind
the curtains or other concealment spot.

The only one requirement for the transmitter is that you need an
amplifier with a high enough gain to drive the LED or Laser Diode.
I have neglected quite a few details, especially with respect to
Laser Diodes which require more power than LED's. If you run into
a situation wheras you cannot get the LED or Laser to emit light,
then the solution is pretty simple. Just add a 1.5V AA or AAA battery
connected directly (in parallel) to the light emitting souce (observe
polarity since LED's and Laser Diodes are polarity sensitive). Do NOT
ever add more than 1.5 Volts as it will just likely give you even more
problems (by saturating the receiver circuit). Like I said, this is a
project of experimentation. If it doesnt work, then YOU figure out a
way to make it work.


------------------------------------------- 1/8th inch jack
| | Headphone Output
|1/8th 1/4 Watt Radio Shack | of Amplifier
---------------|inch Pre-Amplifier. $12 |--------------
| |audio or use any other pre-amp |--------- |
| --------|jack or build your own. | | |
| | | See Radio Shack book on | | |
| | | Op Amp IC Circuits. | | |
| | ------------------------------------------- | |
|--9V---| | |
|Battery| | |
|Parallel (add a 10k resistor in parallel) |-ÛÛ-|
| | (to the 9V battery or you will ) Headphones
|--ÛÛÛ--| (burn out the phototransistor )

Phototransistor (Matched to the same frequency as the transmitter).
Check the specification for the component by calling the manufacturer
and getting the spec sheet. The graph should indicate the efficiency
of voltage conversion for any specific wavelength. The maximum efficieny
(or voltage output) should be within 20 nanometers of the transmitter
output). In reality however, its not all that critical. The reader
should not worry about any of this to great excess. I just mention it
because efficiency and precision is a nice goal to achieve when designing
circuits, but sometimes its more trouble than its worth.

The infrared detector phototransistor that Radio Shack sells, for instance,
is perfectly sufficient for detecting any near to mid-IR transmitter and
also works nearly as well for detecting RED light emitting LED's.

As you might observe in the diagram, the phototransistor detector needs
some external power. A 9V battery with a resistor of 10k ohms hooked in
parralel should work. (I have seen some phototransistors that could take
a 9V battery directly, but I dont recommend it as you will likely
burn it out).

[The author personally uses a Martin L. Keiser "1059 Model Pre-Amp" for
the receiver unit, as it comes with a infrared probe for surveillance
countermeasures purposes. With this unit, the adding of an external
battery is not necessary, as with the flick of a switch on the 1059, the
probe can be fed voltage. This low-noise, high-gain pre-amplifier
(normally used for surveillance and countermeasures) costs about $250
and is a worthy investment for anyone into surveillance or TSCM. ]

It is highly recommened that the receiver unit have a simple
lens assembly to drastically increase efficieny and light gathering
abilities. In its simplest form, all that is required is a single
lens costing less than $3. Even a simple magnifying glass lens
is sufficient. Only requirement is that the lens should be at least
5 - 8 centimeters (2 to 3 inches) in diameter. At the focal point of
the lens (usually a few inches away, you would place the phototransistor).
An even larger lens will provide much better results. This is particularly
of importantce if you plan to use your light transmitter at ranges of
1/2 mile and greater. For such ranges, a 5 - 8 inch lens might be best
provided you can afford it $30 - 160.)

The reason for the lens assembly, is that at great distances (anything
over 300 feet), the light beam starts to diverge noticeably. This is true
not only for LED systems, but also for laser diodes. At 300 feet it is
not uncommon for a Laser Diode to have a beamwidth of 4 inches or more.
(the original beam diameter is approximately 1/30th of an inch). By having
a large lens, it allows all the light to be collected that spread out
over the distance of transmission.


Daytime / Nightime
RED Standard L.E.D. approx. 300-600mcd : 30/60 45/70
INFRARED (I.R.) Standard L.E.D. : 40/65 50/75
RED "Jumbo" L.E.D. 5000mcd light output: 50/90 65/105
IR or RED Laser Diode,635,670,720nm;5mW: 400/900 600/1300

The aforementioned figures are approximations which may vary greatly
depending on ones set-up. All figures are conservative in nature. With
proper design, distances of 30 - 100 percent greater can be achieved
over stated figures. This is especially true if a collimating lens is
added when using Red or IR LED's. In the latter case, distances up to
300 feet can be attained. I.R. always works much better than does
RED transmissions due to less interference. This is especially true
in the daytime, however both RED and IR work nearly equal at nightime.
A lens on the receiver should always be used. In order to see the difference,
just grab a camera lens (a 28mm lens will work best for a demonstration
because its easier to aim at the light source. When the camera lens is
placed in front of the phototransistor a dramatic increase in reception
range and clarity will be apparent.)

Project 2: Laser Listener System

The ability to surreptitiously monitor a conversation in a passive nature
presents one of the greatest opportunities for the eavesdropper. Not only
do passive attacks involve less personal risk; as such methods do not
require any form of covert-entry to install electronics. Passivisity also
has the advantage of providing near-equal results and can be performed
with a minimum amount of set-up time and planning.

Their exists a number of methods to passively monitor a target. A few of
of the most popular methods are:

(1) Shotgun or Parabolic directional-microphone to intercept sound upwards
of 500 feet away with fair degree of clarity.
(2) Trans-structural monitoring using a High-Gain Audio system. In such a
set-up, a target is monitored from an adjacent structure such as
another room using a series of microphones (contact, spike, tube or
other specialized mics). Since the target does not directly control
the adjacent structure he cannot perform any countermeasures
techniques to detect the monitoring.
(3) Laser or Microwave "pick-off" to intercept audio from a target.

Although, technically it is debateable whether laser listening is truly a
"passive" technique because such methods can be detected by the target,
it is generally considered to be such, because it doesnt involve any
elaborate plan to penetrate the target by physical means. Shotgun and
parabolic mics on the other hand are truly passive techniques and are
totally undetectable.

THE CONCEPT: An electromagnetic signal of a high degree of coherence with
low divergeance characteristics is targeted at any object within the vicinity
of the subject being surveilled which has a high degree of resonance to
acoustic waves and which could act in a similar manner to the diaphragm of
a microphone; and which has a high degree of reflectance to the originating
electromagnetic signal. The signal reflects off of the diaphragm at an
angle which is inversely proportional to the angle of incidence at
which point it travels to a receiver element for demodulation and

TRANSLATION.... You aim a LASER (or microwave) beam onto something such as
an outer glass window (or something inside the targets area). When a person
speaks, his voice sends out acoustic pressure waves in the air. These
waves of pressure vibrate everything in the surrounding area. The thinner,
larger, and harder something is, the more likely it will be vibrated by
voices. The aforemention description happens to perfectly describe a
glass window. Glass is: Large, thin, and hard. And as such, it is an
excellent conductor of audio waves. When you speak; your voice vibrates
the windows in the room a minute amount. Although you ceartainly cannot
see the glass vibrate with the eye, with electronic pick-up equipment
such as a microphone or a laser; that tiny vibration becomes a big
vibration once it is run through a small amplifier.

So the laser beam is aimed at the window, and as the target speaks, and
the window vibrates,... that vibration (which is very much like the
vibration of a diaphragm on a microphone) will "modulate" the laser
beam with the voice in the room. So essentially, when the laser beam strikes
the window is the exact point in which the targets voice is being "put onto"
the laser beam.

The laser beam gets reflected off of the window, and gets sent to a receiver
element which picks up the light from the laser, and turns it into a
small electrical signal. The is the same principle as a solar cell turning
sunlight into a small bit of electricity. This is the same technique which
our first project above (the lightwave transmitter) worked. The pick-up
element can be a phototransistor or a series of them which only cost a
few dollars at electronics supply stores. However, when dealing with the
field of lasers, their have been developed some ultra-sophisticated
detectors which can output large amounts of power at specific frequencies.
In out project, we will not be using any of these sophisticated detectors
as some of them cost well over $100 dollars. We can suffice with an I.R.
photodiode or phototransistor bought at Radio Shack.

Once the phototransistor receives the light and turns it into a small
electrical signal, it must be amplified. This is the job of an audio
amplifier or pre-amplifier. The difference between the two aforementioned
is negligible. Pre-amps are generally anything that outputs less than
1/2 Watt of audio (500mW) and amplifiers (or "power amplifiers) can
output anything from 1/2 Watt to 500 Watts. For this project a
1/4 - 1/2 Watt (.25 - .50W) is fully sufficient. You can purchase the
amplifier from RADIO SHACK. Their 250mW (1/4 Watt) amplifier costs only
$12. I absolutely recommend chaining two of these together for a total
output of about a half watt.

That is the concept. It is quite simple and requires only a minimum number
of components. The skill required to build this is negligible, however some
of the optics can be tricky to design. Optics is the key element in
designing these units and can make the difference between an amateur
system, and a pro-system which is as good if not better than the
law-enforcement grade stuff. You can build the project even without a
lens element on the receiver, but the range is going to be shorter and
its going to be more difficult to align the receiver onto the beam.
Likewise, on the transmitter, optics are also required for better results.
What is needed, is a "beam expander" to increase size of the laser
beam. (note: if you read a book on Optics, dont be confused. They often
refer to a beam-expander as a device which can reduce the size of a laser
beam. Its the same thing, it all depends which way you shoot the laser
beam through the optics.) You'll want to expand the beam between 10 - 20
times its normal size, but that varies VERY VERY greatly, and also depends
on how far away you are going to be using the laser listener from.
The next element thats needed on the transmitter is a "collimator" however
that device is not really necessary because a beam expander has a
collimator built into it (but its not as precise as a dedicated collimator)
The collimator limits the divergence of the beam.


ÜÜÜÜÜÜÜÜÜÜÜÜ 2 - 8 power spotting scope
²²² 2X "barlow scopes" can be bought for $10
ON/OFF>>ÛÛÛÛÛÛÛÛÛ ÛÛÛÛÛÛÛÛÛ=====wire====°°°°°° |||||
switch -----------------------------------------------

ÛÛÛÛÛÛÛÛÛ ÛÛÛÛÛÛÛÛÛ = 2 AA batteries (3 Volts total)

°°°°°° = 5mW LASER Diode (50 - 120 US Dollars)
The 5mW laser diode is pretty much the
standard. It is usually 3 - 5mW depending
upon the input voltage. As far as laser
diodes go, the most they make (for a reasonable
price) is a 10mW model but that costs
$200 US Dollars. They make some 30mW laser
diodes, but their upwards of a thousand
dollars. For a laser listener their is no
need for anything over 5 - 10mW.
If you wanted to make a lightwave communicator
(like in project 1), and you wanted a range of
several (2 - 10) miles, then the most economical
solution would be, not to use laser diodes
but to switch to Helium Neon laser tubes
which can provide 30mW for only $600. BTW:
I might point out, as far as long range
communications goes, you are limited by the
horizon line, and believe it or not its
alot closer than you think. Anything more
than a few miles and your going to have
major problems and will have to install the
unit on a roof upwards of 30 - 80 feet.

635,670nm RED laser best for amateur use.
The reason I recommend a 635 or 670 RED
laser diode for the amateur is because it is
the easiest to use. Since the laser beam is
visible, it is easy to line up the receiver
with the beam. On the other hand, with an I.R.
laser diode, you cannot see the beam with the
eye, and as such.. well the problem is obvious.
If you cant see the beam, you cant align the
receiver and hence youll never get the system
working. If you want to use the IR lasers
(which you really should do) your going to have
to cough up an extra couple hundred for either
an Infrared viewer, a night vision scope, or
even better a Black & White video camera,
most of the latter have the ability to see deep
into the I.R. spectrum (some up to 2800nm.)

720nm is in the area between RED and I.R.
This is better to use than the visible red
lasers in terms that it is better received
by phototransistors and you'll get a greater
range with this 720nm device.

808 - 824 nanometers is in the mid-infrared
range and is the frequency used by most law
enforcement laser listener systems. It has
the advantage of being less detectable to
the target (it cant be seen by the naked
eye like RED lasers can, but can be seen
with a B&W Vide camera, night vision scope,
or countermeasures receiver with an IR probe.)
Another advantage is that it allows the
greatest range with the clearest reception.
This is because photodiodes have a very high
efficency at the mid I.R. range. I should
mention, that ordinary photodiodes dont work
particularly well in mid-IR range (not bad,
but not great). You can buy special $100
detectors for 800 nanometers that are ten
times more sensitive than the I.R. detector
(phototransistor) you can buy at Rat Shack.

||||| = Beam Expander. This is optional for the
first time builder. You can purchase
beam expanders from companies that specialize
in LASERS and optical components or you can
get them from low end resellers like
EDMUND SCIENTIFIC (a very popular company
that sells all kinds of science "stuff"
as well as lasers and extensive optics.)

The amount of expanasion is a complex matter
of design and I cannot get into it without
dealing in complicated opical calculations
and giving a lesson on the physics and
mathematics of optical components..
Read a book.. Do trial-and error. Its also
heavily a factor relating to the distance
at which your going to use your laser
listener. Not having the right beam
expansion can leade to distortion in the
recioeved audio due to a "chopping" effect
of the received signal. On the authors
personal unit, I use a complicated variable
expansion components with a seperate
collimator unit. And that allows me to use
the device at any distance without
distortion. I also have a similar variable
zoom optical set up on the receiver which
dramatically improves efficeincy, but again
its not necessary for a first time project

= PVC Tubing. The housing for the transmitter
----------- section can be made of simple 1 - 3 inch
PVC tubing. You can custom fabricate a metal
housing if you have access to a machinist
willing to do it at reasonable price. Paint
the housing flat black (standard procedure)
as it gives the unit less visibility and
detracts attention from the unit.. not to
mention makes the unit look nicer.

If you plan to design optics into the unit
a 2 or 3 inch wide PVC pipe is necessary,
and if your going with a straight battery
and laser diode set up, then the size is
miniscule and you can get by with either
1 inch or even 3/4 inch PVC tube.

The TRIPOD unit is an absolute necessity. It is imperative
that the laser beam not be moved, and a tripod helps
stabilize the unit. A heavy and expensive tripod MUST be
used. You can go with one of the cheapie $45 tripods
that weight 4 pounds but your going to likely get bad
results because the unit is going to vibrate alot causing
serious distortion at the receiver. What you need ideally
is a heavy photograohic or telescope tripod that weighs
like 15 pounds but such units cost upwards of $250 [USED].
Whether you choose to use a lightweight inexpensive tripod
or a heavy duty professional one, you need one that can
extend to at least 5 feet high. That is an absolute
necessity in most situations. The unit has to ideally be as
high as the middle of a typical residence window.


The user should NOT put a scope on the receiver! Even though the author
has one and finds it very convenient. This is EXTREMELY DANGEROUS and
I recommend it to no one. In my personal set-up I use a visible 635nm
diode laser as a spotting and alignment beam, in addition to the
primary 820nm laser. When I'm done aligning I shut the 635nm unit off.

Lasers are very dangerous items that can blind you in a matter of seconds
if you look straight into the beam. Even though 5mW is such low power it
could not burn a hole through paper (you need at least 2 Watts to do that),
it still has enough intensity to permanently blind you or damage your
retina. Lasers are not toys to be screwed around with. Use caution...
I might also add, that your devices should contain the proper CDRH stickers
which identify their class. (5mW I.R. lasers are a class IIIa threat).
These stickers (not only may be required by law, but their also their
to remind you of dangers as well as to serve as a reminder to any nosy
individuals that screw with your equipment.)

The user should tap holes for a scope mount (a 2X - 4X scope should
be sufficient) and should be mounted on the top of the tube and another
hole should be tapped for a tripod mount (on the bottom). Drilling holes
will not crack the PVC piping in any way.

²²² ðð
Û || this space is ÛÛÛÛÛÛÛÛÛÛÛÛÛÛðð
Û 1-2 9V batteries Audio Amplifier || focal length of ÛÛÛÛÛÛÛÛÛÛÛÛÛÛðð
Û 250 - 500mW || lens assembly. ÛÛÛÛÛÛÛÛÛÛÛÛÛÛðð
²²² ðð
^^Headphone output jack. ±±±±±±±
1/4 or 1/8th inches. Û Û Û ðð = Mechanical Iris Assembly.
Û Û Û This has many uses, and can
Û Û Û help control distortion under
Û Û Û ceartain circumstances by
Û Û Û governing the received beam.
Û Û Û || = Photodetector Module.
Û Û Û use simple Radio Shack
Û Û Û Phototransistors for most
Û Û Û applications. Specialized
Û Û Û Far-IR detectprs can also
Û be utilized at greater expense

The lens assembly can be designed in hundreds of ways. You could use a
simple double convex lens such as an ordinary 3 inch "magnifying glass"
(although such lenses dont pass mid to deep infrared efficiently.)
If you are gouing to use a deep infrared laser, you need special
IR optics to gain a 95 percent transmissivity rating. A better system
is to use single plano-convex lens which will reduce the amount of
stray light being received (will focus more "straight ahead" at the beam.)

Another great yet simple technique is to use an INFRARED FRESNEL grating
LENS. Use the circular type about 3 inches in diameter and place it
1/2 inch in frot of the sensor. This will intensely focus the received
IR light onto the photodetector.

Another important thing to use, and especially if you choose to use no
optics at all, is an INFRARED FILTER (if your using a IR laser source).
This will reduce the noise floor of the received signal, especially in
the daytime by cutting out unwanted signals in the visible light range
which the photodetector will demodulate.

The power source should be 1 or two 9 Volt batteries. Preferably the
latter but it depends on ones design. 3 - 9 Volts has to be fed to
the photodetectors (varies with each detector) and then the photodetector
has to be fed into the input (microphone jack) of an audio amplifier.
You can use either two seperate 250mW amplifiers chained together or
can use a 1/2 Watt (500mW) amplifier. You can either use an amp from another
manufacturer or you can design your own units. the Radio Shack units are
best for hobbyists. They seem to have a higher signal to noise ration
(better clarity) than do some of the professional surveillance
amplifiers costing 10 - 20 times as much.
You can design your own dual or triple-stage amplifier using the 386 chip(s)
and some miscellaneous capacitors and resisors and 1/8th inch phone jacks
for a mere $10 dollars the results will be better than most commercially
available amps. You could also use the 741 Op-Amp chip. I prefer using the
DC-09 chip myself. I cascade 5 of them in sucession. This chip is one of
the best amplifiers on the market and has an extremely low distortion

Lastly, are the control mechanisms for the receiver unit. All you need is
an 1/8th inch or 1/4 inch output phone jack for the headphones and you
need to install a variable resistor (potentiometer) [with a logarithmic
audio taper not a linear taper] for the volume (gain) control. While were
on the subject, might I point out that the proper word to use is
"gain", and not "volume". I hate it when people call it a volume control;
although thats what it is. That vernacular is used alot by anyone who
works with electronic equipment and especially those who work with
surveillance equipment. The two aforementioned components are best placed
on the back of the receiver. Then of course, you need a SPST toggle switch
for the POWER, also placed in the back of the unit.


Directions for use are straightforward. You turn both units on. Aim the
transmitter at the target window at an angle. The beam bounces off at
the exact inverse incident angle at which it struck. The receiver is placed
in the exact position so it picks up the reflection of the beam. Your
turn the receiver on and listen to the amplifier and the intercepted audio,
and if need be, then make minor adjustements to the tripods pan and tilt,
up or down. Or if need be, move the entire tripod for the receiver altogether
until maximum clarity at the receivers amplifier is achieved.

| |
| Target |
| |
| |
/ \
/ \
/ \
/ \
/ \
/ \

I will make two last observations before I conclude. First, I shall say that
their is alot of physics behind designing these units, but it is not of the
utmost important to design the ultimate system. For instance, each type of
glass (and their are hundreds of types) each has its own characteristics
of reflectance and transmittance. The Infrared wavelengths used in
commercially available and law enforcement units are NOT the ideal frequency.
But it is the best overall compromise and thats why it is used.
Some of the reasons it is used are the fact that it is invisble to the human
eye and hence is harder (yet not impossible) to find in a countermeasures
search by the target. Secondly, infrared units work better than RED laser
systems in many instances, thirdly is the cost factor. Infrared transmission
is the cheapest and most laser type of all frequencies including UV, and
visible light. Fourthly, is the fact that at the current time in laser
technology, you can produce a higher power infrared beam, in a more
compact unit at a cheaper price. Each reason in itself is not all that
meaningfull, but when combined together, infrared lasers definately are
more usefull for communications.
The biggest difference in glass characteristics is between standard glass and
windows which are coated with a metal compound (silverized, yittrium-gold
coatings, etc..) Also, old fashion glass used in some houses (glass that was
made before 1950) has very difference reflectance characteristics than modern
glass. But as stated, it all works to a reasonable degree. The amount of
reflectance has a direct bearing on the distance at which your unit will
be capable of operating from. Another fairly complex topic is the angle of
incidence which varies with different types of glasses. Their are some
angles which are scientifically the ideal angle which provides the highest
degree of reflectance, and hencely would provide the greatest range. But from
a surveillance technicians point of view, the technicalitie are all really
trivial and do not take precendece over matters of practicality. Your unit
will work from most angles, you dont need to spend the time finding the
ideal angle which provides the highest degree of reflectance.

Also, do not be dissapointed by the sound quality from your unit. The
laser or microwave listener is not a miracle tool, as is the same with
all surveillance equipment. You will get a mild degree of intelligibility
from these units; and that is all that a technician really needs. Their are
many factors which contribute to a degredation of audio clarity. First
and foresmost is the fact that wind vibrates the windows causing quite
a large degree of distortion. On windy days, the unit can get to the point
of being unuseable. This is where filtering comes in. That will not be
discussed in this article. Another factor is due to internal pressures.
A door closing anywhere within the targets facility causes a pressure
disturbance that can be picked up by your receiver. Vibrations from passing
automobiles, and aircraft also causes pressure disturbances as well as
vibrational disturbances. Vibrational disturbances occur through solid
structure such as the ground, while pressure disturbances are directed
through the air.

Lastly, I might point out that the device need not be aimed at a window,
because in reality, many times that may not be the best option. Just to
peak your creativity, for example, you can aim the device inside of the
room at objects, you could even install a tiny mirror (they sell wafer thin
mirrors from LASER suppliers which are only 1mm - 5mm square) and you can
place said mirror onto objects in the room, even a radio speaker which
makes for an excellent diaphragm and conductor of sound. These may not
be practical solutions in most situations, but I merely point to the fact
that the targets window does not have to be exlusively used as the


Edmund Scientific
101 E. Gloucester Pike
Barrington, NJ 08007-1380

Edmund Scientific is a major supplier of scientific related equipment
for schools, industry, hobbyists, etc. Their list of products is
immense, and this is a catalog that you simply must. Not only scientific
equipment, but video equipment, night vision devices, LASER systems,
an extensive line of optics, electronic equipment, laboratory equipment,
all kinds of meters and scales, microscopes and telescopes, and anything
ekse you can damed well think of. I recommend you get your optics from
this company as they have one of the largest selections in the country
of individual components and assembled units. Request free catalog, or
specify that you want their complete optics catalog. The regular catalog
has some optics, but their is a seperate catalog with hundreds of pages
of everthying you could need with al the specifications.

MWK Industries
1269 W. Pamona
Corona, CA 91720

This company is a small time, yet very popular mail-order company that
sells mainly to hobbyists as well as industry. This company purchases
alot of equipment in bulk and as surplus so they can bring equipment
to you that may be quite a bit cheaper than through other companies.
This company has one of the best selections of Helium Neon laser tubes,
aklternate frequency HeNe's, laser diodes of all types and frequencies,
as well as some of the mid range systems. CO2 lasers, Argon, Excimers,
etc.. They sell quite a few LASER and OPTICS books if your into that.
Plus they also sells informational packets on different topics
to hobbyists. Building high powered lasers upwards of a 100,000 Watts
peak pulse power, 20Watt metal; cutting CO2 lasers, How to build your
own copper vapor, nitrogen, or ruby laser. And yes, they even have plans
on making lightwave communications devices and laser listeners. Call or
write for free catalog.

Merredith Instruments
Post Office Box 1724
Glendale, AZ 85301

This is another mail-order catalog company. Not as big or diverse as MWK,
but their a favorite of mine because they always seem to have hard to find
laser items at rock bottom prices. They carry a nice selection of laser
tubes, particularly the Helium Neon type, also carry power supplies for
the lasers. They have a decent selection of alternate frequency laser diodes,
optics, light show equipment, etc..

Radio Shack

I hate to give these overpriced losers any recognition, but they do have a
few of the items which you need for building the above two projects. They
have the LED's in both RED or Infrared, they have the phototransistor
detectors, they have the 250mW (1/4 Watt) audio amplifiers which you need,
as well as any miscellanous components such as jacks, headphones, wiring,
batteries and such, or the components to make your own amplifiers. I might
also point out that they DO also have the laser diodes in the form of a
"laser light pointer" (for between $49 - 79 dollars) you can hack the
things up, or use the whole pointer as-is. The only problem with their
laser pointers are that they are not of the infrared variety. But again,
for a beginners project, it may be best to use a visible Red Laser to
aid in aligning the receiver with the beam.

[end of page]


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