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"). FOREWORD: 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. DIAGRAM OF INFRARED TRANSMITTER: 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 ллл=============|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 |--->>>---| | | | | |--ллллл--| 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 does 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. DIAGRAM OF INFRARED RECEIVER: ------------------------------------------- 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) |-лл-| | | (to the 9V battery or you will ) Headphones |--ллл--| (burn out the phototransistor ) ллл PHOTOTRANSISTOR ELEMENT 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. -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- REAL-WORLD CAPABILITIES: Reception Range: WITHOUT LENSES RECEIVER LENSES 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 amplification. 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. DIAGRAM OF LASER TRANSMITTER: мммммммммммм 2 - 8 power spotting scope ВВВ 2X "barlow scopes" can be bought for $10 ----------------------------------------------- ON/OFF>>ллллллллл ллллллллл=====wire====АААААА ||||| switch ----------------------------------------------- ВВВ БББББББ л л л л л л л л л л л л л л л л л л л л л л л л л л л л ллллллллл ллллллллл = 2 AA batteries (3 Volts total) АААААА = 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 builder. ----------- = 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. DIAGRAM OF LASER RECEIVER: WARNING: WARNING: WARNING: 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. ББББББББББББ ВВВ №№ лллллллллллллллллллллллллллллллллллллллллллллллллллллллллллллллллллллллллл№№ л || this space is лллллллллллллл№№ л 1-2 9V batteries Audio Amplifier || focal length of лллллллллллллл№№ л 250 - 500mW || lens assembly. лллллллллллллл№№ лллллллллллллллллллллллллллллллллллллллллллллллллллллллллллллллллллллллллл№№ ВВВ №№ ^^Headphone output jack. БББББББ 1/4 or 1/8th inches. л л л №№ = Mechanical Iris Assembly. л л л This has many uses, and can л л л help control distortion under л л л ceartain circumstances by л л л governing the received beam. л л л л л л л л л || = Photodetector Module. л л л use simple Radio Shack л л л Phototransistors for most л л л applications. Specialized л л л Far-IR detectprs can also л 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 rating. 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. USING THE DEVICE IN THE REAL-WORLD: 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 | | | | | |_______========__________| /\ / \ / \ / \ / \ / \ / \ TRANSMITTER RECEIVER 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 reflector. ======================================================================== SUPPLIERS OF EQUIPMENT ======================================================================== Edmund Scientific 101 E. Gloucester Pike Barrington, NJ 08007-1380 U.S.A. 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 U.S.A. 1.909.278.0563 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 1.602.934.9387 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.