System Application, Safety, and Legality

These fully working modules may be packaged by the builder into a compatible housing that will allow micro-mechanical adjustments. You can use a visible red laser module for short-range demonstration or sighting and alignment purposes, The illuminating laser transmitter must be precisely in alignment with the optical axis of the laser receiver and be mechani­cally fine-tuned via micrometer head screws to receive and process the scattered reflections. The laser transmitter described in this project uses the test-tone circuit that greatly simplifies rough optical alignment to a far-field surface. Output of the laser transmitter may be preset to 2 to 4 milliwatts for class 3a or at 8 to 10 milliwatts for class 3b. A collimator is included that greatly extends the potential range of the system. Optional use of the telephoto lens retro­fitted to the optical receiver will greatly enhance dis­tance and performance. The prebuilt modules may be set on tripods to verify performance integrity or for demonstration purposes before being enclosed.

The lasers used can be class 3a to class 3b visible or invisible. Protective eyewear is positively required in case you look into the dircct reflection. Using the scattered reflection mode is less dangerous.

Get permission of those parties you are listening to! Experimental demonstration of this system should not pose a legal problem, nor should it be used for applications not involving oral interception.

Setup Using Direct Reflection

The direct reflection method requires the following


Bill of Materials

R1 1K1/4 W (BR-BLK-RED)

R2 (1) 390K 1/4 W (OR-WH-YEL)

C1 (1) 100 M / 25V vert electro capacitor

C2 (1) 1 M / 25V vert electro capacitor

C3,4 (2) -01 M / 50V disc (103)

И з.

11 (1)555 DIP timer IC

S2 (1) Slider SPDT switch

CL1 (1) Battery snap clip

Test tone output

to a laser


Assembly board layout

WR1 (30") #24 vinyl hookup wire PB1 (1) 1 X 1.5” .1 grid perf board

System Application, Safety, and Legality

Figure 13-14 Test-tone circuit

System Application, Safety, and Legality

1. Obtain two video camera tripods and secure the laser transmitter to one and the laser receiver to the other. Use duct tape, bungee cord, electrical tape, and your own ingenuity.

2. Remove the rear cover of the laser receiver and install a 9-volt battery into the clip.

3. Determine the target window. Select an easy one that is nearly “normal” and on the same

level where you are located. Place a loud radio on the opposite side of the window.

4. If your laser is a pointer or gun sight, it will be

necessary to apply pressure to the trigger switch. This can be accomplished using a paper clip or clothespin to clamp the switch.

5. Position the laser transmitter tripod so the angle is as close as possible to the normal, reflected surface. This will allow minimal sep­aration between the transmitter and receiver.

Note that this is not necessary for proper per­formance hut is easier until you are familiar with overall system alignment.

6. Locate the position of the reflected “laser spot” resulting from the direct reflection of the laser beam as it bounces back from the window. This will depend a lot on the relative position as in step 5, since the angle of reflec­tion will equal the angle of incidence (Snell’s Law).

7. Carefully adjust the positioning of the laser receiver so that it intercepts the spot from the direct reflection. The final position where the reflected signal is incident on the phototran — sistor as viewed through the view hole.

8. If you are using the extender focal lens, adjust it so that the reflected signal is about the size of a penny as viewed on the phototransistor and the white baffled disc. This lens is not nec­essary for ranges below 50 feet.

9. Tiirn the amplifier on via the control and adjusl tl to a comfortable audio level. Opti­mum results may require “tweaking” to an actual signal. A rough adjustment requires detecting a weak optical signal source in total darkness and adjusting for the best perform­ance/noise figure. Note that the unit will not work correctly if not properly set.

10. Carefully adjust the position of the laser receiver for maximum clarity and volume. Note that only a slight adjustment can make a world of difference in performance. Experi­ment with the lens assembly when using ranges over 50 feet. Note the laser beam spot profile on the surface of the white baffle disc. You will see interference bands or fringes consisting of light and dark sections.

Note that clarity, volume, and general performance depend on many factors. The size of the window; the setting of the pane; and even the vibration picked up from window air conditioners, motors, pumps, oil burners, and so on can seriously degrade a usable signal.

Serious experimenters may want to interface the system with an audio equalizer to filter and enhance the usable signal. Again, experiment and experience is the best solution to quickly set up and obtain opti­mum performance.

A setup using the scattered reflection utilizes the light detected off the optical axis. The signal will be weaker over a given distance and will require more careful alignment.

You now have a choice of using the individual modules mounted on tripods for a demonstration of the concept and experiment. You may also choose to retrofit the modules within a sturdy housing similar to what is shown in the following data, providing a usable, field-worthy system of medium performance.

The objective is to allow an optical alignment between the laser impact point on the window and the return signal being coincident (coming back on same axis) to the optical axis of the receiver. Once this initial alignment is accomplished, it is only neces­sary to “tweak” the micro adjust screws that secure the laser optimizing the signal from any reasonable distance. It is assumed some signal is always detectable once initially aligned.

A test-tone signal modulates the laser at 1 to 2 kHz. This scheme provides easy access to aligning the optical axis. You now carefully search for the test tone, which is clearly detected with the optical receiver. Night vision equipment also can be an aid in initial alignment.

Mounting the laser in our test prototype involved floating it inside a stable housing, allowing several degrees of both vertical and horizontal adjustment for final “tweaking” using vernier adjust screws. The lens used on our prototype was from a low-cost video camera and screwed into a mating adapter plate firmly attached to the housing.

Obviously, a certain amount of mechanical ingenu­ity will be required in finalizing the system. The sug­gested assembly of this integrated system is shown in Figures 13-15.13-16, and 13-17.

Exact dimensions are not given, as this might limit you due to the access of materials and the use of your own ingenuity. We show our approach to be used only as a guide that may be closely or partially fol­lowed.

System Application, Safety, and Legality



Figure 13-15 Suggested system assembly—side view

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