Build A Laser
Use a light beam to listen in
to anything, anywhere, any time...
Breaking and entering to plant
a listening device is one way to "bug" a room.
Unfortunately, it can earn you a long
jail term. A safer way to bug a
room is to use a laser beam
eavesdrop on a window from across the street...
This data is presented
for information purposes only. It is legal to own but illegal to use
in the described manner. Consult legal counsel before attempting to
construct this device.
The laser listener...
The sound waves generated by nearby conversation will
cause the glass in a window to vibrate very slightly. If a laser
beam is bounced off the window, its reflection will be modulated by
All that's needed to hear what is being said is a
demodulating device that extracts the audio from the reflected laser
beam That technique is used by sophisticated "surveillance experts,"
but you can easily duplicate that feat by using a hobbyist's laser
and the inexpensive Laser Listener demodulator shown in Fig. 1.
If you need something a little more sophisticated it
can be made part of the riflescope aimed laser-bug system that is
shown in Fig. 2.
Extra precautions must be taken because of a laser
beam's intense concentrated energy. Among other factors, the hazards
presented depend on the power density, the frequency of the beam,
and the time of exposure. Guidelines have established the
classification of lasers.
A brief description of the classification is as
Class I: Low-power beam. Not known to produce any
to the eye or skin.
Class II: Reserved for visible-light lasers only. They
are limited to less than 1 milliwatt output. Eye damage will result
if stared into for longer than 1 second.
The normal blink
response of the human eye will provide protection. Eye damage will
occur if the beam is viewed directly by optical instruments. Direct
(specular) reflection, as from a mirror, should be considered to be
the direct beam.
Diffuse reflection of the light may be viewed.
Class III: Instantaneous eye damage will occur if
exposed to the direct beam.
Class IV: Both direct exposure or direct and diffuse
reflections will produce eye damage. Exposure of the skin to the
beam is hazardous. The beam is considered to be a fire hazard.
Early light-wave communications
Communication using a modulated beam of light isn't a
new idea. In the 1880's, Alexander Graham Bell experimented with
something he called a photophone; a device that modulated a beam of
sunlight. It had a Mouthpiece that concentrated sound energy on a
reflecting di-aphragm, which, in turn, modulated a beam of sunlight
that was aimed at the diaphragm. When a remote receiver consisting
of a photovoltaic cell and a sensitive earphone was positioned in
the beam, the voice could be heard clearly from the receiver.
The aiming problems presented by the movement of the
sun, and the interruptions due to clouds and night, probably
prevented the commercial exploitation of the device. But by using
coherent light-such as that produced by a continuous-wave laser-the
principles used by Bell's device may again be applied in a
meaningful way. After all, terrestrial lasers aren't influenced in
any way by sunlight or clouds. And perhaps more important, unlike
acoustic sound-detection devices, lasers aren't usually subject to
interference originating between the sound source and the receiver.
For example, remote sound-pickup devices in the form of directional
microphones have been available for many years.
Unfortunately, any sound generated between the
listener and the sound source usually renders the device useless
because the interference is heard at the receiver, and it can be
even louder than the source. On the other hand, lasers are not
sensitive to sound of any kind between the source and the receiver.
However, lasers may be subject to other kinds of interference: For
example, AC-powered incandescent lights can produce a hum; gas
discharge devices such as fluorescent, mercury, sodium vapor, and
neon lights might produce a buzz; and direct sunlight might swamp
the laser detector device.
Also, where unusually long distances are involved, air
currents can add flicker to the laser beam, which on windy days can
result in a noise that is similar to that of blowing into a
microphone. (But even though sensitive to some kinds of
electrically-generated noise, laser- listening devices have an
advantage: They can seemingly hear through walls or closed windows,
and even selectively monitor only one window of a building from
several hundred feet away.) Commercially available laser sound
pickups use a laser device having an output in the infrared region.
Because infrared is below the visible portion of the
light spectrum, it cannot be seen by humans. However, some
commercial devices have a power output rating as high as 35
milliwatts. At such a power level there is clear potential for eye
damage if someone in the target area unknowingly stares into the
beam, or if the laser is operated carelessy by the user.
Although the details underlying the generation of
laser light are beyond the scope of this text, an understanding of
some of the characteristics of a laser beam as compared to ordinary
light will be helpful in assembling a laser-listener system. Light
is considered to be comprised of packages of energy particles called
However, light is also electromagnetic radiation and
behaves like radio waves, although at a much higher frequency. The
perceived color of visible light is determined by the radiation's
wavelength, which is usually given in micrometers. The shorter
wavelengths are perceived as violet, the longer wavelengths as red.
The spectrum below the visible portion is called infrared; the
spectrum above is called ultraviolet. The light emitted by a
conventional incandescent or fluorescent source contains a wide
range of frequencies, and the photons are emitted randomly and
spontaneously in all directions.
On the other hand, in a laser light source the photons
are released in one direction, at one frequency, making the laser
light highly directional and pure in color. (An analogy would be to
liken ordinary light to white noise, while the laser is likened to a
sinewave-a single pure tone.) Since all of the light emitted by a
laser is coherent (has the same frequency), constructive or
destructive interference occurs when two beams of laser light meet
at the same place and time (Fig. 3).
As shown in Fig. 3-a, the beams cancel each other when
out of phase (destructive interference). As shown in Fig. 3-b, the
beams are additive when in phase (constructive interference).It is
the interference between the beams that enables the movement of any
reflectingsurface to be sensed by a device called a interferometer.
An interferometer is a beamsplitter usually a piece of
partially-mirrored glass that deflects only a small part of a beam
aimed through the glass. As shown in Fig. 4,
it can be used to reflect both the source and the
reflected laser beams so that their phasing or amplitude can
becompared by a receiver. The major problems with using
interferometry for eavesdropping is that only a part of the laser's
energy is directed at the target, limiting the working range, and
the interferometer is sensitive to the diffusion of the sounds
target's reflections caused by tremors in the mountings of the
interferometer, the laser, and the reflective target. For
super-snooping, a direct reflection from the target is preferred
because the collimated nature (parallelism) of laser light also
allows modulation of the beam to occur just as Bell`s photo-phone
modulated the sunlight.
The prototype's laser Regardless how we choose to
eavesdrop, we must start out with a laser, so we' ll cover the
prototype laser bug's unit first. It's a (CENSURED) unit having an
output power of 0.9 milliwatts. It has a beam divergence of 1.64
milliradians, which produces a spot of light 11/2 inches in diameter
at 200 feet. Although 0.9 milliwats doesn't appear to be much power,
it can cause extreme eye damage if allowed to shine or be reflected
directly into the eye, or if viewed directly through any optical
device such as a telescope, binocular, etc. The beam may be safely
viewed only if projected onto a non-reflective surface such as a
white sheet of paper. It' you want to keep costs at rock-bottom, or
just want the excitement of a complete home-brew project, another
alternative is to assemble the helium-neon laser shown in the June
1986 issue of Radio-Electronics. Also. if you want to build a laser
from your own design, helium-neon tubes are often available from
The Laser Listener's receiver is relatively easy to
build and adjust. It is designed to drive a 4-20-ohm headphone or
speaker. Which permits just about any high-fidelity or Walkman-type
headphone to be used for monitoring. The circuit shown in Fig1, uses
a photo transistor (Q1) for a sensor, and has a meter (M1) that
indicates the relative signal strength of the reflected laser beam.
Because the Meter responds only to the amplitude modulation of the
reflected laser beam, it is unaffected by ambient light and the
relative intensity of the laser beam. An adjustable polarizing light
filter can be installed in front of Q1 to avoid swamping of the
phototransistor by very high ambient light. Phototransistor Q1 is an
inexpensive type usually called an IR detector, which Means that it
is specifically sensitive to infrared light. Tests comparing the
unit specified in the partlist with other less readily-available and
more-expensive devices show no measurable difference in performance
in the prototype receiver. No base connection is used for Q1 because
the reflected laser light controls the collector current. The audio
signal developed across collector load-resistor R1 is coupled by C2
to voltage-controlled attenuator IC1, which has a greater than 30-dB
gain variation; It serves as both a preamplifier and as an
electronic volume control.
Resistor R2 and capacitor C1 decouple (filter) the
power supply voltage to Q1 and IC1. Be sure to take extreme care,
not to eliminate or accidentally bypass the filter because that will
cause unstable operation. The gain of Q1 and IC1 is too great to
permit non-decoupled operation from the power supply. The output
from IC1 is fed through C4 to amplifier IC2. Resistor R4. and
capacitors C5 and C7, tailor IC2's frequency response and ensure
stable operation with varying drive levels and output loads. The
output of IC2 is split into two paths: One goes to output-jack J1
via C6; the other feeds voltage-follower IC3, which drives the meter
circuit consisting of D1, D2, C11, R8, and M1. The time constant
created by the values of R8, C11 and Mi's DC resistance was selected
to provide a comfortable damping of the meter pointer's gyrations.
The value of C11 may be varied to change the pointer's response.
Increasing the value of' C11 provides a smoother response;
decreasing C11's value will cause the pointer to more closely track
the variations in the laser beam's modulation.
The prototype receiver was assembled on a modified
Radio Shack type 276-170 pre-drilled PC board, which has strips of
copper foil on the underside that connect the component mounting
holes. (A board with a parts-placement template in place, as shown
in Fig. 5, is available from the source given in the partlist.)
Nothing about the layout is critical as long as you follow the usual
precaution of keeping the input and output connections reasonably
separated. Check your parts layout against the foil strips on the
underside of the board. If it appears that any will be too long, cut
them to size before mounting any components. Cut each foil strip
exactly as long as needed so that a foil carrying the input signal
doesn't end up running adjacent to an output connection. For best
results when making connection to the foils, use a small
pencil-tipped soldering iron and .040 diameter rosincore solder. If
your layout requires jumpers between component mounting holes, use
#22 solid, bare wire. Insulated junipers are #22 solid, insulated
wire. Connection between the copper foils should be #18 insulated
wire because it's a precise push-fit for the holes in the specified
prototyping board. The enclosure is a 61/2 x 21/8x 15/8 inch
aluminum cabinet. Phototransistor Q1 protrudes from one end of' that
enclosure and is mounted with a dab of household cement. Position Q1
correctly before gluing it in place and be very careful to not get
glue on the surface of the lens. Do not use cyanoacrylate-based
instant glue because it might cloud the transistor's plastic lens.
Output-jack J1, gain-control potentiometer R5, and the meter are
mounted on the side of the cabinet , so as to encourage the user to
face at a right-angle to the source of the laserlight, thereby
lessening the change of looking directly into the reflected beam.
The board is mounted in the enclosure with four 3/4 inch 6-32
machine screws. Use 1/8 inch insulated spacers between the board and
the enclosure to insure adequate clearance between the enclosure and
the board's foil side. A ground lug located at one mounting screw is
soldered to the circuit-board's ground foil to provide the ground
connection between the board and the cabinet. The connections
between the board and the panel-mounted components can be #18-22
stranded, insulated wire.
The optical attenuator assembly, for which
construction details are shown in Figs. 6 and 7, mounts over
Figure 6 shows how it's installed over Q1: Fig. 7
shows the individual details for each component in the assembly.
The front of the assembly is painted flat white so
that the reflected laser beam can be easily seen The attenuator is
built in such a way that the phototransistor can see the laser beam
directly, or through a combination of one or two polarizing filters.
When both filters are in place, rotation of the
large-diameter filter-mount will cause a gradual decrease in light
transmission (to almost total blockage within 90 degrees of
rotation), which allows the receiver to be used over a wide range of
light intensities without swamping the photo detector.
Figure 8 shows the installed assembly and the two
filters. The attenuator has an inner filter and an outer filter made
from brass telescopic tubing. Each filter consists of two sections:
a filter base that is soldered to small mounting plate made from
brass sheet (the painted target), and a filter mount that slips over
the base. Polaroid fillers cut from neutral-tint polarized
sunglasses are cemented to one end of each filter mount to complete
the attenuator. When complete, the entire optical attenuator's
mounting plate is secured on the enclosure over phototransistor Q1.
All resistors are 1/4 watt, 5% unless otherwise noted.
miniature potentiometer with
C1, C6, C9, C10-330 uF, 16 volts, electrolytic
C4-10 uF, 16V volts, electrolytic
C3-0.001 uF, 50 volts, ceramic
C5-0.68 uF, 16 volts, Tantalum
C7, C8-0.047 uF, 50
volts, ceramic disc
C11-4.7 uF, 16 volts, electrolytic
C12-1000 uF, 16 volts, electrolytic
NPN phototransistor (Radio Shack 276-145 or equal)
D2-SK-3090 germanium diode, or equivalent
B1-9-volt transistor-radio type battery
J1-miniature phone jack
Ml-250 uA meter, panel mounting
S1-SPST switch, part of R5
Cabinet, Pre-drilled PC board, brass sheet and tubing,
wire, solder, etc.
The following is available from Dirijo Corp., Box 212,
Lowell, NC 28098. A drilled prototype-board with a component layout
overlay in place, model LXVR-1.
We advise that a small speaker be used rather than
headphones for the initial tests, then, if a wiring error or a
defective component has created an audio oscillator rather than an
amplifier, your ears will not be assaulted by a high-level tone or
squeal. With the volume control fully counterclockwise and
power-switch S1 set to off, install the battery and connect the
speaker. Turn the unit on and point it toward a source of daylight
(not direct sun). Advance the volume control to maximum. Correct
operation is indicated by a frying noise that sharply diminishes
when the light is blocked. The meter-sensitivity control, R8, should
then be set so that the meter's pointer just begins to move off the
zero calibration. Decrease the gain and point the receiver toward an
AC-powered light source, such as an incandescent or fluorescent
light, or even an LED driven by an audio oscillator. Those sources
should produce a loud hum or tone. Sound will be heard if the LED is
driven from an audio amplifier at the correct level. If everything
checks OK, assemble the enclosure.
Remote sound detection
To use the receiver as a remote sound pickup, you will
need a laser and a reflective surface that sound waves will cause to
vibrate; the receiver must be positioned so it can "catch" the
direct reflection of the laser beam (Fig. 9).
A particularly effective reflector for experimental
use is a small piece of mirror (about 1/4 x 3/4 inch) cemented to
the center of a speaker cone (see Fig. 10).
There is no connection made to the speaker. The
movement of the speaker cone caused by sound waves is transferred to
the mirror-reflector. Which in turn modulates the laser beam. Due to
the varying reflectivety and distances of the targets, the intensity
of the light falling upon the detector may vary considerably from
setup to setup. That will be readily apparent if the collector
voltage of Q1 is measured while the illumination level on Q1 is
adjusted. At some point of increasing illumination, the collector
voltage will fall sharply and the audio output from the receiver
will drop or disappear. The small-diameter polarized filter should
then placed over Q1. If more light attenuation is required, slip the
large-diameter filter in position and rotate it for maximum sound
Thin is in
The thinner and more responsive to sound the
reflective medium is. The greater the laser bug's sensitivity. Most
window panes will work. Moving the beam to different spots on the
glass can make a dramatic difference in the sensitivity. For
testing, no additional optics are needed for the receiver, Set up
any convenient reflector-the mirrored speaker. or even an embroidery
hoop holding plastic wrap or Mylar film (see Fig. 10) aim the laser
at the reflector, and then position the reflector so that the beam
bounces back to the receiver. If you speak in the room, or play a
radio or a tape recorder, the sound will be heard in the receiver's
headphones. Another test can be done by modulating the laser with a
1 kHz tone while having an assistant move the target reflector for
maximum tone reception as indicated by maximum volume in the highest
meter reading. A non-adjustable target, such as a window pane,
requires that the operator select a site where a direct reflection
can be caught. That can be done from hundreds of feet away if
conditions are right. Use the modulated beam for setup, and then
remove the modulation to listen in. Double-pane glass and storm
windows tend to greatly reduce sound transmission to the outer
glass. It is possible, however, to aim through the glass to an
object within the room, such as the glass front of a china cabinet
or a hanging picture. The returned reflection is usually modulated.
At long range
At ranges greater than 100 feet or so, or when a high
ambient light level obscures the reflected beam, a means must be
provided to accurately aim the receiver to the reflected laser.
As shown in Fig. 11, the receiving unit of our
prototype laserbug system uses a telescopic gunsight; and that
assembly is, in turn, mounted directly on the laser housing as shown
in Fig. 2 so both the laser and receiver can be aimed as a single
unit. The design of a combination receiver and laser mounting
bracket will depend on the particular laser and scope that's being
used. In general, the mounting bracket should be sturdy and have
provisions for coarse elevation and azimuth adjustments; all gun
scopes have provisions for fine adjustments. The adjustment details
for the prototype mount are shown in Fig. 12.
The scope-to-laser alignment is done in two stages,
First, the distance from the center of the laser beam to the center
of the scope is measured and used as the spacing for the cross marks
of the target shown in Fig. 13,
which is made from dull, white cardboard. Then, the
target is taped to a wall about 50 feet away from the laser
assembly. Next, with the scope's cross-hair adjustments at the
center of their range, position the laser beam at the center of the
lower cross. Looking through the scope, adjust the scope's mounting
bracket so that its cross-hairs are close to being centered on the
target's upper mark. Making sure that the laser beam stays centered
on the lower mark, tighten the mounting bracket's nuts and use the
scope's fine adjustments for the final alignment. In this instance,
the diffuse reflection of the laser beam from the card should
present no eye hazard. When using the laser/scope assembly, remember
that at a range of under 300 feet you must compensate for the aiming
error introduced by the offset between the scope and the laser beam
centerlines. Again, let us stress that under no circumstances should
the laser beam or its direct reflection be viewed through optical
devices of this type because severe damage to the eye can result.
SCAM ALERT: "hackertronics" and
"hackers homepage" claim to sell laser listening devices. Do not
waste your time trying to order from them as they are well
documented scam sites.