May 1970 Popular Electronics
Table of Contents
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ambition of amateur hobbyists has always amazed me. In practically
every area of technology, amateurs have contributed significantly
to the effort of pushing back the frontiers of ignorance by developing
new applications, improving existing products, and demonstrating
to commercial companies a need for new features or products. Many
of those innovative hobbyists make a living in the field they choose
for off-hours recreation. As a heavy reader of technology and hobby
magazines of all sorts, I know this to be true for musicians, boaters,
pilots, radio operators, gardeners, science experimenters, woodworkers,
and many others. Levels of quality of finished products are awesome
in the master class competitions. The amount of money spent by some
for their "hobbies" is shocking. This laser communications article
from the May 1970 Popular Electronics is a prime example of that.
The author uses off-the-shelf components to construct both the transmitter
and receiving components. A quick tally of the most expensive parts
- telescopes, laser module, photocell, power supplies - shows the
project would have cost upward of $400. According to the U.S. Bureau
of Labor Statistics'
, $400 in 1970 money is the equivalent of
about $2,400 in 2012 money. That's an expensive "toy."
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Popular Electronics Exclusive Developmental
Laser Beam Communicator
By C. Harry Knowles
Modulate Our Low-Cost Laser
Communicating by means of a
laser beam is as fresh and new as the tomatoes picked from your
garden tomorrow morning. The mere idea of being able to transmit
information on a beam of coherent laser light suggests all sorts
of possibilities for secret, non-jammable, interference-free communications.
And it is possible today!
Communications by laser beam offers
several advantages over conventional radio links. Neither atmospheric
lightning nor airborne electrical noise affects laser communications
though they can completely ruin radio communications. On the debit
side, however, laser performance is degraded, over any reasonable
distance, by heavy fog, rain, snow, or terrestrial heat.
Unlike radio, in which the signal is "sprayed" out over a wide
area, a laser beam communications system operates on a line-of-sight
basis and the beam is tight enough to provide excellent privacy.
Of course, obstructions cannot be permitted to interrupt the beam
but conventional optical mirrors can be used to bend the light beam
around obstructions if necessary.
PE AT SMITHSONIAN
A pair of POPULAR
ELECTRONICS laser communicators, similar to the one described
in this article, is scheduled to be shown in operation at the
Laser-10 exhibit at the Smithsonian Institution's National Museum
of History and Technology in Washington, D.C. this spring and
summer. Readers living in the area or visiting Washington will
want to see this excellent exhibit, which features a wide variety
of lasers in many unique applications. The POPULAR ELECTRONICS
laser communicators will be set up to carry two-way conversations.
Two approaches to laser communications are described in this article.
The first involves only a simple addition to the basic laser described
in POPULAR ELECTRONICS in December 1969. This system has a range
of about 100 ft, and can be used for experimenting within a room
and provides a "breadboard" for use in understanding modulated laser
action. It also makes an excellent science fair project.
The second approach uses a modulation and receiving scheme similar
to the first but it operates through conventional low-cost telescopes
to achieve a range of several miles (depending on atmospheric conditions).
The light output of a
gas laser such as the 0.5-mw helium-neon type described in our previous
article is a function of the current flowing through the laser tube
(see Fig. 1). At very low currents, the laser becomes unstable and
tends to turn itself off. The light output increases reasonably
linear with tube current up to approximately 5 mA. Above that, the
light output drops drastically and tube life is decreased. If the
current is centered on the middle of the linear portion of the curve
and varied about that point, the light output can be made to swing
in a linear fashion and very high modulation levels can be obtained.
Fig. 1. Light output of the laser is a function of tube current.
Modulation 90% or better can be obtained easily by this modulation
Fig. 2. Negative-resistance characteristic of the laser shows
that a large-value variable resistance is required for stable
operation over the entire operating range.
Fig. 3. The basic amplitude modulator uses a conventional pentode
in series with the laser. The suppressor grid is electrically
connected to the cathode.
The voltage-current curve in Fig. 2 shows that the laser tube has
a negative resistance characteristic (voltage decreases as current
increases). Stable, linear operation thus depends on the use of
a ballast resistor. When the tube is operating at 5 mA, approximately
1100 volts are required. At this point, the negative dynamic resistance
is about 30,000 ohms. As the current is decreased the required voltage
rises until, at about 1 mA, it is approximately 1300 volts. Here
the negative resistance is 80,000 ohms. Therefore, the ballast resistor
must have an effective value well above 80,000 ohms to keep the
basic modulator circuit, using a pentode with a large dynamic resistance,
is shown in Fig. 3. The pentode is in series with the laser tube
and forms a simple amplitude modulator. The dynamic resistance of
the pentode is a function of the applied audio signal on its control
grid. A potentiometer in the cathode circuit of the pentode determines
the basic operating resistance of the tube and, hence, the operating
point of the laser. Once the latter point (located on the curve
in Fig. 1) has been set by the bias potentiometer, an audio input
to the pentode causes the laser current to fluctuate about the operating
point and the emitted light is amplitude modulated.
any type of audio driver can be used to generate the input audio
signal to the pentode. Basic Modulator.
The circuit for converting the original laser project into a light-beam
transceiver is shown in Fig. 4. A photograph of the finished project
is shown in Fig. 5. A complete vacuum-tube system is used simply
because a high resistance device is required and the tube that will
do the job is inexpensive and readily available. In addition, the
+175 and 6.3-volt sources required by the pentode can be used elsewhere
in the circuit.
The modulator circuit can be divided into
two portions. The transmitter (V1) consists of the pentode modulator
driven by the triode half of the tube acting as a microphone preamplifier.
Potentiometer R4 provides modulation level control. The three gas
tubes in series (I1-I3) are 200-volt breakdown lamps which chop
off the high-voltage spikes that trigger the laser. Although the
operating plate voltage of the tube is below its maximum rating,
a much higher voltage spike is used to trigger the laser. The three
gas lamps limit this spike to 600 volts. Unlike semiconductors,
a vacuum tube can withstand an overvoltage for a short time. The
trigger spike here lasts only about one millisecond so no damage
can be done to the tube. If you can't locate the gas tubes called
for in the Parts List, use any combination of conventional neon
lamps that add up to approximately 600 volts.
portion of the modulator consists of a three-stage conventional
audio amplifier driven from the output of the solar cell. Unlike
a conventional light-dependent resistor, a solar cell generates
a voltage that is a function of the amount of light striking the
photosensitive surface. Construction.
you built the original laser project, the same metal chassis may
be used. Drill or punch holes for two 9-pin and one 7-pin tube sockets.
These may be located on the top of the chassis, next to the laser
tube. (Be sure to remove the laser tube when doing mechanical work
on the chassis.) On the wall opposite the high-voltage laser power
supply, mount the three potentiometers (R6, bias; R4, modulation
level; and R12, receiver volume), the microphone input jack (J1),
and the photocell input jack (J2) (see Fig. 5). Mount power transformer
T2 on the outside of the chassis using the same mounting hardware
as were used for the original 600-volt transformer. (It was T1;
now it is T3.)
Fig. 4. Other than the basic pentode
modulator (V1B) circuit, either vacuum-tube or semiconductor audio
amplifiers can be used for the remainder of the circuit.
Once all the components are installed, wire up the
circuit point-to-point (using terminal strips as required) following
the circuit shown in Fig. 4. Of course, it is not necessary to use
vacuum tubes for the microphone amplifier. You can use the 6AU6
pentode for the laser driver and, for the amplifier, anyone of several
commercially available transistor amplifiers. The author used one
of the new RCA IC kits - the KC4000 microphone preamplifier - in
one model and found that it worked fine. The solid-state receiver
consisted of a KC4000 microphone preamplifier for the photocell
preamplifier and a KC4003 1/2-watt audio amplifier to drive the
Fig. 5. The prototype was built on the original laser chassis
(December 1969 issue). Any other layout will do as long as the
pentode modulator is as close as possible to the laser.
Internal layout of the prototype transceiver (above) showing
the laser power supply mounted on one wall with the rest of
the components occupying the remaining space. The internal arrangement
of the telescope electronics (shown below) shows the modulator
tube and its associated components arranged within its smaller
The complete telescope system can communicate as far as a 12-inch
target can be clearly seen via the telescope. At night, this
target will have to be illuminated. In good visibility, range
can be very great but is dependent on certain conditions (see
text). To assist distant communications, an optional 1-kHz audio
oscillator is used to modulate the transmitter, and both ends
must be "juggled" until the received audio tone is at a maximum.
To get around opaque objects, a large-size front-surface mirror
(not a ladies compact mirror) may be used to reflect the laser
The receiving photocell in this simple light communicator is mounted
at one end of a dark plastic tube. (A cleaned out container of Polaroid
print coater works very well.) If you use a cardboard tube, paint
the interior a dull black before installing the cell. For testing
and experimentation, make up a microphone cable with a phono connector
at one end. Use a phone jack to make the connection to the earphone
output of a conventional transistor radio. The radio is silent when
the earphone jack is plugged in and produces a non-tiring audio
signal for testing. Testing.
volume, modulation, and bias potentiometers in their minimum resistance
positions. Connect up the speaker, photocell, and radio and turn
on the power. The laser tube will start to blink at a low level
until the modulation pentode warms up. Once the tube is hot, the
laser will operate at its full brightness. A slight increase in
the resistance of R6 should cause the laser beam to dim slightly.
This shows that the bias control is operating properly. Now set
the control for full brightness. Increasing the volume control should
produce some hum in the speaker. If conventional room light is allowed
to fall on the sensitive face of the solar cell, it will produce
a distinctive hum. This is the reason the solar cell should be mounted
in a dark tube.
Separate the laser and the solar cell by
a few feet and aim the beam at the receiver. Alternatively, aim
the laser beam at a mirror so that it is reflected back to the cell.
(The beam must be aimed straight down the cell tube and not at the
With the laser beam shining on the solar
cell at full brightness, turn on the radio, tune to a station, and
plug in the earphone jack. On the laser chassis, turn up the receiver
volume control and note that, as the hand is passed through the
laser beam, a thump is heard in the speaker.
the bias control to dim the laser a little, and turn up the modulation
control slightly. These two controls interact somewhat so you will
have to "juggle" them for best modulation.
Make sure that
the radio volume is turned up sufficiently.
Once the communicator
is working, you can experiment with the controls and the circuit
(always retaining the pentode as the laser modulator) to increase
your understanding of laser communications. Optical
Depending on how you want to use it, the laser
communicator can be set up with anyone of three optical systems.
The simplest, which can be used for point-to-point communications
around a room (to a total of 100 ft round trip), is as described
above, without any lenses. To improve the reception somewhat, a
simple lens can be placed in the beam path at the receiver end to
reduce the size of the diverged beam.
The second type of
optical system, requires the use of a set of binoculars, one eyepiece
for the transmitter and the other for the receiver. Simple toy telescopes
may also be used. The range for this type of system is a few hundred
For communicating over greater distances, a reasonably
high-power telescope is necessary. Such a telescope, attached to
the laser communicator, acts like a high-gain antenna on a conventional
radio system. In both cases the transmitted and received signals
get a boost from the "antenna." And in both cases, the telescope
or antenna is used for both transmitting and receiving through a
simple mechanical switching process.
How far can you transmit
using a telescope? It depends on a number of factors, the most important
being beam divergence and atmospheric conditions. As the beam travels
along its path, it tends to enlarge (diverge). This means that,
although the beam leaving the laser is quite small (1 millimeter
in the POPULAR ELECTRONICS laser), it does enlarge considerably
- though not as much as a comparable beam of conventional light.
Using a telescope improves this condition considerably.
Atmospheric disturbances of the laser beam cause it to wander.
As the beam of light is projected over a long distance,
it may encounter various forms of air turbulence, such as localized
temperature changes. In each of these turbulences, the density of
the air changes and each change in density acts as a prism as the
beam passes through it, changing the beam's direction slightly.
The amount of wander can be as much as several feet per mile. In
the still, relatively even temperature of morning, before the sun
has had a chance to warm up the air, beam wander may be as little
as a few inches per mile.
In using a reflector telescope
such as that described later in this article, the beam should be
collimated as closely as possible to the distant receiver, allowing
for thermal refractive variations for the time of day and the atmospheric
conditions. If the air is still and of an even temperature, the
beam will wander only a few inches per mile. In this case, also,
the beam may be focused so that at the receiver, the beam diameter
has diverged only about one foot per mile. If the atmosphere is
clear, there is little absorption by airborne particulates (smoke,
dust, etc.) ; and the overall result is that about 3 to 5% of the
transmitted beam power is obtained at the receiver. This extremely
high efficiency is one of the many attractive features of laser
communications that will help make it the system of the future.
Reflector Telescope Construction.
system is shown in Fig. 6. The laser tube is supported by a pair
of view-finder ring mounts attached to the telescope tube. The laser
is positioned within the mounts so that the light-emitting end is
almost directly over the telescope eyepiece. (Check your laser tube
to make sure whether the light beam comes out of the anode or the
cathode. Some models are one way; some the other.)
up an L-shaped length of heavy bus-bar with the long side about
2 1/2" - the other about 1" long. Cement (with epoxy) the short
end of the bus bar to the relay armature so that it swings back
and forth as the relay is energized and de-energized. Position the
relay about 90° from the telescope eyepiece so that when the long
end of the bus bar is placed through a slot cut in the telescope
tube and with the relay energized (talk position) the end of the
bus bar is out of the beam path. With the relay de-energized (listen
position) the wire should be in the beam path. Remove the telescope
eyepiece to watch this.
Fig. 6. Complete telescope communicator showing
the use of semiconductor audio amplifiers. Any neon lamps may be
used for I1, I2, or I3 if their breakdown totals up to about 600
The main chassis for the telescope communicator
mounts the relatively heavy power supplies (except for the laser).
all controls, and is connected to the telescope electronics via
a multi-lead flexible cable. Microphone plugs into the rear.
On the solar cell called for in the Parts List of Fig. 4, the
black side is the sensitive area. Cement the shiny side of the cell
to the bus bar and then slide the cell and relay assembly into position.
Make sure that the cell switches cleanly in and out of the beam
path as the relay is operated. The two leads from the solar cell
are taken out of the same slit and terminated on a two-lug terminal
strip mounted near the relay.
Mount the empty half of the
two-piece electronic chassis on the telescope tube, just below the
two laser mounting rings, drilling mating holes in both chassis
and telescope tube. Use short mounting hardware so as not to interfere
with the beam path. Recheck all mechanical work and tighten the
telescope tripod screws.
To keep weight to a minimum, only
the modulator pentode and the laser power supply are mounted in
the chassis on the telescope. This is necessary to reduce the possibility
of oscillation in the circuits.
Mount the power supply on
the inside of the chassis, using an insulated spacer (about 1/4")
at each corner. Be sure that the high-voltage end is far enough
from the metal to avoid arcing. The seven-pin tube socket for the
pentode is mounted at one end, while a multi-lug terminal strip
supports the ends of the wiring. A 1/2" grometted hole should be
provided for the incoming cable.
The circuit for the scope-mounted
electronics is shown in Fig. 7. Only the relay, solar cell, and
laser are external to the chassis. The circuit above SO1 is mounted
at the scope. The lower portion is built in a larger conventional
View looking into end of telescope shows how the solar cell,
in transmit condition, is out of beam path from laser to diagonal.
In the receive mode, the cell enters the beam path between diagonal
and eyepiece. Make sure that sensitive side of solar cell faces
In the simple transceiver, the solar cell is mounted within
a tube having a dark interior - in this case, it's a clean Polaroid
print coater. Cell is affected by ambient light so that it must
be shielded during use. Any method of mechanical mounting may
be used to position the cell correctly.
Once again, either vacuum-tube or semiconductor amplifiers may be
used. The latter save quite a bit of work. Connections between the
two chassis are made with multi-lead cable, with the exception of
a small coaxial cable for the solar cell leads. Make the connections
long enough to allow plenty of space between the telescope and the
other chassis. The cables may be taped at intervals to keep them
When all electronic work is finished, attach
the second half of the chassis to the one on the telescope. The
cable should be placed where it will not interfere with scope operation.
Fully open the ring mount thumb-screws and slide the laser
into position as described above. Tighten the thumb-screws gently
to avoid damaging the tube. Attach the plus side of the high-voltage
supply to the laser anode and the negative side to the cathode.
Make up a phono connector to connect the solar cell leads
to J1. Connect the two leads to the relay.
A small 90°
prism is cemented to a plastic block to aim the laser light at the
telescope eyepiece. The plastic block is press fit to the laser
The transmit-receive relay is mounted to the telescope
tube with the solar cell and rod passed inside through a hole cut
in the telescope tube wall. Setup.
the far end of the multi-lead cable to the main chassis, along with
the solar cell and microphone connectors. (You can substitute a
radio for the microphone for testing.) The push-to-talk button may
be temporarily shorted to keep the solar cell out of the beam path
during the following optical alignment.
IC audio kits were used for all stages except the pentode modulator.
Power supplies are mounted under the chassis. Telescope cable termination
is on rear apron.
It is assumed that the telescope optics
have been set up as described in the telescope operating manual.
On the main chassis, set bias control R4, volume control
R5, and modulation control R6 to minimum resistance. Plug in the
117-volt line cord and turn on the power. The laser tube will blink
a few times until V1 warms up. After the laser starts to glow at
full power, allow the entire system to stabilize for a few moments.
Adjusting the bias control should cause the laser glow to diminish
a little. Set this control for maximum laser brilliance.
Place the 90° plastic prism over the protuberance at the laser
exit hole and adjust the prism so that the laser beam is reflected
down the telescope eyepiece. Aim the telescope at a wall and keep
adjusting the prism - and if necessary the position of the laser
- until a red circle, with the diagonal mirror shadow centered in
it, is clearly visible on the wall. At this point, the laser has
been properly set up and should not be moved.
If you have
to keep looking at the laser beam, a pair of blue sunglasses may
be worn to reduce the red glare.
To test the system, aim
the telescope at a distant mirror and reflect the beam back to a
duplicate solar cell that has been connected to the main chassis.
You can also use the second telescope of the communications system
if you have built it at this time.
With the light beam shining
on the solar cell, make sure that the radio is playing at a reasonable
volume and turn up the laser volume control R5. If artificial light
falls on the solar cell, a hum will be heard; so for best reception
keep the ambient light dim. Slowly adjust the bias control (R4)
until the laser dims a little. Then bring up slightly the modulation
control (R6) until music is heard from the main chassis speaker.
Since R4 and R6 are interlocking in their action, you will have
to adjust them together to get the desired results. If R4 is set
for too low a beam level and R6 is set too high, modulation peaks
may extinguish the laser. The automatic power supply will retrigger
the laser, but the controls should be adjusted to prevent the drop-out.
Once clean modulation has been obtained, the radio can be replaced
by the microphone and R6 adjusted for this type of input.