For an updated version of this same design that allows
for a somewhat simpler circuit design and (optionally)
manual gain control, see the web page about the "Version 2/3
of the Mini Pulse Width Modulator." - link
Why another Pulse-Width Modulator:
Why is PWM better for
driving laser diodes?
Laser diodes - unlike LEDs - have a
fairly limited current range over which they will
lase. While the optical output of an LED is
very strongly related to the current through it in a
linear way, a laser diode is not: Too low a
current, the laser diode either does nothing or
lights up like an LED without actually lasing while
a little too much current will destroy the
laser almost instantly! This becomes tricky
with analog modulation since it could well be that a
voice peak could come along that is just a bit too
high for the laser to survive.
The trick would be to finding the range of current
over which a Laser diode will function. The
problem with this is that this current varies from
device to device and even with the same device over
temperature and you can never really be sure if you
are in the "safe" range without testing it - and
possibly blowing up the laser diode! In
addition to this problem, there's no guarantee that
over this "window" of safe operating current the
laser's light output will bear a linear relation to
the current and this could cause significant
distortion in the audio.
If you want to effectively modulate the light
source, you need to be able to vary its current from
a low level - where the light output goes pretty
much to zero - to "full brightness" where the
current is at maximum. If your modulation
doesn't go to these extremes, you are short-changing
yourself in that the "modulation depth" is reduced
and your laser link's performance will be diminished
because you aren't using the full dynamics of the
laser to convey the information.
Using PWM conveniently side-steps this issue in most
respects: You simply set the "on" current to a
known, safe value and since the laser diode is
either "on" or "off", you can safely utilize all of
the laser's dynamic range. What's more is the
fact that since the laser itself is on or off, the
linearity of its modulation is that of the PWM
generator rather than the awkward, nonlinear
current-versus-brightness curve of the laser!
After constructing and using my previous Pulse-Width
I decided to build the future modulators using
analog current-modulation techniques. The reason was that
the "current sink" circuit used is very easy to construct and
offers excellent performance and the PWM circuit offered no
great advantage over the current-sink version - except that the
PWM circuit could easily drive Laser diodes. Since there
wasn't the need to routinely modulate laser diodes in our
experiments - and since I already
had a PWM unit to do
this - there was little impetus to build yet another modulator.
Why go back to PWM again? There was the need to construct
a very simple, but full-featured modulator. As it turned
out, in late 2008 I received an email from Henry, KB7NIE, one of
the members of a Tucson-based amateur radio group and this
group's main interest was microwave frequencies. Since
some of their number are attached to the University of Arizona's
optical department and/or have general interest in things
optical, we got their attention - and an invitation to visit.
After several months everything came together and we paid them a
visit and were given detailed tours of some of the microwave and
optical facilities in the area (think telescopes - radio and
As part of the visit we demonstrated an
across-the-valley optical QSO.
As part of the effort, we (Ron, K7RJ and I) decided to put
together a very simple optical transceiver using foam-core
posterboard, "page-magnifier" Fresnel lenses, and the necessary
electronics. Ron kindly assembled a "Version
3" optical receiver
of his own layout and construction
while I put together an LED modulator. Not wanting to do
just the same as before I decided to start from scratch,
hardware-wise, and I built something that was very simple, yet
had a few nice features such as a tone generator. Part of
keeping the hardware simple and versatile involved the use of
Unfortunately, we ran out of time before we could get everything
focused and aligned - although a preliminary test with the
transmit portion was done over a 7 mile/11 km path: The
unit was simply left with our host when we returned home so that
he could, in the future, conduct tests and experiments of his
What is PWM?
Pulse Width Modulation (PWM)
is widely used nowadays in
low-power audio amplifiers and the so-called "1 bit" D/A
converters and the operation is simple:
- Imagine that you wished to produce a voltage that was 1/2
of the maximum. For this, a 50% duty cycle square wave
is generated at a frequency several times higher than the
highest-frequency component in the audio being
reproduced. While this frequency could theoretically
be as low as just twice the highest audio frequency, it is
usually several (to many!) times higher than that to
simplify lowpass filtering to cut costs. Since it
spends 50% of it's time "on" and the other 50% "off", the average
voltage is also at 50%.
- To increase the output voltage, the duty cycle of this
square wave is increased, with 100% being "full on."
Conversely, to decrease the voltage, the duty cycle would be
decrease, down to 0% being completely off. In reality,
most PWM circuits avoid getting too close to either 0% or
100% as either extreme would produce objectionable "hard"
- The PWM output is filtered to average out the square wave,
the ultimate result being a voltage that is directly
proportional to the duty cycle of the original square wave.
As it turns out, the linearity of a PWM generator could, in
theory, be absolutely perfect: The duty cycle is timed
precisely using digital counters - which are - for practical
purposes - absolutely precise. What is necessary
is that there be enough timer resolution in order to provide the
needed resolution of duty cycles.
Take, for example, a 10 bit PWM converter. Because 10 bits
represents 1024 steps, it would be necessary that the original
timing clock be 1024 times that of the sampling rate. If,
for example, our original clock were 20 MHz, one 1024th of that
would be 19.53125 kHz.
Diagram showing how PWM is produced and how it is
detected and converted back to an analog form.
In practical terms, the frequency response of the circuits in a
typical highly-sensitive optical receiver are not able to
the PWM frequency so they tend to
average out the PWM switching frequency and leave only the
modulation, with the result being a voltage that is very close
to the original analog signal applied to the modulator.
Again, this technique works provided that the optical receiver
being used (and/or its following amplifier stages) don't have
the frequency response characteristics necessary to reproduce
the original PWM waveform: This could be through design
that intentionally reduces frequency response, or it could be
the result of additional low-pass filtering.
Some caution should be exercised, however: If the optical
have the bandwidth to recover the PWM
signal, or if there is a reduced (but still sufficient) response
of the audio chain at the PWM frequency, this could play havoc
with downstream audio devices in several ways:
- The audio amplifier may be capable of amplifying the PWM
signal, robbing power from the audio-frequency
components. In this situation, the audio amplifier is
putting out its normal power, but some of it may be wasted
at the PWM frequency and be inaudible to human
hearing. In this case, the audio amplifier may
overload at lower-than-normal volume levels.
- Aliasing artifacts on digital audio devices.
Computer sound cards and digital audio recorders may not be
able to sufficiently filter the PWM frequency from their
inputs and this may result in odd aliasing artifacts, which
may include noise, distortion, or odd mixing effects.
While the above are possibilities, I have not experienced these
effects when using my Version
3 optical receiver (even with the lowpass filter switched
with digital audio devices - but the fact that this
receiver is designed to roll off severely above about 7 kHz is, no
doubt, a mitigating factor. The caution here is that all
equipment should be tried out before going out into the field
for serious work to verify that there is compatibility.
Again, it should be noted that to detect a PWM signal, the
receiver does not
need to be able to respond to
the switching frequency, but only to the rate at which the pulse
width is being changed - that is, the audio modulated atop
the PWM. A "slow" receiver will simply integrate (or
together the PWM waveform into a form that closely
resembles the signal that was originally fed into the modulator
in the first place.
A PIC-based Pulse Width Modulator:
Note that this is a prototype version: I will
add a few more feature to it as time permits - if someone
Like the previous PWM modulator, this is also based on a PIC -
but a different one: The PIC12F683. This is an 8-pin
device and has a number of useful onboard peripherals, such as a
10-bit A/D converter, a hardware PWM generator and an on-board
CPU clock - plus enough RAM and program memory to do some useful
The PIC is programmed to provide several important functions:
- Digitization of audio. The inputted audio
is is used to vary the PIC's onboard PWM generator to
modulate the light source.
- Audio AGC. An automatic gain control
compensates for a wide variety of input audio levels.
- The generation of audio tones. When aligning
an optical path it is very useful to have distinctive audio
tones being transmitted to more-easily point the receiver
Refer to Figure 2 for the schematics of the
circuits described below.
The heart of the modulator is U2, the PIC12F683.
Starting with the output, the PWM signal is applied to the gate
of Q2, a power MOSFET, which drives the light source by varying
the duty cycle: The higher the duty cycle (e.g. the more
the more light is output, and vice-versa. It
isn't really important what the light source is, as it could be
a laser, a high-power LED or even a light bulb!
Since Q2 turns on the light source simply by grounding one end
of it, all that is important is that the light emitter be driven
appropriately: In the case of an LED, one could simply use
a series-connected current-limiting resistor. For a laser
- such as that from a laser pointer - one would connect the
"minus" end of a laser module to Q2, while using the appropriate
voltage supply (around 3 volts for a typical laser pointer)
"positive" side of the laser. In all cases it is
recommended that C16 be present to make sure that the PWM signal
and the modulated audio doesn't find its way into the unit's
Comment: While you could use a light
bulb, their filaments are comparatively slow to respond to
audio, the result likely being rather muffled and weak audio.
The bottom diagram of Figure 2 shows how various light
sources may be connected.
Moving to the other end of the circuit, there's Q1, wired as a
amplifier with about 20dB of gain. The sole purpose of
this stage is to boost the audio level from a standard electret
microphone (the sort used on computers and headsets)
to a level
that is high enough to be usable on a "line level" input.
Shown on the diagram is microphone that is "built in" to the
unit, but there's also J1 to allow an external microphone to be
used: Through experience I have found that it is best to
include a built-in microphone in the gear just in case one
forgets to bring one along! The output of the microphone
amplifier goes to J2, a "line level" input which allows one to
feed audio from a portable audio player or computer.
Because J2 is a "disconnect-type" jack, the microphone is
disabled when a device is plugged into it. Note the
presence of R5 and R6, which are used to "mix" two audio
channels of a stereo source together: I have found that
simply shorting "Left" and "Right" channels of audio sources
together often results in objectionable distortion, as the two
audio amplifiers used in computers and portable audio players
often "fight" each other!
Automatic Gain Control (AGC):
The audio from the mic preamp and line input go to U1A, a
variable-gain audio amplifier. Using R8, the feedback
resistor, along with R9, R10, R11 and R12, the U2 can adjust the
audio gain of U1A as needed by appropriately shorting the
related pins to ground or leaving them "open" and the resistor
values have been chosen to provide over 13 dB of gain adjustment
in steps of 4dB or less. U2 keeps track of the audio input
level and adjusts the gain as necessary to keep the audio level
driving the PWM generator near maximum. In addition to the
13dB of gain adjustment using U1A, the software in U2 can
automatically provide an additional 12dB of signal boost
internally, providing an overall range of approximately 25dB of overall automatic
gain adjustment capability.
Top: Schematic of the "Mini" PWM LED/laser
Bottom: Interfacing the modulator with LEDs and
laser diode modules.
Click on either image for a larger version.
An AGC circuit can be quite useful as it helps maintain an even
transmitted audio level, even if one is close to or farther from
the microphone. If signals are weak, having a "high" audio level
prevents them from lost in the noise if someone talks too
quietly. The downside of an AGC circuit is that there can
be annoying "pumping" as the background noise can come up during
periods of quiet. Unless one is using headphones and being
somewhat careful, this can also result in audio feedback during
"silent" periods (no talking)
. It is important to note
that having too-little audio is probably worse than having a bit
too much - particularly if signals are weak!
In the "newer" version of this
modulator noted at the top of the page there arebe provisions to completely disable the AGC
and allow the use of a manual gain control. While one
could replace R9-R12 with a potentiometer on this circuit, the
"12dB boost" will still be present and would be very
Lowpass and highpass filters:
U1B, along with R7 and C10, comprise a simple lowpass filter
designed to remove audio above about 3 kHz to prevent aliasing
effects. C9 and R18 form a simple high-pass filter network
that also removes audio content much below 200 Hz to prevent
overmodulation due to the user "popping" the microphone with
consonants or from wind noise.
SW1, the "Mode" switch:
The modulator has two modes: The "Audio" mode which, as
you would expect, transmits audio from the microphone or "line
input" and the "Tone" mode which is used to generate a variety
of tones for alignment and testing. Because the PIC has
only 6 pins that can be used for inputs and outputs (the other
two pins supply power!)
it is necessary to "re-use" at least one
pin - in this case, Pin 7.
When in "Audio" mode, SW1 does two things: It sets a
mid-supply bias (2.5 volts)
on Pin 7 (through R18, using the
voltage divider comprised of R19 and R20)
so that the
analog/digital converter can work properly, and it enables
another mid-supply bias (5 volts)
to provide a reference for
U1A, U1B, and power for the microphone amplifier, Q1.
When in "Tone" mode SW1 re-routes R21, a potentiometer, through
R18 so that instead of audio, a variable voltage (0-5 volts)
be applied to pin 7 to select the tone mode. Additionally,
the 5 volt mid-supply bias is removed from U1A and U1B to
disable those stages as well as removing power from the
microphone amplifier to prevent audio that may be being inputted
from "breaking through" and appearing at pin 7.
In "Tone" mode, sine waves are produced using digital
techniques. In this mode, there are three possible
- When the wiper of R21 is near ground (below 0.5 volts)
a tone of approximately 1 kHz is generated. Having a
very repeatable, constant tone frequency can allow one to
use a computer to detect tones buried in the noise.
- When the wiper of R21 is near +5 Volts (above 4.5 volts) a
distinct tone sequence is generated. These tones
(musical notes C4, A5#, F4# and E6, repeatedly) are intended
to stick out of the noise. Having a sequence of tones
prevents the "ear fatigue" that might result if one is trying to
hear just a single-frequency tone.
- When the wiper of R21 is between 0.5 and 4.5 volts, a
variable-frequency tone is generated, from a few 10's of Hz
to about 2.5 kHz.
- Frequency accuracy: Because the PIC is using
its own, built-in oscillator instead of a crystal (which
would have taken up two pins) the frequencies aren't
extremely precise and can vary by +-2% or so.
- "Audio breakthrough": Note that in "Tone"
mode, even with the bias disabled, it is still possible that
very high audio levels inputted to J2 can disrupt the tone
generator. This can occur if audio "breaks through" U1
and appears at Pin 7 of U2, upsetting the voltage set by R21
causing the tone to change frequency and/or mode. If
this happens, simply disconnect the audio - which might be
from a computer or audio player - from J2!
The as-built prototype:
Top Left: The as-built prototype for the
Simple PWM optical transmitter along with its LED and
secondary lens - see text for a description.
Top Right: To house the transmitter and its
lens, a box was constructed using "foam core" posterboard
held together with thermoset ("hot melt") glue.
Here, several sets of hands hold everything steady while
the glue sets as the box begins to take shape. Here,
Clint (in the blue shirt) is being assisted by Ron, K7RJ
(left) and Dr. Nofziger, WB8SVK (right).
Bottom Left: In the lab, the box being
assembled with the assistance of Elaine, N7BDZ.
Bottom Right: Finishing touches on the box.
Click on an image for a larger version.
shows the modulator, complete with LED and
secondary lens. All of the circuits are mounted on a piece
of foam-core board on a removable "back panel" for the simple
enclosure (also seen in Figure 3)
constructed for our Tucson friends - one similar to the "cheap
In this picture one may see the main board to the right, mounted
to some wooden rails that had been glued to the foam-core board
as a substrate. The small circuit board on the left is the
microphone amplifier (Q1)
and the jacks, switch and
potentiometer may also be clearly seen.
Interfacing LEDs and laser diode modules:
See Figure 2, bottom for information on interfacing
the modulator with LEDs and laser diode modules.
The LED used for the prototype was a Radio Shack #276-020 high
brightness red "power" LED: This LED is also known as the
Liteon P/N: LTL912SEKSA and is capable of a continuous current
of 70 mA.
A 68 ohm resistor was used in series with the 12 volt supply to
limit the peak current to about 170 mA. Since the average
duty cycle of the LED is 50%, this averages out to about 85
mA. In referencing the data sheet for the LED, it was
noted that the "absolute maximum current" rating was 100mA, but
since the copper to which the LED is mounted acts as a heat sink
- and since it isn't expected that the LED will be powered up
for thousands of hours - there should be no real problem.
If you are worried, change the resistor!
Almost any LED may be used and in general, one can select a peak
current of twice the LED's rated average current and be fairly
safe. It should be noted that red (or infrared) LEDs are
recommended over other colors as silicon photodetectors (such as
photodiodes or phototransistors)
are far more sensitive to red
than other colors - not to mention the fact that red/infrared
light is less-affected by atmospheric losses.
If a white LED is used remember that it uses a phosphor to
convert some of the blue light produced internally by the LED to
yellow and the "persistence" of this phosphor will "slow down"
the LED's response. This may cause the frequency response
to suffer noticeably, possibly causing "muffled"
audio. Since silicon photodetectors are far more sensitive
to the "red" end of the spectrum than the "blue" end, it will be
primarily the yellow-red light from slowly-responding phosphor
that the detector would see rather than the fast-responding blue
LED - a fact that could further-degrade the audio response.
Figure 2, bottom
also shows how to interface with a laser
diode module such as one obtained from a laser pointer.
These modules typically contain a current regulator that
establishes a "safe" current for the laser diode's operation and
are designed to operate from a 3 volt source such as a pair of
There are also some "raw" laser diodes - often with lenses -
that are commonly seen at parts supply houses: Sometimes
these do not
come with a regulator board and if
they do not have a built-in regulator, they should not
be connected as shown, without some sort of external
current regulator! If you have one of these modules,you
will need to consult the laser diode's data sheet or a web site
such as Sam's
to find out to to drive these devices
without blowing them up! Again, if you remove the
laser from a battery-operated device such as a pointer, it
will probably have the necessary regulator circuit already
If you wish to use a single "ordinary" low-power LED - that is,
common, epoxy-packaged LEDs, it can be driven to about 25
milliamps using U2 directly. Because the maximum current
of these LEDs (typically in the 25-35 mA range) is a good "fit"
for the output current capability of U2 (also about 25
one can dispense with the extra transistor, using
only a resistor to limit the current to the LED and an example
of how this might be done is shown on the right side of Figure
Note: It is possible to get more drive current
(say, somewhere around 50mA) out of the U2 directly, but this
current level exceeds the official ratings of the chip.
While this probably won't damage the chip, you are on your own
if you exceed the chip's ratings by that much!
Also shown in figure 3
is a "secondary lens." When
using LEDs, this optical component is absolutely
for good transmission efficiency as it
directs the LED's light toward the Fresnel lens and without it,
most of the LED's light would simply be lost, having spread out
over an area larger than the lens itself, "missing" the Fresnel
entirely and not getting radiated! This lens was obtained
and came from one of their
but there are other places through which
lenses may be obtained - see the "Sources" web page
There was nothing special about this lens except that it is was
a fairly "strong" Plano-Convex type: For practical reasons
the lens used here should have a low f/D-ratio
- preferably smaller than 2, and close to 1 is even better,
still! The fact that it was slightly scratched and chipped
was of no real importance in this application, and for mounting,
it was simply epoxied into a hole cut in a piece of glass-epoxy
circuit board material with a hole saw.
To adjust "focus" and lateral alignment, some pieces of #12
copper wire were used to hold the lens mount as seen in figure
: The wire could be bent to precisely align the
lens over the LED and the "focus" onto the back of the Fresnel
lens was accomplished by moving the lens closer and farther away
from the LED, soldering it in place once the final position was
determined. Once soldered, the lens was held quite firmly
in place and would maintain alignment under normal handling
The "final position" of the secondary lens may be determined
empirically provided that one knows two things: The focal
length of the primary (Fresnel)
lens, and the size of that
lens. Simply put, one places a piece of paper at the focal
length distance from the LED and adjusts the spacing of the
secondary lens until the "spot size" of the light from the LED
is the same size as the Fresnel lens. Because the spot is
round and inexpensive "page magnifier" lenses are rectangular,
it is best if one adjusts the size of the spot to be slightly
larger than the "small" dimension of the lens - but slightly
smaller than the "large" dimension. Once the spacing of
the secondary lens to the LED is determined in this manner, one
secures it into place by soldering, as described above and seen
in figure 3
. For information on the "final"
focusing - that is, the determining of the exact distance of the
Fresnel lens from the LED, see the page, Optical
enclosure - first version
on this site, but in
short, this is done simply by moving the nesting boxes in and
out and the securing with screws and glue once the final
position is determined. The paraxial position (left/right,
up and down) may be adjusted by moving the plate on which the
LED transmitter is mounted so that the beam shoots out the the
dead center of the lens.
In practical terms, the preferred lenses are "PCX" -
Plano-Convex - (e.g. flat on the LED side, bulging out toward
or, even better, "PMN" - Positive Meniscus - (the
concave side toward the LED)
over more commonly-seen
lenses due to more-favorable
geometry. Ideally, one would use the "strongest" lens PCX
or PMN lens that you can find so that it is as close to the LED
as possible to intercept the maximum amount of light.
Comment about the secondary lens depicted in Figure 3:
If you look at the upper-left image in Figure
3 (click on the image itself for a larger
version) you'll note that the secondary lens is
spaced some distance away from the LED. If one does the
math, it will be noted that more than half of the light
emitted by the LED actually misses this
secondary lens and spills out, past the secondary lens, around
its edges. With the lens shown, if we where to have
moved it closer to the LED than we did, the "circle of light"
would have been much wider than the Fresnel lens, and even more
light would have been wasted.
What we really need, then, is a secondary lens
that practically (or actually does)
touch the LED itself to avoid this "spillover" - and this
As noted above, a better lens would have been a Plano-Convex
(PCX) or Positive-Meniscus lens with the flat (or concave)
side toward and just (about) touching the LED to "capture" all
of its light - but the trick is to know exactly how "strong"
that lens should be. The general rule-of-thumb is for a
main lens (in our case, the Fresnel) with an f/D ratio on the
order of 1.0-1.25 that the secondary lens's f/D ratio should
also be in the area of 1.0-1.25. The problem is that
finding such a lens can be rather tricky, particularly if you
don't have access to a wide variety of surplus lenses, so one
could make do - albeit with a slight loss of efficiency - as
- A "Stronger" lens (e.g. one with a higher "diopter"
rating or a lower f/D ratio) or
- Multiple lenses
Using "found" (e.g. unknown) lenses and making them work can
be a bit tricky, but you should still be able to get
- Place a smaller, diameter and fairly "strong" PCX
(Plano-Convex) lens or PMN (Positive Meniscus lens, if
you can get it) right against the LED.
- Place a much larger diameter lens between the LED and
first lens and the Fresnel, adjusting that lens' distance
for the appropriate size of the "circle of light" to match
the "long" dimension of the Fresnel.
A "Simpler" Foam Core enclosure using rigid "Page
Magnifier" Fresnel lenses
If you look carefully at the pictures in Figure 3
will see the the processes of the construction of another
optical enclosure using black, foam core board and "hot glue" -
this time, based around "rigid" page magnifier Fresnel
lenses. Similar to the other enclosure described on this
page - See the page Optical
Enclosure - Cheap Version
it uses rigid
lenses rather than the flexible, clear vinyl lenses described on
page was originally written, I did
some testing on the optical qualities of various Fresnel lenses
- and the results may be seen here:
And there you will see that these inexpensive, rigid
page magnifier lenses can actually work admirably in terms of
optical performance, equal to or better than Fresnel lenses that
come from other sources that cost several times as much!
The obvious advantage of these inexpensive, rigid, page
magnifier lenses - aside from being cheap - is that they lend
themselves to being easily built into an inexpensive,
lightweight enclosure using the same foam core posterboard
described in the aforementioned page - and since they are
rigid, a picture frame is not
required to hold
them flat: They will support themselves. What is
required is that they be installed flat and perfectly parallel
to the rays exiting the enclosure - a precaution that is easy to
manage at the time of construction with a bit of care.
Unfortunately, not a lot of photographs exist of the "build"
depicted in Figure 3
, but here are a few details worth
- There are two enclosures: One for transmit and one
for receive, both being identical.
- Each enclosure consists of two pieces: The "front"
piece containing the lens is slightly larger than the rear
piece to which the transmitting gear (e.g. LED) or receiver
attaches, and the rear piece slides into the front piece in
a telescoping manner to allow focusing and to increase
- The Fresnel lens is placed couple of inches (a approx
5 cm) back from the front opening to protect it during
handling to prevent it from being scratched as it is the
"grooved" side that faces outwards.
- For each enclosure, there is a piece of foam core board -
the same size as the lens - that is placed in front of the
lens to protect it from dust and damage and exposure to
direct sunlight. Attached to this piece is a piece of
tape and/or string so that it may be removed once it is
installed. This is a very important
safety measure as even brief exposure of the Fresnel to
sunlight can pose a fire hazard and would instantly
destroy the installed electronics - not to mention the
backside of the enclosure itself!
- The front half of the enclosure is constructed first using
the lens as a form for the front, and a piece of foam core
"blank" which exactly the same size as the lens - which is
later used as the protector - in the back. The top and
bottom pieces are cut slightly oversize so that there are
exterior, overlapping hot-glue joints. Because the
rear section slides into the front section, there are no
glue joints inside the front section.
- Once the front half of the enclosure is constructed,
the rear section is constructed using the front section as a
form so that they fit snugly and perfectly so that they may
slide in and out.It is on the rear section that interior
glue joints are used. Make sure that you do not
glue to the two boxes together: If you are worried
about this, put some paper between the two boxes before
sliding the un-glued pieces into place.
- To add strength and rigidity to the rear section, small
corner "gusset" triangles may be cut and installed, provided
that they do not block the light path.
- For mounting the electronics to the back panel, it is
recommended that either several layers of foam core be
attached to the back side, or some thin paneling/plywood be
used as the back panel and that any holes through which the
electronics protrude be made before attaching
At some point I hope to produce a web page showing the steps in
making a simple, rugged foam core optical transceiver using the
above methods, but if you have any questions, please feel free
to ask them via the link found at the bottom of this page.
"How far will this go?"
The prototype was constructed using the Radio Shack LED
mentioned above and briefly tested over a distance of a bit more
than 7 miles (11km)
At the time we made some audio recordings at the "receive" end
that demonstrate its performance. These were "open mic"
recordings (with some AGC effects of the digital audio recorder
so what you hear are the people on-site (7 miles from
and the voice coming over the speaker via the
LED communications link using THIS
- Recording of the
"Simple PWM" transmitter. (39 seconds,
.MP3 file, 306k)
- 0:00 - 0:15: Aiming the transmitter.
What you are hearing is the dissonant tone sequence
generated by the transmitter to aid in identifying and
- 0:15 - 0:39: Speech via the simple PWM
transmitter over the 7 mile optical path.
As you can hear, signals were excellent over the path.
Since the unit wasn't 100% complete, it was being hand-held
rather than tripod-mounted so signals were varying.
Judging by the audio and the reports there was at least 30dB of
link margin available on the path and even though the LED wasn't
as powerful as the other optical devices used that evening, it
was very conspicuous among the other city lights! As you
can hear from the audio clip, even though it was being captured
by microphone from the receiver's speaker, audio quality is good
and the unit's AGC is well-behaved.
Judging by the the excellent signals at that distance I have
little reason to doubt that the unit built described as above,
with a good-quality receiver (e.g. the "Version 3" mentioned
along with a "page magnifier" type of Fresnel lenses for
both receive and transmit, much longer distances may be easily
attained in clear air: Based in prior experience, I would
guess that distances of well over
25 miles (40km)
should be easily attainable with this LED - probably much
As time permits, I hope to revisit this
circuit and make some enhancements and simplifications.
- On this unit, a simple Plano-Convex "secondary" lens was
used to more-efficiently couple the light from the LED to
the Fresnel. Ideally, a "stronger" lens would have
been used to allow decreasing the distance between the LED
and the secondary lens itself, minimizing losses from the
"side" of the LED's illumination pattern.
Even though it isn't "complete" - that is, I hope to add a
few more features - programmed PICs are
available: If you have interest in this project,
please feel free to contact me using the link below.
Return to the KA7OEI Optical
communications Index page.
- Other things, as I think of them!
If you have questions or comments concerning the
contents of this page, or are interested in this circuit, feel
free to contact me using the information at this URL.
beam, modulated light, optical communications,
through-the-air optical communications, FSO
communications, Free-Space Optical communications,
LED communications, laser communications, LED,
laser, light-emitting diode, lens, fresnel, fresnel
lens, photodiode, photomultiplier, PMT,
phototransistor, laser tube, laser diode, high power
LED, luxeon, cree, phlatlight, lumileds, modulator,
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