LASER BEAM
COMMUNICATOR
Ramsey Electronics Model No. LBC6K
• Uses a standard pen-laser diode to transmit voice and sound over
several hundred feet through the air, and several miles with good
optical fiber.
• Uses a pulse-width modulator running at 20kHz for good audio
reproduction.
• Built-in AGC for sensitivity to low level sounds and good
transmission of high level sounds without distortion.
• Pen laser included! Use even better laser modules for farther
transmission.
• A lot of fun to transmit to your friend’s house through the window!
• Powered by any 9 - 12 VDC source.
• Complete and informative instructions guide you to a kit that works
the first time, every time.
Have you ever wanted to communicate in a new and interesting
way? Forget stringing wires and tin cans between your house
and your neighbor’s! Now you can send messages on a beam
of light! Pretend you’re on one of those space adventure shows
we’ve all watched on TV. Laser beam communication for the
new millenium!
PARTIAL LIST OF AVAILABLE KITS:
RAMSEY TRANSMITTER KITS
• FM10A, FM25B FM Stereo Transmitters
• FM100 Super Pro FM Transmitter
• AM1, AM25 Transmitter
RAMSEY RECEIVER KITS
• FR1 FM Broadcast Receiver
• AR1 Aircraft Band Receiver
• SR2 Shortwave Receiver
• AA7 Active Antenna
• SC1 Shortwave Converter
RAMSEY HOBBY KITS
• SG7 Personal Speed Radar
• SS70A Speech Scrambler/Descrambler
• TT1 Telephone Recorder
•TG2 DTMF Tone Grabber
• SP1 Speakerphone
• MD3 Microwave Motion Detector
• PH14 Peak hold Meter
• LC1 Inductance-Capacitance Meter
RAMSEY AMATEUR RADIO KITS
• HR Series HF All Mode Receivers
• QRP Series HF CW Transmitters
• CW7 CW Keyer
• PA Series VHF and UHF Power Amplifiers
• QRP Power Amplifiers
RAMSEY MINI-KITS
Many other kits are available for hobby, school, scouts and just plain FUN. New
kits are always under development. Write or call for our free Ramsey catalog.
LBC6K LIGHT BEAM COMMUNICATOR INSTRUCTION MANUAL
Ramsey Electronics publication No. MLBC6K Rev. 1.2
December 2002
COPYRIGHT 2002 by Ramsey Electronics, Inc. 590 Fishers Station Drive, Victor, New York
14564. All rights reserved. No portion of this publication may be copied or duplicated without the
written permission of Ramsey Electronics, Inc. Printed in the United States of America.
LBC6K • 3
LASER BEAM
COMMUNICATOR KIT
Ramsey Publication No. LBC6K
Manual Price Only $5.00
TABLE OF CONTENTS
Introduction .............................. ...............4
Parts Layout Diagram .............................9
Parts List ...............................................10
Assembly Instructions ...........................12
Using the LB6K .....................................18
Troubleshooting .............................. ......20
Receiver Board Schematic ...................23
Transmitter Board Schematic ...............24
Ramsey Kit Warranty ............................27
KIT ASSEMBLY
AND INSTRUCTION MANUAL FOR
LBC6K • 4
INTRODUCTION
You and your friend have a pair of radios you use to talk to each other, but
did you notice that sometimes people listen in when you don’t want them to?
How can you have a conversation that nobody can listen to but you and your
friend? Well you could encrypt the audio of the radio and decode it, but that
is an expensive option, and if someone has a decoder they can listen too.
But if we send the audio over a laser beam, the only people who can hear
your conversation are people who can see the laser beam itself. You don’t
even have to encrypt the audio!
Ever wonder how sound is sent over a beam of light? There are several ways
to do it. One is to AM modulate the light beam. This means that we will vary
the brightness of the beam along with the audio, and the detector will give us
an output according to brightness. This is probably the simplest way, but
can’t really be done very well since many lasers don’t have a very broad
range of AM. This means that if the audio level is too low, the laser turns off!
This is also prone to interference from other sources that vary in brightness
like fluorescent lights which turn on and off at 60Hz.
A laser module is better used for turning completely on and off rather than
trying to vary its brightness slightly. Solid-state lasers perform better this way
than with AM modulation. The detector then will see the on and off state very
easily. If we turn a laser on and off at a known rate, usually very quickly, we
can ignore all other possible interfering signals on the receiver like
fluorescent lights. For example our LBC6K is turning the laser on and off at a
rate of 18kHz. If we just look at 18kHz, and filter out all other frequencies, we
filter out all other potential interfering light sources too.
So how do we get audio out of an 18kHz signal which is simply an on and off
signal? Well we could encode the data in a data stream, much like what a
modem does, meaning we would have to decode it digitally on the other end.
Otherwise we could simply pulse width modulate the 18kHz signal, and use a
low-pass filter to demodulate the audio.
Pulse width modulation (PWM); what the heck is that? Well, it is exactly what
it sounds like. If you have an 18kHz square wave you would normally have a
duty cycle of 50%. This means the on time is exactly that of the off time per
cycle. PWM means that you are varying this duty cycle according to the data
you wish to transmit; in our case it is audio. A high signal level would have a
longer on time vs. off time, and a quiet signal would have shorter on time vs.
off time. The total time per cycle always remains the same.
See the following diagram and you will see the waveforms in the LBC6K. The
top signal is the 18kHz clock. The middle signal is the clock converted to a
LBC6K • 5
PWM signal which has an encoded signal in it. The bottom signal is the
decoded PWM of the middle PWM signal. Note how when there is a lot of on
time (high) on the PWM signal, the resulting decoded signal is higher, and
when there is a lot of off time (low) the signal is lower.
The receiver module of your LBC6K is pretty simple. It consists of a very
sharp low-pass filter at 6kHz and an audio amplifier. The filter is sharp to
reject the 18kHz PWM clock, but let the audio encoded from 6kHz on down
through. 6kHz and below is perfect for transmitting voice signals.
Below you can see the various stages of filtering in the receiver. The top
trace is the unfiltered received PWM signal right at the collector of Q1 (the
signal is small, but this is what it looks like). The middle trace is after the first
stage of filtering, which is close to what you would see on pin 14 of U1.
Notice how even through it has the 18kHz signal on it, it more closely
resembles the final filtered output of the bottom trace, which is seen at pin 7
of U1.
The receiver is a bit more complex and many of the more complex parts are
contained in the microcontroller. The microcontroller (U3) performs the AGC
LBC6K • 6
for you, and also digitizes the incoming audio from the microphone with an
internal analog to digital converter and converts it to our 18kHz PWM signal.
Before we convert the audio from the microphone however, we have to first
boost the microphone’s small output of only a few milivolts up to a usable
level of a few volts. This is done with U1A, a high gain stage. This stage has
a gain of 100, so a small signal of only 100uV will be amplified to a signal of
10mV. The next two stages are low pass filtering. If you compare this filter
with the one on the receiver you will see that they are the same. It is a 6kHz
sharp low-pass filter.
Why do we low-pass filter the audio before digitizing it? It is to prevent
signals more than 1/2 of the sampling rate from being digitized and then
transposed when they are received. While this may be what you want for a
voice scrambler, it is not what we want for transmitting audio. This magical
1/2 of the sampling rate is called the Nyquist frequency. Any incoming
frequency above this 1/2 point is not decoded as expected. For example, if a
10kHz signal came in and was digitized at a rate of 18kHz, we would actually
get a 1kHz signal at the output of the receiver instead of the intended 10kHz.
You can figure this out by realizing that with a Nyquist frequency of 9kHz
(our sampling rate is 18kHz), anything over the Nyquist will be seen as the
(incoming frequency - Nyquist frequency). So 10kHz - 9kHz = 1kHz.
So this may be more information than you really wanted to read, but the
Nyquist frequency is very important in many aspects of digital sound. When
you are playing MP3s on your computer, they are usually listed as having a
certain sampling rate, usually 128kHz. This means that the best frequency
response of the audio clip is 64kHz, which is pretty good. The practical
reproduction is actually more like 80% of the Nyquist. You may find some
audio files which are sampled at 32kHz, which means the highest they can
reproduce is 16kHz, this means that the 80% mark is 12kHz, which for
decent music reproduction is pretty poor. Since we are only reproducing
voice we can stay down at a sampling rate of 18kHz, so the highest we can
reproduce is 9kHz, and we filter down to 6kHz to hit that 80% mark for
decent voice audio.
Ok, so now we have filtered audio ready to sample. The microcontroller
samples the filtered audio at a rate of 18kHz, but how is it performing the
automatic gain control (AGC)? It does this by looking at the largest values of
the incoming samples, and turning pins 11, 12, and 13 into high impedance
(off) or low impedance (on) to vary the gain of U1:B, which is set up to be a
non-inverting opamp with variable gain. The gain is found by the simple
formula of Av = 1 + Rf/Ri. Rf is R20, which is a 100k reistor, and Ri can be
any combination of R17, 18, 19, and 24. With all three pins on, R18, 19, 24,
LBC6K • 7
and 17 are all in parallel, making U1:B have a gain of (Av = 1+ 5.6k/100k) or
17.6. When all three pins are off, only R17 is in circuit making the gain (Av = 1
+ 100K/100K) or 2. This gives a decent range of amplification for small to
large signals, and is controlled by the microcontroller.
The AGC of the microcontroller tries to keep the signal within the range of the
analog to digital converter so that we have the best possible data
reproduction, so the gain is always changing on the last stage to keep it there.
Once the sample is taken in the microcontroller, it is tested for level and the
AGC may or may not be adjusted. It is then converted to a PWM pulse and
sent to pin 9, where Q1 is used to turn the laser module on and off for the
single sample/cycle.
Now the laser beam is sent to the receiver where it is detected and converted
back to an audio signal where you can listen to it on a speaker or earphones!
We know this may be a bit confusing, but there are a lot of principles of digital
data and reproduction packed into this one small kit. If you are interested in
learning more, there are a lot of resources on the internet and in your local
library.
RAMSEY Learn-As-You-Build KIT ASSEMBLY
There are numerous solder connections on the LBC6K printed circuit board.
Therefore, PLEASE take us seriously when we say that good soldering is
essential to the proper operation of your transmitter!
• Use a 25-watt soldering pencil with a clean, sharp tip.
• Use only rosin-core solder intended for electronics use.
• Use bright lighting, a magnifying lamp or bench-style magnifier may
be helpful.
• Do your work in stages, taking breaks to check your work. Carefully
brush away wire cuttings so they don't lodge between solder
connections.
We have a two-fold "strategy" for the order of the following kit assembly steps.
First, we install parts in physical relationship to each other, so there's minimal
chance of inserting wires into wrong holes. Second, whenever possible, we
install in an order that fits our "Learn-As-You Build" Kit building philosophy.
This entails describing the circuit that you are building instead of just blindly
installing components. We hope that this will not only make assembly of our
kits easier, but help you to understand the circuit you’re constructing.
LBC6K • 8
For each part, our word "Install" always means these steps:
1. Pick the correct part value to start with.
2. Insert it into the correct PC board location.
3. Orient it correctly, follow the PC board drawing and the written
directions for all parts - especially when there's a right way
and a wrong way to solder it in. (Diode bands, electrolytic
capacitor polarity, transistor shapes, dotted or notched ends
of IC's, and so forth.)
4. Solder all connections unless directed otherwise. Use enough heat
and solder flow for clean, shiny, completed connections.
SINGLE SIDED COMPONENT SOLDERING INSTRUCTIONS:
You’ll notice that the circuit board contains plating on only one side. This
makes soldering relatively easy for even the inexperienced kit builder. Just
take your time and be sure to apply enough heat to the connections. Don’t be
too afraid of overheating a component; most are fairly hardy and a weak
connection will prevent your kit from working properly.
In all RF kits it is a good idea to keep the components as close to the board as
you can and trim off the excess lengths of the component legs. However, in
this kit the highest frequency on the board is around 4MHz, so this isn’t as
critical as it would be in a higher frequency circuit.
LBC6K • 9
LBC6K PARTS LAYOUT DIAGRAM
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