There's something rather futuristic about talking 'over' a laser beam, which is what this inexpensive project allows. It will easily give a communication distance of several hundred metres, and
with a parabolic light reflector, up to several kilometres. It transmits high quality audio and the link
is virtually impossible for anyone else to tap into.
by PETER PHILLIPS
ELECTRONICS Australia, July 1997
page 62
In the February 1993 issue, we described a laser beam communicator
project developed by Oatley Electronics. It was an extremely popular project, but
this latest version not only makes the device better and simpler, but cheaper as
well. Unlike the previous version, a visible laser diode (5mW 65Onm) is used as
the transmitter. This makes alignment between the transmitter and receiver
much simpler, as you can now see the beam. As well, the laser has a greater output
power. The circuitry is also simpler, and uses basic components.
As before, there are two sections: the transmitter board and the receiver board,
both powered by a separate 9V battery or a fixed voltage power supply,
depending on your needs. The transmitter board has an electret microphone
module at one end, and the laser diode at the other end. The electronics modulates
the intensity of the laser beam according to the output of the microphone. The
laser diode has an inbuilt collimating lens, and is simply a module that connects
to the transmitter board. The previous design required brackets for the
laser diode assembly.
The receiver uses a photodiode as the receiving element,
and the onboard amplifier powers a small 4-36 ohm speaker. This board is therefore a
high gain amplifier with a basic audio output stage.
But what about results - are they better? Sure. Because this design uses
a higher power (and visible) laser beam, the range is improved, and
alignment is easier and not all that critical, especially over a few hundred
metres. The quality of sound transmit ted by the link is quite surprising.
As a simple test, we set up the prototype with the transmitter microphone
near a radio. The received sound was clear and seemed to cover the full
audio bandwidth. We haven't tried feeding an audio signal directly to the transmitter,
but that will undoubtedly give even better results.
So clearly, this project is ideal for setting up a speech channel between
two areas, say adjacent houses, or offices on opposite sides of the street.
Or you could use it as a link between the work shop and the house. For duplex (two way)
communication, you'll obviously need two laser 'channels'.
An important feature of transmission by laser beam is privacy. Because a
laser beam is intentionally narrow, it's virtually impossible for someone to tap
into the link without you knowing. If someone intercepts the beam, the link
is broken, signalling the interception. Fibre-optic cables also have high
security, as it's very difficult to splice into the cable without breaking
the link. However it's theoretically possible; so for the highest security,
you probably can't beat a line-of-sight laser beam.
You can also use an infrared laser, as in the previous project.
While this gives even better
security, as you can't see the laser beam without special IR sensitive
equipment, it also makes alignment more difficult. (An IR laser diode is
available for the project; see end of article for details.)
The Transmitter fig.1 (above): The circuit for the transmitter. The output of
the microphone is amplified by IC2a, which feeds the modulating transistor Q1,
which varies the laser current according to the signal. The quiescent
current of the laser diode is set by VR1.
The Receiver fig2 (above): The circuit for the receiver, where light from the transmitter
is detected and converted to a voltage by the photodiode. The signal is amplified
by Q1 and IC1, which drives the speaker.
Where the transmission distance is no more than metre of so, a LED (or two
for increased power) can be substituted for the laser diode. For instance,
where the link is being used for educational purposes, such as demonstrating
fibre-optic coupling, or the concept of communication over a light beam.
Obviously the security of the transmission is much lower as LEDs transmit
light in all directions. While this laser link can be adapted for use as a
perimeter protector (as in the previous version), Oatley Electronics
has developed a project especially for this purpose. Contact Oatley
Electronics for further details if that is what you are really after.
Now to a description of how it all works. As you'll see, it's really very
simple. We'll start with the transmitter...
Transmitter
A laser diode needs a certain value of current, called the threshold current,
before it emits laser light. A further increase in this current produces a
greater light output. The relationship between output power and current in a
laser diode is very linear, once the current is above the threshold, giving a
low distortion when the beam is amplitude modulated. For example, the 65Onm
5mW laser diode used in this project has a typical threshold current of
3OmA and produces its full output when the current is raised by approximately
1OmA above the threshold to 4OmA. Further increasing the current will
greatly reduce the life of the laser diode, and exceeding the absolute
maximum of 8OmA will destroy it instantly. Laser diodes are very fragile
and will not survive electrostatic discharges and momentary surges!
However, if used within specifications, the typical life of one of these
lasers is around 20,000 hours. In the transmitter circuit (Fig.1) the
laser diode is supplied via an adjustable constant-current source.
Since the lasing threshold also varies with temperature, a 68ohm NTC
thermistor is included to compensate for changes in ambient temperature.
Note that the metal housing for the laser diode and the lens also
acts as a heatsink. The laser diode should not be powered without the
metal housing in place. The quiescent laser diode current is controlled by
Q2, in turn driven by the buffer stage of 1C2b. The DC voltage as
set by VR2 appears at the base of Q2, which determines the current through
the transistor and therefore the laser diode. Increasing the voltage at VR1
reduces the laser current. The setting of
VR1 determines the quiescent brightness of the laser beam, and therefore the
overall sensitivity of the system.
The audio modulation voltage is applied to the cathode of the laser diode,
which varies the laser current around its set point by around +/-3mA. The modu-
lation voltage is from the emitter of Q 1,
which is an emitter follower stage driven by the audio amplifier stage of
1C2a. Diodes D4 to D7 limit the modulating voltage to +/-2V, while C4 and
C5 block the DC voltages at the emitter of Q 1 and the cathode of the laser diode.
The audio signal is coupled to the laser diode via R10, which limits the maximum possible variation in the
laser diode current to a few milliamps.
LED1 gives an indication of the modulating voltage. Diodes D2, D3 and
resistor R8 limit the current through the LED and enhance the brightness
changes so the modulation is obvious. The LED flickers in sympathy with the
sound received by the microphone, giving an indication that a modulating volt-
age is present.
The inverting amplifier of 1C2a includes a form of compression, in
which the output level is relatively constant and independent of how soft or
loud the audio level is at the microphone. This is achieved by FET Q3 and
its associated circuitry.
The cascaded voltage doubler of C9, D8, D9 and C8 rectifies the audio signal
at the emitter of Ql, and the resulting negative DC voltage is fed to the gate of
Q3. An increase in the audio signal will increase the negative bias to Q3,
increasing its drain-source resistance. Because the gain of 1C2a is determined
by R7 and the series resistance of R5 and Q3, increasing the effective resistance of Q3 will lower the gain.
Since the compression circuit takes time to respond, the clamping network
of D4-D7 is still needed to protect against sudden voltage increases. This
system is rather similar to the compression used in portable tape recorders.
The electret microphone is powered through R1 and is coupled to the
non inverting input of 1C2a via C6. This input is held at a fixed DC
voltage to give a DC output to bias Ql.
The supply voltage to the transmitter circuit is regulated by ICI, a 5V three
terminal regulator.
Receiver
The transmitted signal is picked up by the photo detector diode in the
receiver (shown in Fig.2). The output voltage of this diode is amplified
by the common emitter amplifier around Ql. This amplifier has a gain of
20 or so, and connects via VRI to ICI, an LM386 basic power amplifier IC
with a gain internally set to 20.
This IC can drive a speaker with a resistance as low as four ohms, and
35OmW when the circuit is powered from a 9V supply. Increasing the sup-
ply voltage will increase the output power marginally.
The voltage to the transistor amplifier stage is regulated by ZD I to 5.6V, and
decoupled from the main supply by R2 and C2. Resistor R3 supplies forward
current for the photodiode. (Incidentally, the photodiode used for
this project has a special clear package, so it responds to visible light, and not
just infrared.)
Construction
As the photos show, both the transmitter and the receiver are built on silk-
screened PCBS. As usual fit the resistors, pots and capacitors first, taking
care with the polarity of the electrolytics. IC sockets are not essential,
although servicing is obviously made
easier if they are used. In which case, fit
these next, followed by the transistors,
diodes and the LED.
Take care to use the right diodes for D8 and D9. These are larger than the
1N4148 types, and have two black bands (the cathode end) around a glass
package. Note that the regulator IC has the tab facing outwards.
The photodiode is mounted directly on the receiver PCB. When first
mounted, the active side of the diode (black square inside the package) will
face towards the centre of the board. You then bend the diode over by
almost 180' so the active surface now faces outwards.
The polarised microphone element solders directly to the transmitter PCB.
The negative lead is marked with a minus sign and is the lead that connects
to the metal case.
The laser diode is also polarised, and has three leads. Of these, only two are
used, shown on the circuit as pins 2 (cathode) and 3 (anode). Take care
when soldering the laser in place, as too much heat can destroy it. The diode can
be mounted on the board, or connected with leads to it.
Finally, connect the speaker and 9V battery clips, then check over the boards
for any soldering errors or incorrectly installed components.
Testing
First of all, it's most important that
you don't look directly into the laser
beam. If you do, it could cause perma-
nent eye damage. Also, you are respon-
sible for the safety of others near the
laser, which means you must stop others
from also looking into the beam, and
take all necessary safety steps. This is
covered by legislation.
Both the receiver and the transmitter
can be powered by separate 9V batteries
or suitable DC supplies. Before apply-
ing power to the transmitter PCB, set
VRI to its halfway position, to make
sure the laser current is not excessive.
To be totally sure, you could set VRI
fully anticlockwise, as this setting will
reduce the laser current to zero.
Then apply power to the board. If the
laser doesn't produce light, slowly
adjust VRI clockwise. The laser diode
should emit a beam with an intensity
adjustable with VRI. At this stage, keep
the beam intensity low, but high enough
to clearly see. If you are not getting an
output, check the circuit around IC2b.
You should also find that LED 1
flickers if you run your finger over the
microphone. If so, it indicates that the
amplifier section is working and that
there's a modulation voltage to the
laser diode. You won't see the laser
beam intensity change with the modu-
lating signal.
To check that the system is working,
place the two PCBs on the workbench,
spaced a metre or go apart. You might
need to put a sheet of paper about
2Omm in front of the photodiode to
reduce the intensity of light from the
laser beam. Set the volume control of
the speaker to about halfway. If the
volume control setting is too high
you'll get acoustic feedback.
Move the laser diode assembly so the
beam points at the receiver's photodi-
ode. It's useful to adjust the beam so it's
out of focus at the photodiode, to make
alignment even easier. You should now
be able to hear the speaker reproducing
any audio signal picked up by the
microphone. When the receiver and
transmitter are in close range, the
strength of the beam can cause the
receiver to respond even if the laser
beam is not falling on the photodiode.
Setting up a link
Once you've tested the link, you'll
probably be keen to put it to use. For a
short link of say 100 metres, all you
need do is position the receiver so the
laser beam falls on the photodiode.
Once the link is established, adjust VRI
higher the laser current, the
shorter will be its life.
If you have an ammeter,
connect it to measure the
current taken by the trans-
mitter board. Most of the
current is taken by the laser,
so adjust VRI to give a total
current consumption of no
more than 45mA.
Also, focus the laser so all
of the beam is striking the
photodiode. At close range,
there's probably no need to
focus the beam. In fact,
because of the high output
power (5mW) of the laser
diode, excellent results will
be obtained over reasonably
short distances (20 metres or
so) with rough focusing and
quiescent current adjust-
ments. But the longer the dis-
tance between the transmitter
and the receiver, the more
critical the adjustments.
For example, for distances
over 20 metres, you might
have to put a piece of tube
over the front of the photodi-
ode to limit the ambient light
falling on it. This diode is
responsive to visible light, so
a high ambient light could
cause it to saturate. For very
long distances, say a kilome-
tre, you'll probably need a
parabolic reflector for the
laser beam, to focus it direct-
ly onto the photodiode.
For short ranges (a metre
or so), or for educational or
testing purposes, you can use
a conventional red LED.
Adjust the quiescent current
with VR1. The light output
of a LED is not focused, and
simply spreads everywhere,
so a reflector might help the
sensitivity.
Warnings
The laser diode in this
project is a class 3B laser
and you should attach a
warning label to the trans-
mitter. Labels will be sup-
plied by Oatley Electronics.
Remember that, as for any
hazardous device, the owner
of a laser is responsible for
its proper use.
Resistors
All 1/4W, 5% unless otherwise stated:
Rl 680 ohm
R2 22 ohm
R3 4.7k
R4 39k
R5 3.gk
R6 10k
R7 1 k
R8 220 ohm
Rg 4.7 ohm
VR1 50k trimpot
Capacitors
Cl,2,5,7 10OuF 16V electrolytic
C3,4 1 uF 16V electrolytic
C6 15nF polyester
Semiconductors
Qi BC549 NPN
ici LM386 power amp
ZD1 5.6V 40OmW zener
Miscellaneous
PCB 36mm x 64m; photodiode with clear
casing; 9V battery and battery clip, 4-16 ohm
speaker; 8-pin IC socket.
A kit of parts for this project is available from:
Oatley Electronics
Phone (02) 9584 3563
Postal address (mail orders):
PO Box 89, Oatley NSW 2223.
Prices:
Both PCBS, all on-board components, photodiode, speaker,
battery clips and high intensity LED $29. (aud)
5mW/65Onm visible laser diode and housing $30.
IR laser diode and housing $20 (aud).
Packing and postage charges $6(aud Australian destinations only).
Copyright to this project is retained by Oatley
Electronics.
