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a Laser MCW, NBFM, SSB System
By John, K3PGP 03/18/97
This article will attempt to show how to modify the Lyte
Optronics 670 nm laser light pen for use with CW, FM and SSB.
First off if you haven't ordered your laser pen I suggest you
get busy and order it before this offer disappears! Remember you
get FREE SHIPPING when using this order number!
Lyte Optronics Laser Pointer Pen
Model # CPP2
<5 mw 670 nm
NOTE: These pens typically put out 1 to 2 mw. The power can be increased to as much as 5 mw by using three or four 1.5 volt cells instead of two. However, it you raise the voltage too much you will end up destroying the laser diode. Most pens will tolerate 4.5 to 5 volts very well. Keep in mind though that the difference between 2 and 4 mw is only +3 dB. If may not be worth it. If you do try this I suggest it be done in the first 60 days as the pen will still be under warranty!
Uses 2 AAA batteries (included)
Available from: Damark 1-(800) 827-6767
Quote the following Item # and
get FREE SHIPPING! ($5.99 value!)
Item # B-50051-509141
Price: $24.99 Non members / $22.49 for Damark Members
Remember: Shipping is FREE with the above item number and
Damark has a no questions asked 60 day return policy! So if you
the pen isn't as expected, put it back in the box and send it
back for a complete refund! Of course this assumes you haven't
drilled holes in it and have wires hanging out of it :-^) !
When I got my laser pens they came ready for use with
batteries already installed. I suggest that you experiment with
it for a bit to make sure all is well before starting to proceed
with the modifications.
Although there is nothing wrong with disassembling the laser
pen my goal was to come up with a simple way to modulate the
internal laser diode WITHOUT modifying the pen in any way!
What you will need is a piece of small diameter insulated
wire. I found a piece of copper wire from a 25 pair telephone
cable to be ideal. I believe this is appox. 26 gauge with a
flexible plastic coating. Strip about 3/8 of an inch of the
insulation from both ends of a piece of wire about 6 inches long.
Lay this aside.
Carefully open the laser pen. Do this by holding the back part
(with the clip) and rotating the front part until the unit opens.
When you have it apart place both batteries in the front assembly
if they aren't already there. Carefully stick one end of the
telephone wire into the hole in the back assembly under the clip.
It should be inserted to a depth of about a 1/2 inch (some
insulation (approx. 1/8 inch) sticking into the hole).
Now hold the back assembly and carefully re-insert the front
assembly. When the first battery hits the protruding wire it will
bend backwards and you will feel a bit of resistance as the wire
is forced between the case and the battery holder assembly. Screw
the front end back in by holding the back end and rotating the
front end. Congratulations, you have now added a modulation input
to your laser pen WITHOUT modifying it in anyway. Your 60 day
warranty is still valid! (Just don't tell anyone what you did
'cause they would never believe you even if they did understand!)
Before proceeding test the laser pen by touching the bare end
of the protruding wire to the clip. The laser diode should come
on. If it doesn't you will need to disassemble the unit and
figure out what went wrong. Although I have had no trouble doing
the above with a couple of pens, I suppose that something could
go wrong to prevent the wire from contacting the internal metal
battery holder especially if you are using a different diameter
Once everything appears to be working correctly you can
proceed with the rest of the circuit. You will need an NPN
transistor. The exact type doesn't seem to be critical as it will
be required to carry less than 50 ma. of current. The emitter is
attached to the wire and the collector is attached to the clip
using a small alligator clip. Yes, you could probably solder to
the clip but remember our goal is to NOT modify the laser pen in
Connect a 10 k resistor from base to emitter. Connect one end of a 1 k resistor to the base.
Touch the free end of the 1 k resistor to the collector of the
transistor. The laser diode should come on. If it doesn't short
the transistor from collector to emitter. If this works you know
the wire is still connected to the laser pen correctly and the
problem is with the transistor. Make sure the transistor isn't
connected backwards. As a last resort try another transistor!
The free end of the 1 k resistor is the modulation input and
can be DC coupled to a 50 % duty cycle square wave generator.
This can be a simple 555 timer, a flip flop divider chain, or any
other source of square waves. If your goal is modulated CW set
the frequency of the square wave to an audible rate. Although
most stations using laser communications have been using 800 Hz
as a standard I find that my ears peak at a much lower frequency,
around 575 Hz.
If you want to use the pen for voice work you should set the
frequency of the square wave to around 25 KHz. This isn't at all
critical as the pen will work with frequencies up to approx. 100
KHz but there will be a drop off in performance with the higher
frequencies due to the action of the internal power regulator
circuit in the pen.
The square wave generator should have the capability of being
fm modulated linearly over a range of +- 3 to 5 KHz. There are
many function generator VCO chips on the market that are made
specifically for this purpose but there is nothing wrong with a
simple 555 timer circuit. However, if you are going to use a free
running VCO I suggest that you use a decent capacitor that is
temperature stable. A ceramic disk capacitor is a NO-NO here
unless you want to do remote temperature sensing by reading the
frequency of the 25 KHz subcarrier!
When you apply the square wave to the base of the transistor
the laser diode should come on and look just about as bright as
it did before the modifications. If you are using the lower
frequency for MCW you can verify that the laser diode is being
modulated by rapidly waving the pen back and forth and looking at
the projected beam on the wall or ceiling. You will see that it
is being rapidly turned on and off as indicated by the short
dashed lines! You most likely will have difficulty observing this
effect when using the 25 KHz subcarrier unless you can wave you
hand a lot faster than I can!
RECEIVING THE SIGNAL
Since the purpose of this article is to describe how to modify
the Optronics CPP2 Lyte Pen for use with MCW, FM and SSB I will
not go into detail as to how to build an optical front end. There
have already been many good articles on this in the literature.
If you are using MCW you can simply dump the output of the
optical front end into an audio amplifier and speaker. However,
you will most likely need some help with the FM voice system if
this is your first experience using low frequency subcarriers!
My favorite system which I use to compare all FM demodulation
schemes to is a VLF converter setup feeding a communications
receiver. This setup is tuned to the frequency of the subcarrier
in use, in this case 25 KHz. Although it's very desirable to use
a receiver with a built in NBFM demodulator don't worry about it
if yours doesn't. You can detect NBFM modulation by using slope
detection on most AM receivers. Simply tune off to one side and
you will hear the NBFM modulation. However, using an AM system to
demodulate NBFM is far from ideal and should be used only if
nothing else is available!
A word of caution on using the VLF converter setup. Be careful
to NOT overload the input of the VLF converter. Most of these are
quite sensitive. Many optical front ends can put out several
volts of signal! You may need to attenuate the signal before you
connect it to the VLF converter. You can do this with a simple
pot hooked up just like a volume control between the optical
front end and the VLF converter. Once you find the correct value
you can either use the system like this or remove the pot and
solder in the correct resistors. If your S-Meter on the
communications receiver is against the pin you definitely need an
Once you have this going I'm sure you will want to make a
portable setup so you don't have to drag the VLF setup around
with you! I have had very good results using a single stage
op-amp filter feeding a CD4046 PLL used as an NBFM demodulator.
If you go this route avoid the temptation to beef up the filter
ahead of the PLL. The system will actually work extremely well
without the filter. If you make the filter too sharp the entire
system will suffer or fail to work at all. The filter should only
be down approx. -3db at 20 and 30 KHz to properly pass the NBFM
signal to the PLL. This doesn't require much in the way of a
filter and for most laser QSOs you won't notice any difference
with it in or out since the PLL itself is a fairly good filter
when used as an NBFM demodulator. However, weak signal
performance is slightly better with the op-amp filter ahead of
It is also possible to upconvert like the above VLF converter
system. For this setup I like to use a double balanced mixer with
NO RF stage. You can either buy one of these already made up or
make one out of 4 diodes and a couple of toroids. I'm using two
junk box toroids and four 1N914 diodes. The choice of a local
oscillator depends on the receiver that you want to use. Some
nice choices are 3.5 MHz or 7 MHz when using HF equipment,
100.000 MHz when using an R-7000 general coverage 25 to 2000 MHz
all mode receiver, (If using 100.000 MHz you will most likely
have to shield everything to keep out strong FM stations!) or
144.000 MHz when using an FM handheld. In the later case you can
use a 48.000 MHz crystal into a tripler to get 144.000. (I chose
144.000 MHz to keep strong FM repeaters out of the system! This
is extremely nice for mountaintopping as you can always come up
with a two meter handheld! To use this with the laser NBFM system
above you would tune to 144.025 MHz.
It is also possible to use the above setup on SSB! All that is
required is to bias the NPN transistor on the laser pen so that
the beam is approx. half brilliance. Unfortunately determining
where this is isn't that easy but in actual practice it isn't
that critical. You can NOT do this by measuring the current into
the laser pen. If you do you will run into an effect called laser
threshold. This is a very sharp knee where the diode changes
characteristics from an expensive LED into a laser diode. In the
case of the CPP2 laser pen this occurs around 23 to 25 ma. Below
this critical value you get no laser action. Much above this and
you stand a good chance of wiping out the laser diode. Yes,
unfortunately laser diodes live right on the edge of self
destruction! You won't have to worry about over current with this
circuit though because the internal power regulator in the pen
will prevent this.
What you need to do is connect a third resistor from base to
collector of the NPN transistor. Adjust it's value till the laser
appears to dim down a bit. With proper test equipment you will be
able to set this to optimum. I won't go into details here since
it isn't that critical when getting the system up and running.
The SSB signal is applied to the free end of the 1 K resistor
coming from the base of the NPN transistor through a coupling
capacitor. The actual value isn't critical just so it is large
enough to pass the subcarrier frequency. A low voltage 0.1 mfd is
fine for 25 KHz but you might want to make it a bit bigger if
using baseband voice or MCW. In that case I would suggest
something like 10 mfd.
When modified as above the laser pen can be modulated with any
type of signal including sinewaves, baseband voice and CW and
since it is linear it can handle more than one subcarrier
simultaneously! To put SSB on it you will need as source of VLF
SSB at approx. 25 KHz. Remember the above description of the
double balanced mixer? Well if you turn it around and transmit
into it (at very low power of course < 1 milliwatt !) you will
DOWNCONVERT to 25 KHz! Feed this downconverted signal into the
base of the NPN transistor and you're now on laser SSB! You will
have to optimize the bias on the transistor and set the level of
the 25 KHz SSB signal feeding the base. About the same procedure
as setting up a linear amplifier!
For receiving you will need an upconverter and SSB receiver.
The SAME double balanced mixer can be used for both transmit and
receive if using a transceiver! If you have a separate receiver
and transmitter you can either use relays to switch the system or
you might want to think about two double balanced mixers, one for
transmit and one for receive using a common local oscillator
I was fortunate to come up with a pair of two meter multi-mode
handhelds many years ago. These are very nice since one radio can
be used for NBFM, SSB, and direct two meter contact.
In one experimental QSO I had three signals on the laser pen
simultaneously. At 25 KHz I had the NBFM subcarrier system. At 67
KHz we were talking on SSB. On the baseband feeding directly into
the NPN transistor) we had a couple of 28.8 kb/s modems running
in leased line mode doing file transfers via computer!
The input to the NPN transistor is extremely sensitive when
biased in a linear fashion. It is quite easy to detect signals of
less than 1 microvolt. With that in mind, during one memorable
QSO I had the distant station pull all the subcarriers off the
system and substitute a long wire antenna. On the receive end I
was able to hear on the air stations from VLF through the
short-wave broadcast band at 9 MHz! This test was done using a
laser diode transmitter with a power regulator circuit that does
NOT exhibit the roll off at 100 KHz that the CPP2 pen does.
Unfortunately the optical front end that I was using rolls off
around 10 MHz so I don't know what if anything was being
transmitted above that frequency.
Keep in mind that the CPP2 pen does roll off quite severely as
the frequency goes up so if you have interest in using higher
subcarrier frequencies you will have to completely tear the pen
apart and rebuild the internal power regulator. In my mind you
would be better off to leave the CPP2 laser pen system alone and
pick up a couple of IR or 670 nm laser diodes at the next hamfest
and start from scratch! I'm working on a cheap source of IR laser
diodes and detectors so stay tuned!
So there you have it. A $25 laser pen system capable of transmitting MCW, NBFM and SSB!
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