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Building 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 wire.

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 any way!

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 attentuator!

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 the PLL.

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.

LASER SSB

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 feeding both.

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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