[Laser] 5 mw laser transceiver kit

[n5gui at cox.net]

An interesting suggestion Tim.

I started this reply comment to suggest that rather than trying to dismantle a camera for lenses, one would have more predictable results and probably far less work to order "educational" lenses from Surplus Shed.  They have a series of various focal length double convex lenses in 50 and 38 mm diameter.  The 50 mm lenses are currently available for $1.50 each or 10 for $10. These are not commercial quality, but certainly good enough for what you have suggested.  Available focal lengths are 50, 75, 150,  200, 250, 300, 500, 667, and 1000 mm.

The 50 mm lenses fit nicely inside 2" schedule 40 PVC plastic pipe, although for a receiver system it might be better to consider the black equivalent.  To actually mount 50 mm lenses in 2" plastic pipe, I use a "test cap" as a lens holder.  To hold the lens I cut a groove along the inside edge using a drill press and rotary file.  An alternative that only requires simple hand tools is a pair of rings cut from Sched 20 ( thin wall ) PVC.  The outer ring can act as a spring to hold the lens in place, so that different lenses can be exchanged.  For permanent mount, glue both rings.  


I should also point out that the Ramsey Electronics Laser Beam Communicator kit is still available, although the price is now nearly $55.  For those not familiar with the LBC6K, it has separate receiver and transmitter boards, each of which will fit inside 2" plastic pipe.  I built a pair of them about a decade ago, but the kit seems to not have changed, other than price.  I was disappointed that the instructions did not suggest any way of mounting the boards, other than to say they would fit in 2" plastic pipe.  The transmit circuit uses a quad op amp and microprocessor to provide an amplifier for the electret microphone, band pass audio filter 150 to 6,000 Hz, automatic level control, and pulse width modulator to a FET driver for the "bullet style" pocket laser pointer.  The receiver uses a photo transistor, similar audio amp and band pass filter feeding an IC audio amp for earphones or small speaker.  It works.  It also has a lot of room for experiementing and improvement - mechanically, electronically, and optically.  As for the price, compare commercial kit suppliers of UHF or QRP amateur radio equipment to the group buys of projects sponsored by various clubs.

The LBC6K has an advertised range of 1/4 mile.  The phototransistor it uses has a 5 mm clear plastic housing, so estimate the performance using that as the sensor size.  A 50 mm lens is 10X the diameter, therefore 100X the area.  If the laser pointer beam's spot expands to 10X the diameter, it will have 100X the area, hence 0.01 the brightness.  It is a fair assumption that beyond 1/4 mile the beam expands linearly, so at 2.5 miles, the beam should be expanded 10X diameter.  The beam will also weaken due to atmospheric absorbtion in a linear fashion, but that may not be important for this example.  This only shows what I mean by a lot of room for improvement.


I would like to see what sort of ideas come up.


James
 n5gui





---- Tim Toast <toasty256 at yahoo.com> wrote: 
> Hi All
> 
> It's great to see someone has put a kit together. That is sure to 
> spark some interest in optical coms. In the interest of a little 
> fun competition, it would be nice to have a simple kit for laser 
> com's too. One obvious advantage (or not so obvious) for laser 
> over LED, is size and power consumption. A simple laser transceiver
> could be made very small and portable, even pocket-sized and 
> battery powered. Under 90 ma. for most 5 mw diodes.
> 
> It would help if everyone could start with the same size, easy to
> obtain, enclosure. One that comes to mind is the old "Tuna Tin 2" 
> kits. You could actually use a tuna can (Tuna Tin THZ or 
> Tetrazzini?) but someone suggested to me to use one of those 
> "Altoids" candy tins. They have an easy access door and complete
> sheilding, plus plenty of room for a circuit board and batteries
> inside. These are the large 5" x 3" x 1" inch size boxes. They 
> also come in a couple smaller sizes and even round ones, all with
> resealable metal lids or doors.
> 
> I was looking at some amplifier circuits today and saw this high 
> gain hearing-aid design that uses 3 transistors and one battery 
> (1.5v). If you switch the microphone with a photo detector front 
> end, you have a high-gain receiver plus part of the transmitter 
> too. A laser transceiver could be made very small and energy 
> efficient using a low voltage like 3 volts (2 cells). A 3, 6 or 
> 9 volt design for the small light-weight battery sizes. With a 
> dual 555 timer chip, you can have a simple high quality PWM AM 
> for the laser and a tone generator for MCW or alignment. The 
> goals being low cost, light wieght, easy to put together etc..
> 
> Relatively cheap optics that work well:
> If you are willing to take apart some junk equipment you can find 
> lots of optical goodies. The old VHS Camcorders can supply most of 
> the optical parts. An old DVD player will get you a 5 milliwat red
> laser diode (these are often dual wavelength visible/IR diodes with
> three pins, or four pins if it has a monitor diode) and some small
> lenses to use. These even have one lens you can use to collimate 
> the laser fairly well (the laser pickup "output" lens is a 5mm or 
> 6mm f0.9 or faster type) or it can adapt the beam for use with a 
> larger lens. The main large glass lens on the camcorder is usually 
> a 25 to 50mm f1.0 type when used by itself. It can work for either
> transmitter or receiver. Some of the older ones have a fine 
> adjustment twist focus. 
> For short range use, it might be nice to be able to defocus the
> transmitter and receiver some - up to 10 degrees or more maybe - 
> these twist focusing lenses would be great for that. 
> Inside the camera zoom lens are a few smaller glass lenses in both 
> positive and negative types plus a special "eyepeice" type assembly 
> - similar to a 15 - 20mm Kellner telescope eyepiece. This could 
> make a nice finder when paired with a larger objective lens and a 
> cross-hair. The video viewfinder eyepiece is also adjustable and 
> could be used for either one or a finder. It is usually a 20mm F1 
> or F2 plano convex plastic lens. 
> The performance won't be excellent obviously with these small 
> lenses but should work for the shorter ranges ok. (under 50 miles 
> or so?? :) you can always break out the big optics for better 
> performance.
> 
> 
> tim toast