[Laser] 5 mw laser transceiver kit

[Tim Toast]

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