[Laser] 5mw laser tranceiver kit

[Tim Toast]

Hi all

I drew this schematic up tonight just for the heck of it. It has 
not been built yet so i have no idea how well, or even if, it 
works. I thought it would make a good weekend project sometime to 
find out. It is the push-to-talk type :D i know, i know... full 
duplex is where it's at, no question. But here's something super 
simple just as an example of one starting place among many. I'm 
sure this combines several of the bad ideas from the past to avoid,
into one tiny gadjet. A good transmitter might be relatively simple,
but a good receiver is sure to be more complicated.

http://www.aladal.net/toast/5mwkit1.jpg

It's based on a simple 1.5 volt hearing-aid amplifier i found 
online. The parts values shown are for the original hearing aid 
circuit except the battery which was changed to 3 volts. Something 
may need to be adjusted first to get normal hearing-aid operation 
back due to the voltage change, but i assume it would make it have 
a little more gain if anything.. maybe not. I left the transistor 
part numbers out but i have them if anyone wants them. I assume the
best transistors to use would be something besides what they have 
used originally anyway. It's possible they are the older germanium 
type instead of silicon - due to the extremely low voltage. Maybe 
that won't matter when using 3 or more volts. The microphone is an
electret type.
As a hearing-aid, i suspect there is a certain amount of current 
flowing all the time through the earphone and the audio makes it
alternate up and down. If this current can be adjusted for a 
sutable resting current to flow through the laser diode, it could 
be biased at a proper level for relatively clean AM. Hopefully 
this would also be compatable with the earphone in receive mode 
without too much trouble. The trouble with this is there's a big 
chance of interaction between the transmitter and receiver 
adjustments.

If this sort of thing can work well enough, it's better to just 
keep the transmitter and receiver completely separate with their 
own circuits. This would pretty much double the parts count, but
provide full duplex and ease optimizations and modifications etc..


tim toast