[Laser] Reasonable Sensitivity Checks ?

[TWOSIG at aol.com]

>My goals at this point aren't quite clear....I need a
>bit of experience first before being able to generate
>specs  !  But I would like primarily to have something
>suitable for ham  radio (ARRL) contests (must use a
>laser transmitter and must work both at  night and in
>daytime and must be capable of > 1 km  communication,
>preferably including during the daytime).  There  would
>be some advantage here for design choices that provide
>ease  of use and quick pointing.  On the other hand it
>might be  interesting to try for distance too....which
>has rather different  requirements.
>
>Steve VE3SMA
 
Please let us know how you progress.  I am particularly interested in  how 
you fare in "ease of use and quick pointing".  
 
I have been giving demonstrations at kid's events with two Ramsey  
transmitter/receiver kits at the range across a gymnasium or similar distance  outdoors. 
 The claimed performance is 1/4 mile.  I have gotten more  than half of that 
with full quieting.  The receiver sensor is a 5 mm photo  transistor with a 1M 
collector resistor followed by a gain stage and audio band  pass filter, then 
an audio amp with volume control.  You could probably get  a similar photo 
transistor at Radio Shack if you wanted to build a  reference detector.  It will 
not compete with some of the other detectors  described on this list but it 
comes close to the range of 1 KM without any  external optical collector when 
the transmitter is a cheap "bullet" laser  pointer, pulse width modulated at 
about 18 KHz with voice audio limited to 6  KHz.
 
The biggest problem that I have is that even a tiny jiggle of the  
transmitter takes the beam off the receive sensor.  Indoors I use a  lens to expand the 
beam to 1 to 3 foot diameter depending on range and light  level.  Outdoors 
and in daylight the system cannot handle the  expanding lens, but I am guessing 
that the sensor is getting way too much  background light, especially IR.  ( 
There seems no filtering of IR and the  sensitivity peaks in the IR spectrum.)
 
Hope this information is useful.
 
 
James
N5GUI