From: whit3rd@gmail.com   
      
   On Saturday, May 18, 2019 at 11:51:29 AM UTC-7, AK wrote:   
      
   > Would this work?   
   >   
   > Silicon Photodiode with Daylight Filter   
   > PIN Photodiode Chip, BPW34....   
      
   For a 'laser alarm', i.e. detecting a light beam, the   
   presence of anbient light  (generally white-ish and not modulated)   
   is a guaranteed interfering signal.   
      
   If you use a photodiode (instead of LDR)  your receiver can pick up   
   any frequency from zero (direct current output) to 1 MHz,   
   as opposed to zero to 1 kHz (for CdS, which has a storage time).   
      
   If you start with a laser, you can use a long black-painted tube to   
   aim the receiver in one and only one direction.   Alignment will be fiddly,   
   though.   
      
   Laser or LED light also has a characteristic color, not common in nature,   
   so some kinds of filtering will reject interference while passing the   
   whole intended signal.   
      
   The best ( easiest) way to make it all work, might be to make the laser   
   flicker at some frequency, and  use a LM567 'tone decoder' that can   
   pick up on one-and-only-one frequency of light-brightness-oscillation.   
   Because both the laser and receiver require power, it could be   
   good to package them together, and use retroreflective tape or   
   other backscatter optics at  the 'target'.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)