[Laser] first time, simple system, but problem

[F1AVYopto at aol.com]

Dans un e-mail daté du 30/08/2006 01:57:42 Paris, Madrid (heure d'été),  
hamfreak2002 at gmail.com a écrit :

hey all,  KB1LQD here, built the simple RX'er out of a 741 op amp, and a 386
amp with  a photodiode. It works, but there is a terrible occilation when the
10k pot  is at a volume point of anything decent, also, it seems like a
signal is  there or not there at all, (No background noise), any tips? the
only long  leads are the leads to the photo diode in the PVC pipe with a
lens. Also,  any tips on making an aiming system for the RX and TX, Also, how
do I make  sure the lens is perfectly in posistion with the photo  diode?




The  oscillation could come from a decoupling problem and a bad  shielding. 
Put a big  capacitor on the DC + line and feed the photodiode biasing circuit 
with an  additional RC decoupling cell. 1K resistor and a 100 MF capacitor 
could be  correct. 
For the good  photodiode position, replace it by a piece of paper and look 
the focusing point  by aiming on a far away powerful light source. 
Al the  circuit and the photodiode must be in a metalic box connected to the 
negative  supply wire. 
Yves