damn damn damn,
if i understand it well then i'm in big big trouble since the optocoupler is part of an already printed PCB as shown in the figure below. the ground of the 5V is already connected to the ground of the input led of the 4N33 as you also see on the figure.
the story you told is just what i was affraid of... when i connect the pin2 of 4N33 to an output of the ULN, the ground of the PCB (where the 4N33 is part of) isn't a ground anymore, so it will disturb also the 5V output of the 4N33... am i right with my suspicions?
i only can test it out friday, but if this is true it is a realy disaster ! :?
is there another solution? can you help me please?
the PCB of the ULN isn't ordered yet, so maybe i can fix something on that side to let the PCB of the 4N33 work correctly this way???
any suggestions???
(since there are already 250 PCB's of that 4N33 delivered (a lot of money), i hope you understand how big my problem is).