Thanks for explaining.
Finally, can you tell me why the other scheme (in my first post) uses a 4.7 k
hm: resistor?
Because the transistor requires far less base current to make it operate than does the 'emitter' diode in the opto ic.
It seems to me that the more ohms, the safer for the PC
Safer from what.?
(although, with an optically coupled circuit, like in my case, the dangers aren't that great anyway).
Thats is not quite correct, the opto still requires a resistor in order to limit the current thru the diode, thats drawn from the port pin.
Is this one higher because the transistor (BC547) is more, lets say, sensitive?
Yes
Thanks again! I'll be trying to build the whole thing (for 4 data pins and 4 lights) later today. I'll be trying it on an old PC, so if I make a mistake there's no crying or cursing