Let's say I have a CMOS (5V) output connected through a resistor to the base pin of a transistor, working as a switch. And collector and emitter pins are working with 12V.

This works but, should I use an optocoupler here?

 

should I use an optocoupler here?

No there is no need for one, The transistor and base resistor provide isolation between the 5V and the 12V.

 

I wouldn't.. Only time I use an optocouplers is if I'm going to be triggering a circuit from something far away like a sensor 50 or more feet from the circuit or in switching high voltage (AC line).

 

No opto is needed. But make sure the base current into your transistor is withing the capability of your CMOS gate. If not, you might need a darlington transistor.

 

Thank you all!

 

I wouldn't.. Only time I use an optocouplers is if I'm going to be triggering a circuit from something far away like a sensor 50 or more feet from the circuit or in switching high voltage (AC line).

This is very interesting.
Are you using the coupler to "drive" a long wire?
Can they "soup up" the output of a standard CMOS/TTL chip?

 

This is very interesting.
Are you using the coupler to "drive" a long wire?
Can they "soup up" the output of a standard CMOS/TTL chip?

Other way around.. Usually have a 12 volt signal feeding the optoisolator from some distance. This isolates the electronics from the possibility of surges on the cable and adds a little noise immunity.

 

Other way around.. Usually have a 12 volt signal feeding the optoisolator from some distance. This isolates the electronics from the possibility of surges on the cable and adds a little noise immunity.

Ok, so it's not a "driver" but rather a pure "isolator".
Thanks!