This is my first time positing to the forums and I am simply trying to use the 4N25 optoisolator to electrically isolate a DC voltage so it can be sensed by a microprocessor on the output side. Here is the basic circuit I am using: This is the same circuit I've seen on other websites describing how to use the 4N25. I have varied the R1 and R2 values from 100 Ohms up to 10 KOhms and varied the input V1 from 0 to 10 V but the output either remains at some constant value in the mV or it is equal to V2. However, it never is equal to V1 which is what it should do. Somebody please let me know what I am doing wrong.