I have used Red and Green LED’s. When node A is positive green LED glows while when node B is positive, Red LED glows. This indicates polarity of the input d.c voltage . 2) LED as a Continuity tester: ...

Optocouplers draw power from the isolated circuit, and switching can be relatively slow and uncertain because of LED aging. Substitutes without optocouplers, such as the ADUM12xx from Analog Devices ...