buffer_overfly:
Chagrin:
There's no need to connect the Arduino GND to the GND of the moduleAgree
Chagrin:
...and doing so will negate the optical isolation.This is what I don't really understand. Why would it negate the isolation? AFAIK when a second source is provided, the Arduino's Vcc is only used for the octocoupler, so why would it hurt having a common ground?
Well it depends on one's definition and requirement for 'isolation', as the relay contacts are already isolate the controlled circuit from the arduino circuit. The optoisolator only effectively if you both don't connect the grounds and you don't use the arduino's Vcc to power the relay coils. I never kind understood this common Asian relay module design because of that issue. If you are using a higher then 5vdc relay coils then the optoisolator kind of makes sense, but really it's the coil to contact isolation that is the real deal with relay applications.