enen92:
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An SSR is a "special kind" of octocoupler, so it has a LED. Does this mean that if the main circuit is on (switched from the arduino or from the mechanical switch) the SSR led will always be on and that the value I get from the arduino digital pin is always HIGH? Or on the other I can only turn the SSR led on from the low DC power side?
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I think I got what you concerned. It is perfect point.
The led of SSR has life span. if SSR is on most of time, your idea will help a lot for extend led life span. and implementation is easy and just inverter opto-isolator. However at practical is no go, the deal stopper is low DC power side power failure will cause high AC side is on. It is not acceptable.