Phew!!!

Was never expecting so much well-thought answers on my question. Thanks all of you for your useful input (some even simulated a proposed circuit, which is really great...).

Sorry for asking, but what is a 'wallwart'? That's an English word I don't know (yet???)... :-(

The problem in my specific situation is, that I have to 'observe' some 40 contactors (or whatever you guys call this switching thing), so I have to be careful that I try to find a solution that both is practical and also cost-effective. That's why I found the input of Sasha quite useful for me at first glance.

If you go for the transformer, then you have the 220V directly on the board (which is also the case for the opto-isolator solution, of course) + there are lots of components to lay out, especially if you need some 40 of those circuits.

I should take care of the price too. I know that those mechanical potential-free 'aids' contacts for the contactor are quite expensive, but looking to the schematic of Kai (for which I really wish to thank him very much!) gives me a feeling that this might be more expensive compared to the mechanical 'aids' contacts.

So, I'm a little bit 'in between'... Don't know exactly what solution to choose. On one side, the opto-isolator would be very simple, but then taking into account the remarks of Erik makes me hesitate.

Even if it's a "hobbyist" project, it must be safe and to be used for another 80 years! It's for our new house, you know... ;-)

If anybody can do more suggestions, I'm more than interested!

Best rgds,

--Geert