Grumpy_Mike:
The purest would say no but i practice it does not cause any feedback problems that I have come across.
In fact they are not isolated despite having an opto isolator on them. You use that extra 5V to insert extra decoupling like the last circuit on this page:- http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html
I was talking about the relays being isolated.. ie that they do not draw any power from the arduino to operate, only the logic signal, with the operating power being provided via a separate 5v input to the relay boards..
That being said, you bring up a good point about noise rejection in the whole project... First of all, is noise rejection required across the relay coils, or is this something that is primarily for discreet components?
As a general rule, should I plan to put a 47uf electrolytic across the output of the power adapter where it supplies power to the entire project?
As for as the individual caps, do these go across the 5v (or 3.3v) to ground whenever supplied to a discreet device? Or is it for the signal/logic paths? (or both)
Thanks,
Mark