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Which deign would you advise. Or maybe can I do this with only 1 mega and use something to extend the number of I/Os ?
You should only need one Mega, Shift Registers are the answer
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Which switches should I use for it to be easy to install (no soldering if possible) and cheap. Can I use any SPST-NO Off-Mom push button ? Do you know of some beautiful ones ?
Go to your Hardware Store, they should have some momentary wall switches from different brands.
(I'm using Busch+Jaeger switches. Don't know if they are available where you live, but you can just plug the cables into them without any soldering.) -
Are opto-isolated relay boards OK for connecting my loads to arduino ?
Yes, they should be fine. But notice you'll probably need a seperate power source for them.