How do I safely connect a light switch button to the secondary side on a board?
For that you would need an isolated DC/DC converter and galvanically isolated transceiver - maybe an optocoupler. 

Perhaps, more easier is to use just an optocoupler with few resistors or capacitors and use 230VAC directly for the switch supply.







simulate this circuit &ndash; Schematic created using CircuitLab

Similar question : Link