Thank you Raghu for your reply,
You have not made any mention about the MCU power supply and also if the 15V for the proximity is galvanically isolated from this. Maybe you would have done that already

The supply to the four layer board i.e. +5V and the supply for single sided board input section i.e. +15V are fully isolated they have separate transformer and totally separate grounds.

Even for 8 inch of cable, would be a good idea to use the buffer.

Thanks for this comment.

And on the 110V PCB , hope you have taken care of the minimum PHYSICAL distance between the 110V traces and 15 V traces.

I am using atleast 2.5 mm separation in between the 110V AC and the 15V DC traces. Is that a good separation or it needs to be increased.

Happy welding.
Thanks.

Regards,
Prahlad Purohit