Power line induced noise risk
OK. Here it is. 



To the cheapo PSU had to be placed a 100uH choke to suppress the noise from domestic appliances.  Note that line voltage goes to Vcc of the MCU. The inputs have pull-down resistors and the input signal comes from the button switch. Two buttons were located on PCB for settings, while the third button was actually a multiple parallel switch buttons located in house wiring (4 switch buttons in parallel). Also there was no go and return path wire, the wires came from existing wiring - the live signal 220VAC.  

I never had any issue regarding input switch signals, but I would probably add capacitors in parallel with pull-down resistors or even a TVS.

EDIT:

The three buttons S1-S3 are on PCB, the input signal from on/of switch goes to the connector X1, pin 4. Have forgot the value of R8. It is base on the app. note : link

EDIT2:

I would make a little upgrade for the input:







simulate this circuit &ndash; Schematic created using CircuitLab