I'm a beginner with electronics and plcs. I understand most electrical theory and components until I run it to circuit board and such. What I have is a bad npn inductive 3 wire switch that is normally open connected to a plc. What I was wondering if I was to connect a physically actuated switch, would I connect the switch between the positive and the signal wire or the negitive and the signal wire. Basically is power coming from the plc input or out of the input. I believe understand that when current drops to zero the plc input is actuated. My thought is that I connect the switch as normally closed and when actuated it would open providing zero current actuating the input. Any help would be appreciated.

 

Post the circuit diagram of what you have.

 

so all you need is a normally open switch from your pic input to the ov pin on the pic.and a pull up resistor to the 5v rail.

 

Like the pic except instead of switch to ground it's to +5v rail. Thanks. ] Got a basic question though, on a plc wired for npn sensors is power coming out of the inputs going to negitive rail via the sensors when actuated or visa versa?