HI all
I am trying to use a PNP proximity sensor 15-30 Vdc into a plc expecting an NPN type sensor as a quick fix. I cannot change the common rail as all the others sensors are NPN, and do not have a small relay available. Any ideas?

 

I think you might be able to use a pull down resistor to hold your input low until the sensor is triggered and a n.c. contact in your ladder logic. Care to give more details? What plc are you using? I assume you are working with 24vdc. Is that correct?

 

the "PLC" is actually not your standard type, it uses +15Vdc/0Vdc as the supply to NPN sensors. 0Vdc is the HIGH when sensors are ON, so off course using a PNP sensor, 15Vdc is switched as being ON, thus the "PLC" thinks the sensor is OFF. So I need to pull the input HIGH, then when my PNP sensor switch's ON with 15Vdc, somehow pull the input LOW.
Does this clarify it better

 

added to the above, I have no means of accessing the logic. Very old Industrial Washing Machine

 

OK, this should work. If you can't get 2N3904, almost any general purpose NPN will work.

EDIT: I forgot to draw the grounds for the sensor and the PLC. All grounds should be connected together.