A control system uses a 24 V DC proximity sensor to detect the presence of a metal object. The sensor is a 3-wire NPN normally open (NO) type. Which wiring configuration would correctly connect this sensor to a PLC input?

Question: A control system uses a 24 V DC proximity sensor to detect the presence of a metal object. The sensor is a 3-wire NPN normally open (NO) type. Which wiring configuration would correctly connect this sensor to a PLC input?

Answer options:

• Brown to +24 V, Blue to 0 V, Black to PLC input, with +24 V also connected to a common terminal on the PLC input module. ✅ Brown to +24 V, Blue to 0 V, Black to PLC input, with 0 V connected to a common terminal on the PLC input module.
• Brown to 0 V, Blue to +24 V, Black directly to 0 V through a load.
• Brown to +24 V, Blue to +24 V, Black to PLC input.

Correct answer: Brown to +24 V, Blue to 0 V, Black to PLC input, with 0 V connected to a common terminal on the PLC input module.

Explanation: For a 3-wire NPN NO proximity sensor, the brown wire is typically +24 V (power), the blue wire is 0 V (ground), and the black wire is the switching output. An NPN sensor 'sinks' current, meaning it provides a path to 0 V when activated. Therefore, the PLC input module must be 'sourcing' (receiving 0 V from the sensor when active) and its common terminal must be connected to 0 V to complete the circuit.