Proximity sensor for tight spaces
The Omron E2E-X3D15-N 5M is a shielded inductive proximity switch with a 3 mm sensing distance, normally open output, and a 5-meter PVC cable. It runs on DC 2-wire, so you get two wires total — power and switching in one pair.
Fit and wiring
Shielded construction means it can be flush-mounted in metal without derating the 3 mm range. The 5 m cable gives you reach to a junction box without a splice. DC 2-wire output leaves one wire free; confirm your PLC input accepts a sinking or sourcing signal from a two-wire sensor — some inputs need a pull-up resistor.