A 3-wire proximity sensor works by detecting the presence of an object without physical contact and sending a clean electrical signal to a controller such as a PLC. These sensors have three wires — one for power supply (V+), one for ground (0V), and one for output signal. The third wire provides a separate path for the output, ensuring better signal stability and faster switching compared to 2-wire sensors.
In an inductive proximity sensor, an internal oscillator circuit generates a high-frequency electromagnetic field at the sensing face. When a metal object enters this field, it creates eddy currents that weaken the oscillation. The sensor’s detection circuit senses this change and switches the output signal. The output (either PNP or NPN) changes state, sending an ON or OFF signal to the PLC, depending on the target’s presence.
In a capacitive proximity sensor, the sensing face forms one plate of a capacitor, while the surrounding environment acts as the other. When a non-metallic object like plastic, wood, or liquid enters the field, the capacitance changes. The internal circuit detects this change and triggers the output, just like in inductive types. The sensor’s output remains stable and fast because power, ground, and signal lines are separated.
In simple terms, a 3-wire proximity sensor detects objects by sensing changes in an electromagnetic (inductive) or electrostatic (capacitive) field. When it detects a target, the sensor’s output wire sends a clear ON/OFF signal to the control system. This design gives excellent response time, higher accuracy, and better noise immunity — making 3-wire proximity sensors the standard choice for modern industrial automation systems.