Whether you can use a PNP sensor instead of an NPN sensor depends on what kind of input your control system or PLC accepts. Both sensors do the same job — they detect objects without physical contact — but they work in opposite electrical ways.
An NPN sensor is known as a sinking sensor, meaning when it detects an object, it connects the output to the ground (0V). The current flows from the load into the sensor and then to ground. A PNP sensor is called a sourcing sensor. When it detects an object, it sends a positive voltage to the output. The current flows from the sensor to the load. Because of this opposite current flow, PNP and NPN sensors are not directly interchangeable unless your controller or input circuit supports both types.
If your PLC or controller is designed for NPN inputs (sinking inputs), it expects a signal that goes to ground when the sensor is ON. If you connect a PNP sensor to that input, the logic will not work correctly because the signal will go high instead of low. In the same way, if the input module expects a PNP signal (sourcing input), an NPN sensor will not work properly because it pulls the signal to ground instead of providing a positive voltage.
In some cases, you can use a PNP sensor in place of an NPN sensor, but only if your control system allows it or if you modify the circuit using an interface relay or signal converter. For example, a relay module can invert the output from PNP to NPN or the other way around. However, this adds extra components, cost, and wiring complexity.
Before replacing an NPN sensor with a PNP sensor, always check your PLC’s input type or wiring diagram. If the inputs are designed for sourcing (PNP) signals, a PNP sensor will work perfectly. But if they are designed for sinking (NPN) signals, a direct swap will not function properly.
Both PNP and NPN sensors are good, but they must match the input type of the device they are connected to. Using the wrong one won’t damage the system but will prevent it from working correctly.