PNP sensors can be either normally open (NO) or normally closed (NC). The term PNP only tells us about how the sensor output works electrically, not whether it’s open or closed in its normal state. Let’s make it clear in simple words.
A PNP sensor is also called a sourcing sensor, which means it sends out a positive voltage when it detects an object. The output wire of the sensor provides current to the connected load, such as a PLC input or indicator lamp. Now, the “normally open” or “normally closed” part simply describes what the output does when there is no object in front of the sensor. In a PNP normally open (PNP NO) sensor, the output is OFF when nothing is detected. The moment an object comes close, the output turns ON and sends a positive voltage to the load. It’s like a switch that stays open until something passes in front of it. This is the most common and widely used configuration in automation systems.
In a PNP normally closed (PNP NC) sensor, the output works the opposite way. It stays ON when no object is present and turns OFF when an object is detected. You can think of it like a light that stays on until something comes near and breaks the signal. This type is used when continuous monitoring is needed, or where it’s important to detect wire breaks or sensor faults quickly, because the signal will disappear if something goes wrong.
So, the correct way to understand it is that PNP refers to the type of electrical output (positive sourcing), while NO and NC describe the output behavior. A PNP sensor can be made in either form, depending on the internal wiring and design.