NPN sensors refer to the type of transistor output configuration the sensor uses. The terms “normally open” (NO) and “normally closed” (NC) describe how the sensor’s output behaves when no object is detected. But it’s important to know that NPN itself doesn’t automatically mean “normally open” or “normally closed.” In fact, NPN sensors can be either normally open or normally closed, depending on how the manufacturer designs them.
Now, let’s break this down in simple terms. An NPN sensor is also called a sinking sensor. It works by connecting the load between the positive voltage and the sensor output. When the sensor detects an object, the NPN output “sinks” current to ground. That means the output wire gets connected to 0V (negative terminal).
In a normally open (NO) NPN sensor, the output remains off (no current flow) when no object is detected. Once an object comes close, the sensor switches on, allowing current to flow to ground. For example, if you have an indicator light connected, it will turn ON only when the target is detected. This is the most common configuration in industrial applications because it’s easy to understand and troubleshoot.
On the other hand, a normally closed (NC) NPN sensor works the opposite way. In its normal state (no object present), the output is ON — meaning current is flowing. When an object comes near, the output switches OFF. This setup is useful when you need to know immediately if a wire breaks or a sensor fails, because the output will stop working, signaling a fault.
So, to answer your question directly: NPN sensors can be either normally open or normally closed, depending on the internal circuit design. The term “NPN” only defines how the sensor output connects to the circuit (sinking current), not whether it’s open or closed in its normal state.