Difference Between NPN and PNP Sensors

Photoelectric sensors provide two output polarities: NPN (current sinks to ground) and PNP (current sources from V+). In PNP sensors when triggered current flows from V+ through the sensor output to the load and then to ground. In NPN sensors when triggered current flows from V+ through the load into the sensor output to ground (GND). Typical sensors have three wires: VCC, GND and OUT.

1. NPN: when triggered the OUT line connects to GND, producing a low-level output.

2. PNP: when triggered the OUT line connects to VCC, producing a high-level output.

The primary difference is polarity: PNP outputs high when active; NPN outputs low when active. Both are switching sensors using transistor saturation/cutoff.

Sensor types (switching outputs)

(1) NPN-NO (normally open) (2) NPN-NC (normally closed) (3) NPN-NC+NO (both)

(4) PNP-NO (5) PNP-NC (6) PNP-NC+NO

PNP/NPN sensors typically have three lead wires: VCC, 0V, and OUT.

PNP behavior

PNP outputs VCC to the OUT line when active. For PNP-NO the OUT is open when idle and connects to VCC when triggered, outputting a high level. For PNP-NC the OUT is at VCC when idle and opens when triggered.

NPN behavior

NPN outputs GND to OUT when active. For NPN-NO the OUT is open when idle and connects to 0V when triggered, outputting low. For NPN-NC the OUT is at 0V when idle and opens when triggered.