What you'll learn
**Symptom:** the line operator reports the conveyor's box-present signal at PLC input X0 has been stuck **HIGH** all morning even when the conveyor is empty. You walk to the panel — the sensor on the conveyor infeed is an **NPN** photoeye (its label plate reads "NPN"). The PSU LED is on; the sensor LED looks healthy from a distance.
Lab time: ~6 minutes.
Lesson briefing
NPN Photoeye Polarity Reversal
Symptom: the line operator reports the conveyor's box-present signal at PLC input X0 has been stuck HIGH all morning even when the conveyor is empty. You walk to the panel — the sensor on the conveyor infeed is an NPN photoeye (its label plate reads "NPN"). The PSU LED is on; the sensor LED looks healthy from a distance.
Important context for NPN sensors: an NPN output is current- sinking. With a target visible the sensor pulls its OUT pin to 0 V, so PLC X0 should read LOW (0 V) at rest with a box on the conveyor. (Compare to the PNP sensor in F1, which sources +24 V to OUT and reads HIGH at rest — the topologies are mirror images.)
Reach for the multimeter on the toolbar and probe the panel.
Hints
Start at the photoeye. Probe across its +V and 0V terminals with the multimeter in voltage mode — what reading would you expect on a healthy install of a 24 VDC sensor?
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This lesson uses 6 placed components on the lab canvas. Components are vendor-neutral (no proprietary trademarks); the wiring rules apply to any equivalent industrial part.
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