Amazon.com: Digital Counter, Easy Connection NPN PNP Signal Input Stable Electric Counters Auto Detection for Industrial Control : Industrial & Scientific
Amazon.com: TRI-TRONICS LER 4 Conductor, Label-Eye, 100 MICROSECOND Response TIME, Push Button Adjustment Type, NPN/PNP Output, 10-30 VDC, 6 FT Cable, Sensor, Polarity Protected, RED LED : Electronics