![]() ![]() If the common terminal is 0 V dc, it’s PNP 24 V dc is NPN. In that case, look at what’s connected to the “common” terminal. Some input modules can be configured as either. POINT MEASURE DISCRETE MANUALIf the PLC manual says “sinking input,” it’s PNP if it says “sourcing input,” it’s NPN. PNP is typical in the U.S., but if there’s equipment from other origins, it’s important to know what the PLC input is expecting. It describes the direction of current flow. PNP versus NPN: This is a required option for all solid-state devices.Once a sensor is chosen that fits mechanically and is the general type, there are other considerations to consider during the selection process: When purchasing a sensor, there are many options. ![]() Parts can be detected either by reflecting light back to the receiver (a reflective application) or by blocking the light beam from reaching the receiver (a through-beam application). Sometimes the light is guided through fiber optic lines and sometimes it’s used directly from the emitter/receiver. These are usually an inexpensive way to track parts in a system. Photo eye (PE) sensors have a light “emitter” and “receiver.” Sometimes they’re in the same package, and sometimes they’re separate. However, this is unusual enough that when a proximity switch comes up in conversation, it’s usually assumed to be inductive. Other varieties exist that function on non-inductive principles (capacitive and ultrasonic) and can sense non-metallic parts. These are two names for a prox that is more sensitive because it has less metal shrouding on the tip of the sensor.
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