Sensor Sense: Photoelectric sensor outputs

March 17, 2005
Dc photoelectric sensors detect the presence or absence of light.

Naming outputs

Sourcing outputs use a pnp transistor to supply a positive voltage to the load giving them another name of pnp outputs. Sinking outputs connect the load to the negative voltage or ground through an npn transistor, earning the name npn output.

4-in-1 Wiring diagram

The 4-in-1 sensor connects loads in any of the four modes of sensor operation by choosing one of two outputs for one load wire, and V+ or V for the other.

They turn an output connection on or off in response. In the light-on mode the output turns-on when the receiver detects its own light source. This is called a normally open (n.o.) output. The second method reverses the n.o. output by turning on when the receiver has no detectable light. This is called dark-on mode and is designated as a normally closed (n.c.) output.

When outputs turn on, power flows through the load. When turned off, the load receives no power. The remaining two modes of operation identify how the load is connected to the sensor to get that power.

A sensor output can source or sink power. Sourcing power means the sensor output supplies a positive voltage when the sensor turns on. This is called a pnp output. The name comes from the pnp output transistors used to control the power.

A sinking output connects the load to the negative supply voltage that is also ground in most systems. These connections are called npn outputs because npn transistors have historically been the active control elements.

When combined, these four modes define the operating qualities of dc photoelectric sensors: n.o. npn, n.o. pnp, n.c. npn, and n.c. pnp.

The four-in-one sensor combines all four output types into one unit. It offers both normally open and normally closed outputs and automatically detects the kind of load connected to see if the load needs an npn or pnp output. A four-in-one sensor operates in any of the four sensor output modes.

 Pepperl+Fuchs Inc. provided information for this article.

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