How do 3 wire proximity sensors work

The different proximity sensors that we find in the market can be classified by the technology they use to feed and obtain the output of the sensor. The different types of proximity sensors can be connected with two wires, three wires or four wires.

Proximity sensor terminals

The cables used for the sensors follow European and international standards that make it easy to distinguish which function each cable has.
The cables or terminals that we are going to find in a proximity sensor, the mission that each cable fulfills and the nomenclature of the cable that comes from an abbreviation of the color of the cable in English is detailed below:
Positive supply (+): The cable is brown and is called BN for brown
Negative (-) supply: The cable is blue and is called BU for blue
Sensor output : The cable is black and is called BK for black
Antivalent or normally closed output : The cable is white and is called WH for white.

Two-wire proximity sensors

This type of proximity sensors that have only two wires to connect, are powered and connect the load in series, so that they work as a normally open or normally closed contact depending on the sensor.
The two cables used are brown and blue, being able to place the load depending on the sensor, always in series between positive and brown or between blue and negative.
Proximity sensors can work with alternating or continuous power depending on the technical characteristics of each sensor, we can find proximity sensors that work with alternating current with other types of colors than those indicated above, but in that case it does not matter how they are connect to having no polarity.
The power supply is usually 15v. at 250v. in dc or ac., taking into account that being a sensor where the power supply and the load are connected with the same wires, we can have a small voltage drop in the sensor at rest and a small current flow to guarantee its operation even if it is in standby the proximity sensor.

3-wire proximity sensors

In proximity sensors that have three wires, we find that they have two wires to power the sensor and a third wire whose exclusive mission is to output the sensor, indicating whether the sensor is detecting or not. The power wires are usually of the colors indicated above, brown for positive and blue for negative, the third wire being black to give the output.
We can find four types of outputs, the output can be based on the PNP or NPN sensor, they differ because the PNP is called positive output connecting the load between the output cable and the negative cable. Sensors with NPN output are said to have a negative output by connecting the load between the output lead and the positive lead.
In turn, we can find that the output is a normally open contact in such a way that we will not have an output signal until the proximity sensor detects it. The other possibility is that the proximity sensor has an output with a normally closed contact, having an output signal when the sensor is at rest and not emitting the output when the sensor is detecting.

Four-wire proximity sensors

The four-wire proximity sensors have the same characteristics that we indicated for the three-wire proximity sensor but adding a fourth wire that is equivalent to another output, with the sensor having one normally open and one normally closed output. This other output is called the antivalent output and is usually the white wire.