Commonly used three-wire sensors, whether NPN or PNP, can be measured directly with a multimeter.
The sensor, 24V power supply and multimeter form a circuit, and the multimeter is set to the DC position. The blue wire is connected to the negative terminal of the power supply, the brown wire is connected to the positive terminal of the power supply, the black wire is connected to the black or red probe of the multimeter, and the remaining probe is connected to the positive or negative terminal of the power supply.
At this time, when the NPN or PNP sensor is working, the voltage measured by the multimeter is the power supply voltage, which should be 24VDC. When the remaining probe of the multimeter is connected to the positive terminal of the power supply and an object approaches the sensor, the multimeter displays 24V voltage, indicating that the sensor is an NPN sensor and is normally open. When no object approaches, the multimeter displays 24V voltage, indicating that it is an NPN sensor and is normally closed.
If the remaining probe of the multimeter is connected to the negative terminal of the power supply and an object is close to the sensor, the multimeter displays 24V voltage, which means that the sensor is a PNP type sensor and is normally open. If no object is close to the sensor and the multimeter displays 24V voltage, it means that it is a PNP type sensor and is normally closed.
So, for this question, if the sensor you are using cannot be identified, it is more reliable to judge by measurement. But remember, the NPN sensor, which is a proximity switch, outputs a low-level signal, which is the negative pole of the 24V power supply, while the PNP sensor outputs a high-level signal, which is the positive pole of the 24V power supply.
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