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This circuit shows the difference between a PNP sensor and a NPN sensor (like a proximity sensor). The PNP types will switch the positive voltage, like 24 VDC. The NPN types will switch the negative voltage, 0 VDC. This is important to know, when connecting the load to the sensor.
*How to use*
On both circuits a potentiometer is connected. This simulates the sensor "sensing" the material to detect.
On the top circuit, which represents a PNP sensor, the sensor is simulated as if it doesn't detect the material. When increasing the resistance, the material is being detected and the PNP transistor swithes on the load with 24 VDC. The wires on these kind of sensors, which are usually 3 wired, need to be connected on a 12 to 24 VDC supply with the brown and blue wire, and the black wire needs to be connected on (for example) terminal A1 on a relay.
On the bottom circuit, which represents a NPN sensor, the sensor is simulated as if it doesn't detect the material. When decreasing the resistance, the material is being detected and the NPN transistor swithes on the load with 0 VDC. The wires on these kind of sensors, which are usually 3 wired, need to be connected on a 12 to 24 VDC supply with the brown and blue wire, and the black wire needs to be connected on (for example) terminal A2 on a relay.
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