I have one of these inductive sensors marked as PL-08N which should work within a 4-30VDC range and trigger at 8mm or less. enter image description here

I wired it as described below, with my multimeter probing voltage between sense and GND and my power supplying providing different voltages: 5V, 12V and 24V.


simulate this circuit – Schematic created using CircuitLab

Now to my question: the device seems to trigger properly, with sensitivity increasing with supplied voltage and sense line being pulled correctly to GND, but it does not recover, meaning the LED doesn't turn off and the sense line doesn't go back to VCC once I move the sensor away from the triggering metal plate.

What is wrong?

Using a GM328 transistor tester to test the component this what is reported:


simulate this circuit

I thought about requiring a pull-up resistor on the sense line, but didn't help either...

Is the sensor faulty or what?


It seems the sensor has no trouble recovering from active state if the supply voltage is 5 V turning back inactive once I move the activation plate a couple of mm away fro the activation point, which seems to be at 6-8mm from the sensor surface. So the question is what makes the sensor latching when supply voltage is relatively high

  • \$\begingroup\$ Please provide a link to the manufacturer's datasheet for the sensor. \$\endgroup\$ – Elliot Alderson Feb 27 at 13:27
  • \$\begingroup\$ I don’t have any info on the provider, it’s clearly a cheap Chinese knockoff of some other device. Also it seems to have no manufacturer marking anywhere, only the part number is displayed, but even that is printed on a sticker. Actually, all the info I reported are printed on this sticker :-/ \$\endgroup\$ – Roberto Lo Giacco Feb 27 at 13:31
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    \$\begingroup\$ If you have no datasheet and the manufacturer is unreliable then you should just discard the one you have and buy a replacement from a reputable supplier. \$\endgroup\$ – Elliot Alderson Feb 27 at 13:35

enter image description here

Figure 1. An NPN proximity switch.

The diagram on the front of the PL-08N (1) switch in your question indicates that the load (3) should be connected between supply positive (2, brown) and output (black). Internally it is using an NPN (or equivalent) transistor to do the switching. These are often referred to as NPN or "current sinking" type sensors.

  • \$\begingroup\$ If with load you mean a resistor, then I’ve tried that with no successful recover after triggering... If you meant something different, I need additional info as I didn’t get you... sorry \$\endgroup\$ – Roberto Lo Giacco Feb 27 at 15:32
  • \$\begingroup\$ Load can be a resistor, lamp, relay, PLC input, etc. Load should be connected between brown and black. If it won't switch off then it suggests that something is latching in the switch. You can expect some hysteresis which means that if it turns on at X mm that it might not turn off until you go back to 2X mm, but I'm sure you've tried that. \$\endgroup\$ – Transistor Feb 27 at 17:20
  • \$\begingroup\$ Yes, I confirm something is latching into the switch. Weird enough that latching doesn’t seem to occur if I lower the supply voltage around 5V \$\endgroup\$ – Roberto Lo Giacco Feb 27 at 18:19

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