PNP inductive not properly working

  • Hi! I just connected a common LJ18A3-8-Z-BY PNP inductive sensor to my delta duetWifi 1.04 controlled: I added R1 and R2 resistors as explained in the guide, but I used 100K and 275K resistors since I haven't 10K and 30K now with me.
    I plugged
    blue --> Duet GND in Power In connector
    black --> on Z_probe_IN, and R1 and R2 between
    brown --> Vin 12V in Power In connector

    When I turn on and pass some metal under sensor, onboard led turns on.
    When I look at Setting --> Machine Properties: endstops triggers well by hand, from "no" to "yes", inductive sensor doesn't trigger even if onboard led turns on and stays "yes" all the time, so G30 doesn't work, obviously.

    What's wrong?
    Too big resistors, so not enough current?
    I upload my config.g in case it could help
    I used
    M558 P5 H4 F120 T3000

  • One more thing:
    measuring voltage between Black wire (sensor output) and GND
    1.32V when not triggered by metal, and led is off
    3.27V when triggered by metal, and led is on

  • administrators

    I think the resistor values are too high. When in digital mode, the Z probe input has a pullup resistor, nominally 100K but minimum 50K. It might work if you put it in analog mode (P1 in the M558 command).

  • Sound strange, just verified: if I connect sensor to 12V external power supply I measure 0V when not triggered, 3.3V when triggered between signal (black wire) and GND (blue wire).

    Will have a try on M558 P1

  • You are right: lowering resistors to 10k and 34k (two 68k resistors in parallel) the inductive PNP NO sensor works as expected, digitally, setting M558 P5.

    FYI: it worked in analog mode too, with previous resistors and setting M558 P1

  • One more question: why does it work with lower resistor and not with higher ones? I would like to understand, anyone can share a simple scheme or drawing? I'm not so expert in electronics...

  • administrators

    Because the internal pullup resistor (nominally 100K) is pulling the input up to +3.3V, so the value of your pulldown resistor to ground needs to be a lot lower than that, in order to hold the pin close to ground potential when your sensor is not triggered. In analog mode (M558 P1), the pullup resistor is turned off.


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