How can I connect a 24v-powered inductive proximity sensor to a 3D printer control board?

Question

I am attempting to connect an inductive proximity sensor to my 3D printer. The sensor I'm using is a Zhongde PL-08N2 sensor, which is an NPN output NC (Normally Closed) sensor. As I understand it, this sensor has an open collector output, and I am powering it with 24v. When powered up, the sensor's indicator light is normally on, and turns off when metal is placed near the sensor (which I believe indicates normal, correct operation).

The board I am connecting this sensor to is an SKR 1.3 controller board, which operates at 3.3v (though the digital inputs are 5v tolerant) and has a 10K pullup on the board for all the endstop connectors:

Of course, I cannot allow the 24v from the sensor to be applied to the input pin, so I used a reverse-biased diode as described at the bottom of this page:

This did not, however, work for me. The board shows that the sensor is always triggered, whether or not any metal is near the sensor. I believe that may be because the setup in that link is an NPN output NO (Normally Open) sensor, and my sensor is NC.

When I have used this type of sensor before, I used a resistor divider to set the sensor output to an appropriate level, but in that case, there was no hardware pullup on the control board I used. Given that pullup, I do not know how to calculate an appropriate resistor divider for this sensor.

How can I connect this sensor to get the appropriate (3.3v/5v) logic output?

Answer 1

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. The simplest solution.

Just be careful that no accidental 24 V connection is made to the signal line. If the microcontroller has internal pull-ups R1 can be omitted.

I think internally to the sensor, the "sense" output will be pulled up to the 24v input when it's not triggered; I believe that would damage the microcontroller?

It would if there is an internal pull-up but you said open-collector. Your link points to a catalog page and there is no datasheet. (This should have been enough to discourage you from a purchase.)

Testing is easy to do:

• Power up the sensor with a high-value resistor - 1 MΩ, say - between the collector and ground and your voltmeter across it.
• Move the target into and out of range. If the voltmeter reading stays close to zero then there is no pull-up. If it increases towards 24 V in either state then there is.

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^{simulate this circuit}

Figure 2. Using an opto-isolator provides level-shifting and prevents accidental connection of 24 V to the microcontroller. This version uses the opto-isolator in active-high mode. Swap the positions of Q2 and R1 for active-low mode.