Inductive sensor with microcontroller

[Veketti]

Hi,

I'm trying to connect PNP inductive sensor pepperl+fuchs NJ5-18GM50-E2-V1 to 5V microcontroller for reading its state. I made following connection to protect the input as the sensor will be supplied with +24VDC and micro 5VDC. However I'm seeing 2,5V in the UC input when sensor is not sensing and not 0,5V of the Vf of diode. If I connect the +24 directly to pin 4 removing the sensor it works as I was expecting. Eg. no supply, uC input 0,5V and 24V supply uC input 5V. I don't understand why it wont work with the sensor. Could someone explain what's wrong?

I could also use voltage divider for the input but I'd see this is safer.

Thank you in advance.

[langwadt]

it is a PNP sensor it only pulls high, when it is off or the sensor disconnected you have a voltage divider between the 15k and the 12k

the 12k needs to be a lower value

[Veketti]

Ah, yes indeed. Stupid me. Thanks.

What would be the ideal/better way of reading the sensor? I wouldn't want to drop the 12k value too much as it would consume much power when sensor is active..

[BeBuLamar]

Check pin 4 of the sensor with the sensor powered up (pin 1 and 3 connected) and the sensor is off (not sensing) and measure the voltage between pin 4 and pin 3 and see if you have any voltage there. There may be some leakage current that cause a small voltage reading when the PNP transitor is off. But to be honest I use those type of sensors a lot and not notice the problem.

[langwadt]

Ah, yes indeed. Stupid me. Thanks.

What would be the ideal/better way of reading the sensor? I wouldn't want to drop the 12k value too much as it would consume much power when sensor is active..

you can reduce the 12K and add another resistor in series with the input from the sensor to limit the current

[Kim Christensen]

The uC could still get damaged from static electricity or if a negative voltage was applied on the input. A much safer method would be to use an optocoupler which would also give you the benefit of eliminating ground loop problems if the sensor was some distance away. Or, if you're not worried about ground loops, a simple voltage divider with a 5V zener in parallel with the lower resistor could work also.

[BeBuLamar]

I think a 5V zener between pin 4 and 3 is a good idea. The sensor is capable of 200mA so with some current going thru the zener isn't bad.