Circuit to detect in which port sensor inserted

Question

I want to build a Counter using a Proximity Sensor. I want to give choice for user to use either PNP or NPN. I will give two ports for the same. I am using PIC16f887 microcontroller. How to detect that which sensor user inserted?

• I dont want to miss any sample just to detect.
• I dont want to put burden on user to select type of sensor from the menu.

Power supply for sensors is 12V DC.

It may be programmatically but automatically it should detect without user intervention. Or it may be electronically , may be using Gate circuit or something.

What is confusing me more is some of them come "open collector" type. When sensor detect the object, they pull down.

Answer 1

This should be possible to do if you use a window comparator, and detect whether the input is pulled 'high' or 'low' from a known midpoint. We take advantage of the fact that both NPN and PNP type sensors use open-collector outputs.

Example (simulate it here):

The window comparator is high when the input voltage is between 1/3 and 1/3 of 12V (that is, between 4 and 8V). If it is pulled above that by the PNP, or below that by the NPN, the output goes low.

The comparators have open-drain outputs and are wire-OR, so this can tie to your logic with the appropriate logic voltage pull-up. I show 5V here; it can be 3.3V or whatever your system requires.

Basically, this looks like an NPN type to your system, so you need only 1 input. If you want to use two, then you can separate the comparator outputs and detect whether you have a PNP or NPN type plugged in.

Like this (simulate it here):

The upper comparator goes low when NPN is inserted and active, the lower one when PNP is inserted and active. So now you know which kind of sensor is in use, if it matters to you.