Electronic – Proximity sensors interfacing with microcontroller

It sounds like you are correct about the low side NPN and pullup resistor inside the sensor unit. You can make the output any lower voltage you want with the addition of a single resistor:

The resistor inside the sensor (R1) and a resistor you add (R2) form a voltage divider. The output high level will be V+ attenuated by the voltage divider:

\$ V_{OUT} = V_+ \times \dfrac{R2}{R1+R2} \$

In the example shown, the output will be 5V when high. The impedance of the OUT line will be the parallel combination of R1 and R2, which is 5 kΩ in this example.

You could also replace R2 with a 5 V zener if V+ might vary. However, my experience with sensors in general unless they explicitly say otherwise is that they can be surprisingly susceptible to power variations. I would at least filter V+, preferably regulate it.

To use this sensor, connect ground and power (6..36V). The third wire will be connected to ground when something is detected, otherwise it will be floating.

Note that it does not output 300 mA, that figure is the maximum current it can handle (sink to ground).

If you have a 4-20mA input you could use a resistor to supply 20mA to it, and use the output wire to ground the 4-20mA input to ground, so it will get 0 mA (or at least a good deal less than 20) when the sensor detects something.

You could use a serial port if you know how to read a handshake signal in your programming language.