SMArd:
Is it possible to connect these standard industrial sensors with a current output to the Arduino?5V/20mA = 250 Ohms. Will putting a 250Ohm resistor between VCC and GND convert the 20mA signal to 5V? Do I need an external 24V (or whatever) power supply? How? Why? Whatever?! It's kind of confusing, anyone care to explain?
The answers to your questions are yes, yes but the 250 ohm resistor doesn't go between VCC and GND, yes, any way that works, because, ?, and probably.