I added the capability to monitor the state of my two balcony doors (open/closed). 

The door state is monitored with a magnet and reed switch combination. The reed switch is in parallel with a resistor and changes the current through the loop.

So the different current states are corresponding with different door states.  It is also possible to detect a short circuit (>10.65mA) or an open loop (wire failure <5.5mA) and all this with just two wires.

Current states with 24VDC supply voltage:

I only had 0.25W resistors, so i had to use some in parallel to distribute the load:

The current input gets analyzed (0...20mA current loop interface) and visualized by the RasPi PLC: