I can't see what it is supposed to do! I presume it is some sort of switchable state indicator for the 12-24v and you don't want it to interfere with the logic of the 12-24v line. Do you have any resistor values yet. I presume that you are intending to use a small signal NPN, in which case you will need a current limiting resistor to stop the magic smoke (or is the 12-24v line already limited).