How do you detect direct current flowing through a wire?

See this link for a very good answer (solution) to your question:

http://www.discovercircuits.com/DJ-Circuits/pumpmotormon2.htm

Posted by: trwtommy@gmail.com

Sorry I just noticed you want this for a DC voltage line, For this you would need to design in a flip-flop circuit to the circuit at the link above. If you need this, just email me and I can do this for you.... at a cost of course ;-)