close a circuit by 0v switch

okay i have a circuit. 12v source, 12v light. i have another circuit that ossilates (spelling..).... from 12v to 0v. over and over. i need my 1st circuit to turn on when the 2nd circuit is at 0v. so its like they are alternating. i'm guessing this would be done with some kind of transistor.. but not sure. help needed please

if you dont understand what i'm asking... here... try this explanation.

say i wanted the 1st circuit to turn on when the 2nd is at 12v. then i could just connect that 12v to the new circuit. but i want to do that backwards, at 0v. hopefully you can understand now..

why don't you use a relay???

how do you make the relay turn on when 0v is applied? i know how when 12v is applied.. just not 0v

Connect the other end of the relay coil to 12v.

when you buy a relay it has two switches.
one which is activated with 12v
and one which is activated with 0v

one which is activated with 12v
and one which is activated with 0v

Click to expand...

Or in other words, "normally open" and "normally closed" (N.O & N.C)