If you have 120vac, use a NO. relay with a 120 vac coil.
The motion detector would energize the coil of the relay by way of the wires that would normally go to the lamp socket(s). This action would close the normally open contact, which has the low voltage dc circuit going across it.
http://www.tlc-direct.co.uk/Products/SR510.htmlTake notice that the contacts on this particular relay is only good for 10 amps at 30 vdc. If you needed to have more lights, say 50 amps worth at 12 vdc nominal then use the above relay in the same fashion except to pull in a larger, higher rated contactor with a 12 vdc coil.
http://www.evparts.com/prod-SL2555.htmI don't know why anybody would want to but, this would allow a simple 120 vac motion detector turn on a whole "bank" of car headlights or more precisely off road lights.
Low voltage dc LED lighting would seem to be the rage now.
http://www.theledlight.com/12volt-led-bulb.html
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