switch that opens when power is applyed

[cooldog]

okay so i have been looking around for a switch that when a second voltage is aplyed it opens

lets say i hade a light hooked up and wanted a switch hooked to my usb that turns it on. so when the usb port gives it 3.3v it opens and the light turns on

note: i don't want the usb powering the light

do you guys know of anything like this or how to make it

[Ro-Bot-X]

You can use a relay turned on by a transistor or a MOSFET.

[paulstreats]

or just use the transistor/mosfet without wasting a relay