Another option.....an N power mosfet.

The mosfet source connects to ground, the mosfet gate goes to your 5V switching input ....and the mosfet drain goes to one side of your 12V relay coil ...on the other side the coil is 12V supply. With no 5V switching waveform, no current flows through the moseft & your 12V relay is denergised. Once 5V switching waveform is appliedto the moseft gate, MOSFET is turned on, 12V flows through your relay coil to ground through the mosfet.

Have a look at this...

http://www.falstad.com/circuit/e-nmosfet.html (but the top of that circuit would be your coil to 12V)

If you want isolation, you can source these with opto isolators built in.