12vdc relay triggered by 5v

You can use a 2N2904 transistor to amplify the current of the 5vdc output and switch a 12vdc load.

The transistor has 3 leads on it - a collector, an emitter, and a base.  Wire the base through a 470 ohm resistor to the 5vdc output.  Wire the emitter directly to ground.  Wire the emitter to the (-) side of the relay coil.  Make sure you use a relay with a diode across the coil - the stripe of the diode should face the 12vdc side of the relay coil.

Whenever the 5vdc output is active it saturate the base of the transistor causing the transistor to switch on.  The ground at the emitter will pass through the transistor and output on the collector, causing the relay to turn on.

Should only be $2 in parts at Radioshack and only $0.25 worth of parts if you have a real electronics store close by.