The protection diode will critical -- unless of course you only want to energize the relay once or twice...
What happens is that passing a current through the coil generates a magnetic field (duh -- this is what makes the relay work). However, when you turn off the current the magnetic field begins to collapse. This collapsing field induces a current in the coil windings that is of the opposite polarity to the voltage that was originally applied. The diode (which is reverse biased when the relay is energized) provides the reverse EMF with a path to ground that doesn't involve destroying junctions inside transistors. A generic fast-switching diode like a 1N914 should be good.
Mike...
