You could certainly use a relay in theory though the 12v trigger output is only rated to supply 100 mA - at least on my G91. Assuming this is the same with the 818, this would not be enough to run most relays and you would in any case still require the 12v external supply.

If you're able to source a 12v relay with a coil resistance of 120 ohms or more, this is what you'd use...

[Linked Image from i45.tinypic.com]

The diode across the relay coil is important to suppress the voltage spike generated when the relay is turned off and so prevent cooking the 818.

Mark
MC200, AC200, M6, 557, G91