This link will give you the schematic if you click on 8+Relay+Module.rar. You must connect the ground to make it work with the arduino. You can switch the VCC side with a solid stage switch or whatever you like. Drive it directly from the mega 5V, when it goes down the relay will open. You will have powered the VCC via the NC contacts so they will deactivate. The input to the relays is a resistor, opto coupler diode and LED in series. Removing VCC disables it completely. You can even use a single relay board (active high input), they are inexpensive. This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!
Gil