I have tested a very simple circuit where I wired up a set of Christmas lights to one relay and a 10 K ohm resistor to a second relay.
I then used a multimeter to measure the AC volts across the 10 kohm resistor while I was running an Arduino program which turned off one relay and then turned on the other relay. After 5s the Arduino switched which relay was on and which was off.
This will be my first Arduino shield so I am excited. I am, however, confused if I could eventually sell this as a DIY kit since it is designed to plug into main power. If anyone has any advice on that it would be much appreciated.