I am looking into switching on and off regular 120v household power with a relay and arduino. (basically make one of these: http://powerswitchtail.com/default.aspx)
The reason I want to use mechanical relays is because It turns out that solid state relays do not shut completely off.
I have been looking around for a bit and I found this circuit
From reading the circuit diagram I gather that an external power supply is needed. Is the true because the Arduino cannot supply enough current? Also, is there anything else I want to take into consideration?