Arduino UNO from 2010
Waveshield from 2010
Relay SainSmart 5v 2 Channel Relay Module With Optocoupler for Pic AVR DSP Arm Arduino for sale online | eBay
I believe this is a representative schematic http://www.vetco.net/catalog/images/VUPN5938_schematic.png
I think the comment about the 5v from the arduino that energizes the relay can't come from the I/O pins, it should easily from from the 5V regulated power supply, no? I have a 9V, 1A wall power adapter. The 5V pin on the arduino powers the stacked wave shield and my Maxbotix EZ1 ultrasonic sensor. I then tapped the same 5V supply pin for Vcc of the dual relay board and left on a jumper to Vin that also energizes the relay when it is pulled closed. I use pin 8 to switch the relay. The relay switches 110volts AC.
The EZ1 draws a max of 3ma at 5v, the Arduino maybe 50, the waveshield 90-100 and each relay (and I only need 1) draws 75. So I am well under what the 5V regulator on the board can handle. I was more concerned with the relay causing spikes that might interfere with the Arduino.
I wired up a test and have had it running for a few days. It seems to work fine so far.