Here's a breakout board for those Songle SRD series relays that are so cheap on Ebay. It fits nicely on the Dangerous Prototypes standard PCB size DP5031.
Electromechanical relays are generally employ small signals to switch much higher voltages and currents. As a result, a nice big gap is required to insulate between the signal and load sides. It's often a good idea to hive relays off to a separate board.
This breakout board is released under Creative Commons Attribution Share-Alike 3.0 License, the schematic and parts of the layout derive from Sparkfun Electronics COM-11041 designed by N. Seidle and Joel Bartlett.
The component values are designed for VCC = 5 VDC. For VCC = 12 VDC, use R4 = 3.3 KΩ.
Look no further than the next post for twin relays on a DP5050 PCB.
Mains electricity can kill. If switching high voltages, make sure to mount this PCB in a well-insulated or grounded enclosure. Use conformal coating to insulate the high voltage traces and components and to protect against environmental damage. Make sure your loads are properly protected by fuses.