0orksecurityBest Answer 9 years ago ReplyUpvoteOf the five connections: Two are to the coil. They're interchangable. Apply and remove power across these to switch the output. One is output common. One is output normally-closed. One is output normally-open. When power is not applied to the coil, common is connected to normally-closed. When power is applied, common is connected to normally-open. As far as which pin is which: The relay may have come with a small diagram labelling the pins, or the may be labels on the relay itself (often just a letter or two -- NC, NO, and so on), If not, you can figure it out with a bit of experimentation... or, if the case is transparent, you can often figure it out by inspection.