These are universal infrared photocells, that are compatible and replaces your Chamberlain 771REV photocells. Works with 12/24V AC/DC power supplies. The relay contacts are: N.C (notmally closed) - COM - N.O (normally open). Manual and mounting screws included in package. For swing gates you need at least two sets of photocells to get proper coverage. Double swing gates are better than singles when using photocells because the distance between the inside and outside set of photocells is less, where as if you have photocells on a 4m swing gate, the distance between the inside photocells and outside photocells must NOT be more than 4m, so if you get a really short car stop between photocells, the gate still could close on it. It's best to place the photocells as close to the gate as possible to reduce the chance of this happening. If you have any questions about your purchase please contact us.