Abstract: A deflection sensor for a taut wire perimeter fence detection system, which can be installed after the fence wire has been installed easily, and the sensor the operates in line with the wire tension. The sensor includes a plate member adapted to be pivotally mounted, a first wire attachment point at one end of said plate member, a second wire attachment point remote from said first wire attachment point and a transducer or sensor element located on said plate member between the attachment points. The taut wire type detection system including at least one taut wire for a perimeter fence supported by a plurality of posts, at least one deflection sensor being pivotally mounted to one of the posts or a support thereon and a sensor processing circuit for interrogating the at least one deflection sensor and to provide an alarm indication on tampering of the at least one taut wire or the at least one deflection sensor.