Edwards Signaling 5520D-AW 5520D Series Duotronic Horn SirenEdwards D2 Class devices are low current, high performance, high decibel audible signals which may be coded from an external source or used as a continuous alarm. They are designed to function as either a horn or siren by setting an internal, tamper proof switch. The 5520D is primarily intended for use in applications requiring electrical supervision of signaling circuit field wiring. These signals may also be used for unsupervised signaling