Use with existing doorbell. Plugs into 110 volt outlet (indoors only). Works on transformers 8 - 24 volt. Different sounds for front and back doors. Select from 6 sound options. 2-1/2 in speaker provides excellent sound. 32 selectable codes block interference from outside signals. Operating range up to 150 ft Compact design leaves other outlet receptacle available. Will not work with model RC3030 or RC3733, only with works with the "D" version: RC3733D, RC3030D.