This unit uses an advanced IC which has been pre-programmed to create a realistic "Ding Dong" sound. Once its power is connected, it will produce a "Ding Dong" twice. It is a useful unit having many applications such as door bells, alarms, telephone ring, clocks, timers, shop entrance bell, etc. Supply voltage: 3VDC / 50mA. Includes 2 x AA size battery holder.