The unit consists of an amplifier circuit with a speaker at input & output. The unit with amplifier and one speaker is called master unit and the unit with only speaker is called salve unit .the master unit is fixed in the flat & the slave unit is fixed either at the door or in the letter box at the basement. The slave unit has a switch which when pressed sounds bell in the master unit . The person near the master unit switches on the master unit. When the unit is switched on the person near the slave unit can talk with the person at master unit . A press to talk switch is provided in the master unit .by pressing the switch the person at the master unit can talk with the person at the slave unit . Facility is also provided to give a bell at the slave unit . By switching on the master unit one can moniter the conversation going on near the slave unit.