flexible intercom system

The criteria for a good domestic intercom system are as follows: First of all, it is essential that each station can call up any other station in the house, without having to route the signal via a `central` or master station. Secondly, there should be as few wires as possible between each station. Thirdly, the system should be capable of being used as a babyphone, without interfering with normal operation. Finally, there should be no possi-bility of `listening in` to stations (other than in the case of the baby-phone). The circuit described here fulfils all the above requirements.