Using BT Telephones for a private communication system

You may wish to connect several BT telephones to your own 2 wire network and be able to use them for communicating between flats and the entrance security door. The following circuit will power the telephones to enable voice traffic between any or all phones but does not make the phones ring. It is assumed that some simple doorbell type arrangement alerts the receiver. The telephone at the entrance security door can be a handsfree speaker phone and be permanently off-hook. The phones at the flat should be on-hook when not in use (otherwise other flats will be able to eavesdrop).

A real BT line has 2 wires with a DC voltage of 50V when the phone is on-hook. When the phone goes off-hook, it draws current such that the BT line supplies 40mA and the voltage on the 2 wires will drop to maybe 9V. If more than one phone goes off-hook, the current remains at 40mA in total showing that the real BT line is acting like a current source of 40mA. Even if there were a short circuit the current would remain at 40mA.

The following circuit uses a 240V to 13V AC mains transformer for isolation. The 13V AC is full-wave rectified to about 18V DC and smoothed with a 1000 microfarad electrolytic capacitor. A 4mA current flows through a 2.2kOhm resistor and is further smoothed by a 100 microfarad capacitor. The 4mA current flows on through a 1.8kOhm resistor and 2 diodes. The voltage drop of the 2 diodes will be a constant 1.4V. This is connected to the base of a silicon NPN transistor. The voltage at the emitter will be a constant 0.7V. This passes through an 18Ohm resistor to give a constant current of 40mA. This 40mA passes to the collector of the transistor which has a high output impedance thus not shorting any AC voice signals passing between phones.

circuit diagram of 40mA constant current source
The phones themselves only need 2 wires. These wires will be the outer wires of the flat cables that come out of the phones, or the outer 2 gold contacts of the standard 4 contact connector (i.e. pins 1 and 4). If the connector has 6 contacts then the pins are pins 2 and 5.

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