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.
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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