Here is a low-cost teleconferencing system that lets you talk
to two persons at a time in any part of the world over two telephone lines. The
circuit makes use of a coupling transformer and some passive components. The
circuit is connected between the two telephone lines.
It works like this: When
‘X’ calls ‘A’ on the first telephone line, ‘A’ puts this call on hold, dials
‘Y’ on the other telephone line (which is free) and keeps this call too on
hold, and slides switches S1 and S2 to ‘on’ position. Now ‘X,’ ‘A’ and ‘Y’ can
talk to one another simultaneously over the two telephone lines. Both the
primary and secondary coils of the coupling transformer consist of 500 turns of
40SWG insulated copper wire. At the secondary side, a small circuit is used for
DC holding.
This circuit is built around transistor T1 (BC547), resistors R2
and R3 (15 kiloohms and 100 ohms, respectively), condenser C3 (22μF, 63V) and
two LEDs as indicators for both the primary and secondary sides. It provides
proper DC characteristic to hold second telephone line in operation even though
no telephone on that line is present. Here, transistor T1 acts like a resistor to
DC and as high impedance for audio signals. The high impedance of the circuit
is provided by condenser C3, which prevents any audio signal from appearing at
the base of T1. Thus any audio voltage appearing across telephone line No. 2
will not cause a corresponding current in the transistor.
SCHEMATIC :
