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Extension
Phone Switcher
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Having
multiple extension tele- phones at home is very conve-
nient. You can make or receive phone calls practically
anywhere in the house. This circuit disables other telephones
connected to the phone line whenever a telephone (either
the master or any extension phone) is in use. The circuit
is inexpensive and is guaranteed to keep the phone conversation
private. The circuit does not need an external power supply.
It gets its power from the telephone line. The no-load
voltage at the telephone line, when the telephone handset
is ‘on-hook,’ is around 48 volts. However, when
the handset is off-hook, terminal voltage drops to between
5 volts and 15 volts. This is due to the impedance of
telephone line and the telephone set. The voltage of the
telephone line is the key factor that controls the operation
of this circuit. diodes D1, D2, D3 and D4 are connected
as bridge rectifier to make the circuit non-polarised.
Lifting the handset causes the terminal voltage to drop
from 48V to about 10V. The drop in voltage does not, however,
occur rapidly. therefore while the terminal voltage is
still high (above the threshold voltage level), both zener
diodes D5 and D6 are turned on. Current flows through
resistor R3, triggering SCR1 and providing a link to the
telephone set connected to lines L1(a) and L2(a). When
the terminal voltage drops below the threshold voltage
of the zener diode, diode D5 reverts to its non-conducting
state, cutting off the gate drive to SCR1. However, once
the SCR is on, it will remain in that state as long as
the current flowing through it does not fall to near zero
level. Thus the link continues. Zener D6 maintains the
voltage across resistor R2 and LED1/LED2 indicates as
to which telephone is in use. The low off-hook voltage
of the line will disable the other extension phones. The
line voltage will not turn on zener diodes D11 and D12,
even if the handsets of the other extension phones are
lifted. Use the following procedure to check up the system
after wiring:
1. Lift the handset of each telephone to see whether the
corresponding LED lights up. Return the handset back in
its cradle; the LED should turn off. Use the same procedure
to check the other phones.
2. Lift the handset of phone ‘A;’ its corresponding
LED should light up. The other phones should be cut-off
(no dial tone).
3. Lift the handset of phone ‘B’, then return
the handset of phone ‘A’ to its cradle. Now
‘B’ telephone’s LED should light up and
the dial tone should be heard through the ear-piece.
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