Specifications

CIRCUIT IDEAS
supply, diode D4 is reverse biased and
acts as an ‘off’ switch, inhibiting
the conduction of diode D3, which al-
lows normal functioning of transistor
T2. The inverter can be switched ‘off’,
when not required, by using ‘on/off’
switch S1.
Low battery voltage indicator cir-
cuit comprises transistor T3, senser di-
ode D6, LED 2, variable resistor VR1,
and resistors R4 through R6. The
low battery indication can be adjusted
from 4.7V to 5V by using variable re-
sistor VR1. When the battery voltage
is above 4.7V, zener diode D6 comes
out of conduction, keeping transistor
T3 at cut-off level. At the same time,
LED2 gives the indication of low bat-
tery volt-age.
The whole circuit can be assembled
in a cabinet of emergency light suitably.
vides DC path for charging of the bat-
tery. But, in the absence of AC mains
PARALLEL TELEPHONES
WITH SECRECY
S.C. DWIVEDI
MANUJ PAUL
O
ften a need arises for connec-
tion of two telephone instru-
ments in parallel to one line.
But it creates quite a few problems in
their proper performance, such as over
loading and overhearing of the conver-
sation by an undesired person. In order
to eliminate all such problems and get
a clear reception, a simple scheme is
presented here (Fig. 1).
This system will enable the incom-
ing ring to be heard at both the ends.
The DPDT switch, installed with each
to the line.
To receive a call at an end where
the instrument is not connected to
the line, you just have to flip the
toggle switch at your end to re-
ceive the call, and act as usual to
have a conversation. As soon as the
position of the toggle switch is
changed, the line gets transferred
to the other telephone instrument.
Mount one DPDT toggle switch,
one telephone ringer, and one tele-
phone terminal box on two wooden
electrical switchboards, as shown in
Fig. 3. Interconnect the boards us-
ing a 4-pair telephone cable as per
Fig. 1. The
system is
ready to
use. Ensure
that the two
lower leads
of switch S2
are con-
nected to
switch S1
after rever-
sal, as
shown in
the figure.
Lab.
Note: The external ringer for the project
as shown in Fig. 2., was designed/fab-
ricated at EFY Lab.
of the par-
allel tele-
phones,
connects
you to the
line in one
position of
the switch
and discon-
nects you in
the other
position of
the switch.
At any one
time, only
one tele-
phone is
connected
100