Specifications

CIRCUIT IDEAS
CIRCUIT IDEAS
T
his add-on device for telephones
can be connected in parallel to the
telephone instrument. The circuit
provides audio-visual indication of
on-hook, off-hook, and ringing
modes. It can also be used to con-
nect the telephone to a
CID (caller
identification device) through a re-
lay and also to indicate tapping or
misuse of telephone lines by sound-
ing a buzzer.
In on-hook mode, 48V
DC supply
is maintained across the telephone
lines. In this case, the bi-colour
LED
glows in green, indicating the idle
state of the telephone. The value of
resistor
R 1 can be changed some-
what to adjust the
LED glow, with-
out loading the telephone lines (by
trial and error).
In on-hook mode of the hand-
set, potentiometer
VR 1 is so adjusted
that base of
T 1 ( BC 547) is forward bi-
ased, which, in turn, cuts off transistor
T 2
( BC 108). While adjusting potmeter VR 1, en-
sure that the
LED glows only in green and
not in red.
When the hand-set is lifted, the volt-
age drops to around 12V
DC . When this
happens, the voltage across transistor
T 1’s
base-emitter junction falls below its con-
duction level to cut it off. As a result tran-
sistor pair
T 2- T 3 starts oscillating and the
piezo-buzzer starts beeping (with switch
S 1 in on position). At the same time, the
bi-colour
LED glows in red.
In ringing mode, the bi-colour
LED
flashes in green in synchronisation with
the telephone ring.
A
CID can be connected using a relay.
The relay driver transistor can be con-
nected via point
A as shown in the cir-
cuit. To use the circuit for warning
against misuse, switch
S 1 can be left in
on position to activate the piezo-buzzer
when anyone tries to tap the telephone
line. (When the telephone line is tapped,
it’s like the off-hook mode of the tele-
phone hand-set.)
Two 1.5V pencil cells can provide Vcc1
power supply, while a separate power sup-
ply for Vcc2 is recommended to avoid
draining the battery. However, a single
6-volt supply source can be used in con-
junction with a 3.3V zener diode to cater
to both Vcc2 and Vcc1 supplies.
T
he circuit described here is of an
electronic combination lock for
daily use. It responds only to the
right sequence of four digits that are
keyed in remotely. If a wrong key is
touched, it resets the lock. The lock code
can be set by connecting the line wires to
the pads
A , B , C , and D in the figure. For
example, if the code is 1756, connect line
1 to
A , line 7 to B , line 5 to C , line 6 to D
and rest of the lines—2, 3, 4, 8, and 9—to
the reset pad as shown by dotted lines in
the figure.
The circuit is built around two
CD 4013
dual- D flip-flop IC s. The clock pins of the
four flip-flops are connected to
A , B , C ,
MULTIPURPOSE CIRCUIT
FOR TELEPHONES
RANJITH G. PODUVAL
YASH D. DOSHI
and D pads. The correct code sequence for
energisation of relay
RL 1 is realised by
clocking points
A , B , C , and D in that or-
der. The five remaining switches are con-
nected to reset pad which resets all the
flip-flops. Touching the key pad switch
A /
B / C / D briefly pulls the clock input pin high
and the state of flip-flop is altered. The
Q
output pin of each flip-flop is wired to D
input pin of the next flip-flop while D pin
of the first flip-flop is grounded. Thus, if
correct clocking sequence is followed then
low level appears at
Q 2 output of IC 2 which
energises the relay through relay driver
SIMPLE CODE LOCK
G.S. SAGOO
G.S. SAGOO
18