Specifications

CONSTRUCTION
SIMPLE TELEPHONE RECORDING/
ANSWERING MACHINE
B.B. MANOHAR
T
his project is intended to provide
you with a simple recording and
answering machine, which in the
absence of the subscriber/owner of the
telephone instrument, responds to the
incoming calls and also records them
automatically.
Description
To understand the overall working of
the system, refer to its block diagram
shown in Fig. 1. The incoming telephone
line pair is terminated into the ring de-
tection unit comprising a monostable
flip-flop followed by a ring counter to
detect the incoming calls. After count-
ing a predetermined number of rings, it
triggers a timer (another monostable
flip-flop) via an inverter. The output of
the timer is used for energisation of a
set of relays, which initiate the follow-
ing actions:
1. Switch
on AC power
to the tape re-
corder.
2. Switch
on DC voltage
to the tape
player.
3. Reset
the ring
counter in the
ring detector
section to
make it ready
for the next in-
coming call/
ring. However,
any fresh call/
ring will be ig-
nored as long
as the timer
output stays
‘high’. The
timer output
also controls
the ‘on’ time of
the recorder and player. The ‘on’ time
can be set as per length of the message
to be recorded/played; say, two to three
minutes.
4. Simulate off-
hook state of the tele-
phone, which is ini-
tially in on-hook con-
dition.
The schematic dia-
gram incorporating
the control circuitry,
including power supply
and relays, is shown in
Fig. 2. The line dia-
gram, including all ac-
cessories used in the
system, is shown in
Fig. 3.
Normally, the tele-
phone lines (in on-
hook position of the
handset) carry 50V DC. However, dur-
ing ringing, the lines carry 133Hz,
80V AC (modulated pulses), as shown
in Fig 5.
Ring detection circuit comprises an
input sensing section followed by
monostable multivibrator and decade
counter. In the input sensing section,
capacitor C1 is used for DC blocking
while 1N4007 diode D1 is used to rec-
tify the AC ringing voltage. The poten-
tiometer formed by resistors R2 and
R3 (shunted by base-to-emitter resis-
S.C. DWIVEDI
Fig. 2: Circuit diagram of telephone recording/answering machine
Fig. 1: Block diagram of telephone recording/answering machine
195