Specifications
CONSTRUCTION
transistor T1 is pulled ‘low’. This low-
going pulse is coupled to trigger pin 2
of timer NE555 (IC1) configured as
monostable (retriggerable). The output
pulse width of IC1 is given by the re-
lationship
Pulse width = 1.1 R4 x C3 … seconds
where R4 is in ohms and C3 in Farads.
With the component values shown,
the pulse width will be roughly 0.36
seconds. This will ensure that the mono
pulse does not end during the pause
period (0.2 sec.) between two successive
rings, so that only one pulse is gener-
ated at the output of IC1 for each pair
of ring signals separated by 0.2 sec-
onds.
The output of IC1 is coupled to
clock input pin 14 of the decade counter
IC 4017 (IC2), which is used for
counting the rings. From the decade
counter one can select any output from
Q0 through Q9. But in this project,
we take the output from Q3, which
goes high at the beginning of third ring
(so you will
hear only two
rings properly).
The Q3 out-
put at pin 7 of
IC2 is inverted
by N1 inverter
gate of IC3 to
trigger timer
IC4 (configured
as monostable),
whose pulse
width can be ad-
justed with the
help of preset
VR1. The pulse
width should be
so adjusted that
the tape player
could replay the
required mes-
sage and the re-
corder could
record the re-
sponse from the
far-end sub-
scriber within
the set pulse
width period. As
stated earlier,
timer IC4, when
triggered, ini-
tiates four dif-
ferent func-
tions. These are
tance of transistor T1) acts as voltage/
current limiting network for transistor
T1. During the positive incursions
of the ringing voltage at the base
of transistor T1, it is driven into satu-
ration. As a result, the collector of
Fig. 3: Line diagram of telephone recording/answering machine showing interconnection
amongst accessories used in the system
Fig. 4: Telephone circuit diagram using single Motorola IC MC34010/MC34011
196