The
given circuit, when connected in parallel to a telephone,
dis- plays the number dialled from the telephone set
using the DTMF mode. This circuit can also show the
number dialled from the phone of the called party. This
is particularly helpful for receiving any number over
the phone lines. The DTMF signal—generated by the phone
on dialling a number—is decoded by DTMF decoder CM8870P1
(IC1), which converts the received DTMF signal into
its equivalent BCD number that corresponds to the dialled
number. This binary number is stored sequentially in
10 latches each time a number is dialled from the phone.
The first number is stored in IC5A (1/2 of CD4508) while
the second number is stored in IC5B and so on. The binary
output from IC1 for digit ‘0’ as decoded by IC1 is 10102
(=1010), and this cannot be displayed by the seven-segment
decoder, IC10. Therefore the binary output of IC1 is
passed through a logic-circuit which converts an input
of ‘10102’ into ‘00002’ without affecting the inputs
‘1’ through ‘9’. This is accomplished by gates N13 through
N15 (IC11) and N1 (IC12). The storing of numbers in
respective latches is done by IC2 (4017). The data valid
output from pin 15 of IC1 is used to clock IC2. The
ten outputs of IC2 are sequentially connected to the
store and clear inputs of all the latches, except the
last one, where the clear input is tied to ground. When
an output pin of IC2 is high, the corresponding latch
is cleared of previous data and kept ready for storing
new data. Then, on clocking IC2, the same pin becomes
low and the data present at the inputs of that latch
at that instant gets stored and the next latch is cleared
and kept ready. The similar input and output pins of
all latches are connected together to form two separate
input and output buses. There is only one 7-segment
decoder/driver IC10 for all the ten displays. This not
only reduces size and cost but reduces power requirement
too. The output from a latch is available only when
its disable pins (3 and 15) are brought low. This is
done by IC3, IC12 and IC13. IC3 is clocked by an astable
multivibrator IC4 (555). IC3 also drives the displays
by switching corresponding transistors. When a latch
is enabled, its corresponding display is turned on and
the content of that latch, after decoding by IC10, gets
displayed in the corresponding display. For instance,
contents of IC5A are displayed on display ‘DIS1,’ that
of IC5B on ‘DIS2’ and so on. The system should be connected
to the telephone lines via a DPDT switch (not shown)
for manual switching, otherwise any circuit capable
of sensing handset’s off-hook condition and thereby
switching relays, etc. can be used for automatic switching.
The power-supply switch can also be replaced then. Though
this circuit is capable of showing a maximum of ten
digits, one can reduce the display digits as required.
For doing this, connect the reset pin of IC2, say, for
a 7-digit display, with S6 output at pin 5. The present
circuit can be built on a veroboard and housed in a
suitable box. The displays are common-cathode type.
To make the system compact, small, 7-segment displays
can be used but with some extra cost. Also, different
colour displays can be used for the first three or four
digits to separate the exchange code/STD code, etc.
The circuit can be suitably adopted for calling-line
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