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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 display |
