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Telephone
Number Display |
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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
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Click
Here For Circuit Diagram
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