The Talking Phone Dial Monitor detects the
tones generated when numbers are dialed
on your touch tone telephone and speaks
the numbers that were dialed. This verifies
that you dialed the correct number and is
especially useful when the keypad is not
well lighted or if the person dialing cannot
see the keypad well for any reason. The
numbers that are spoken can be prerecorded by speaking into a built-in
microphone and can be re-recorded at any
time if desired.
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This design makes use of an integrated
circuit made by Information Storage
Devices that stores spoken words in
memory cells as analog data. Voice can be
stored by speaking into an electret
microphone connected directly to the IC. It
also has a built-in audio amplifier that
directly drives a speaker. Figure 1 is a
schematic of the talking monitor. The
telephone line is connected to the circuits
through capacitor C1 and transformer T1
which isolate the line from the circuits. A
dual tone multi frequency (DTMF) decoder
IC1 decodes the tones generated by your
telephone when a number is dialed. The
decoded tones appear at the output pins of
IC1 as binary information which is fed into a
first in, first out (FIFO) memory circuit IC2.
Capacitor C1 must have a voltage rating of
250 volts to withstand the high voltage
produced by a ring signal. On the secondary
side of T1, zener diode D1 clamps any high
voltage spikes in both the positive and
negative direction that might otherwise
damage MC145436A touch tone decoder
IC1. Resistor R1 and 3.58 Mhz crystal XTAL
together with an on-chip oscillator generate
clock pulses necessary to the operation of
IC1. Data valid (DV) pin 12 goes high when
tones are decoded and valid binary data
appears at output pins 2, 1, 14 and 13.
DV connects to shift in (SI) pin 3 of the
IDT72401L10P FIFO IC2 and increments its
address pointer when new data is ready to
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