DTMF decoder with voice!

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.
DTMF decoder with voice! - schematic

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 be stored....

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