Telephony Communication
DTMF decoder

This circuit detects the dial tone from a telephone line and decodes the keypad pressed on the remote telephone. two distinct frequency tones make up the DTMF(dual tome multiple frequency), and hence the name. The the dialer and the telephone exchange communicate one way through the DTMF communication.the tone generator and the tone decoder consist of a complete communication setup. In this project, IC MT8870DE is used to decode the input dial tone to the dialled numbers. These digital bits can be interface to a computer or microcontroller for further application.

Kit Background
pulse/dial tone method is used in the modern telephone communication. There is a a dial tone generator on the caller side and a unique tone consisting of two audible tone frequency is generated when the user presses a buton. As an example, 697hz & 1209hz will be generated when key '1' is being pressed on the phone. a decoder circuit is present on the reciever side, which decodes the tone to a digital code. For instance, the tone of 941hz + 1336hz will be decoded to 1010. This digital output will be read in by a computer and then the call will be accordingly routed to the dialled telephone number.

DTMF Circuits
The table in the doc file shows the key tones and the corresponding output digital sognal that will be generated. Detection of dial tones is reflected on the bit TOE, while the output Q4, Q3, Q2, Q1 indicate the dial tone that is being detected . The circuit is very straightforward and the components are readily available on the electronics shop. Telephone Ring Detector Circuit Interface to Microcontroller can also be built and the microcontroller can be used to display the number or any such function.
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