DTMF Interface From Phone to Microcontroller

Developing a system to interface the telephone line with a microcontroller is the main purpose of this project which will involve the of DTMF tones.

The device was based on the ATmega8 microcontroller because it contains 512 bytes of EEPROM, ease of programming & implementation, 8Kb of programmable flash memory, and low cost. A 4-binary output characteristic of each tone will be provided by the MT8870 DTMF decoder and whenever a valid DTMF tone is received, a status signal will have logic high. In spite of the variations in the incoming voltage, a constant 5V is provided by the IC 7805 linear voltage regulator. A heatsink will be connected to each IC 7805 for heat dissipation.

To interface the CMD8870 to the microcontroller, an array of seven Darlington transistors ULN2003 is used. The current from the CMD8870 will be amplified by the BC108 NPN transistor which is used as a switch while the latching relay is operated. There are 10-pin contacts in the magnetic latching relay wherein 2 pins are used for latching and unlatching the relay. Even if the power is removed the relay will remain latched to reduce power requirements.

Rest of the project