This project describes the development of a system to interface the telephone line with a microcontroller. Using this interface any device can be controlled using a telephone or cell phone. The system involves the use of DTMF tones commonly used in telephone networks.
Dual-Tone Multi-Frequency (DTMF) signaling is used for telephone signaling over the line in the voice-Frequency band to the call switching center. These are the tones you hear when you dial a touch tone phone.
The standard DTMF keypad is laid out in a 4×4 matrix, with each row representing a low frequency, and each column representing a high frequency. Pressing a single key will send a sinusoidal tone of the two frequencies at the same time, the lower frequency for the row of the pressed key and the higher tone for the column. These tones are then decoded at the receiving end to determine which key was pressed. The use of two tones for a single key ensures that no two keys will generate the same signal. There are 16 possible tone sets but only 12 are used on common telephones.
The IC 7805 is a linear voltage regulator. It provides a constant +5 voltage in spite of the variations in the incoming voltage. Each IC 7805 is connected to a heat sink to dissipate heat.
The ULN 2003 is an array of seven Darlington transistors which are used to interface the CMD8870 to the microcontroller.
BC108 NPN transistor is used as a switch to amplify the current from the CMD8870 and operate the latching relay.
Magnetic Latching Relay
The Magnetic Latching Relay has 10-pin contacts and two separate pins for latching and unlatching the relay. The relay once latched will remain latched even if the power is removed thereby reducing the power requirements and provide and easy and effective way to cut-off the ignition.
Telephone line is connected to the MT88770 DTMF Decoder, the 4-bit decoded binary output is connected to ULN 2003 Bridge whose output is fed into the micro-controller. The status signal from MT8870 is used to trigger an interrupt whenever a new DTMF tone is detected. The microcontroller then reads the decoded output. Once the correct code is received, the microcontroller activates the latching relay via the BC108 transistor. Power for almost any appliance could be controlled this way.
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