Circuit Functioning

Fig.4-1 shows the schematic of the Caller-ID circuit. The circuit is built arount 89c51, MT8870 DTMF decoder, hook detector circuit and a Display module for its operation. The whole circuit is powered by the +5V DC provided by the 7805 regulator IC. The pin INT0 is dedicated to know the decoded digits are available from MT8870 and pin INT1 is dedicated for the on-hook and off-hook detection. port 1 is an bidirectional IO port, in which pins P1.0 – P1.3 are used to receive the decoded digits available at the Q1-Q4 of IC 8870 when STD goes high. The STD signal is inverted by the use of IC 7404 hex inverter.

When the circuit is connected to telephone line and powered, the microcontroller initializes all the variables and displays the message “Have a Nice Day!” at startup. Consider, the incoming call arrives. Now, the IC MT8870 decodes the DTMF tones into 4-bit code available from the exchange and its STD pin goes high to indicate the arrival of digits. the controller receives the binary code and converts into ASCII and displays in the display module until end of number arrives which is “FF". When the handset is lifted, the hook-detector pull downs the INT1 pin of the controller and now the controller falls into infinite loop for checking INT1 to go high. When the handset is placed back in on-hook position after conversation, the controller comes out of infinite loop and again the welcome message is displayed.

Same way when the user dials number in his phone, the controller receives and displays the numbers dialed by the user and again goes into infinite loop until on-hook and then displays the welcome message again.