THE SAID PROJECT IS DESIGNED ON THE PRINCIPAL OF DTMF DECODING.OUR MODERN TELEPHONE AND EVEN OUR MOBILE PHONE USES DTMF CODING FOR NUMBER DIALING. THESE DTMF CODES CAN BE DECODED AND UTILISED FOR USEFUL PURPOSES. THE CIRCUIT UTILISES IC8870 FOR DTMF DECODING  . MICROCONTROLLER 89C51 READS THESE CODES AND TAKES THE NECESSARY ACTION.

CIRCUIT DESCRIPTION: TELEPHONE LINE IS CONNECTED AT CONNECTOR K1. WHENEVER SOMEONE CALLS THIS NUMBER,  THAT TIME TELEPHONE LINE CARRIES ABOUT 70V RINGING SIGNAL  SIGNAL IS RECTIFIED BY THE BRIDGE D5-D8 AND  FILTERED BY C4, R20, AND D9 AND DRIVES AN OPTO COUPLER IC U8 MCT2E . COLLECTOR OF THE OPTOCOUPLER WILL GO LOW WHENEVER BELL RINGS. SINCE THE COLLECTOR IS CONNECTED TO P2.7 OF MICROCONTROLLER 89C51. NUMBER OF TIMES IT GOES LOW IS COUNTED BY MICROCONTROLLER. AFTER SIX TO EIGHT RINGS,P1.0 WILL GO HIGH AND SWITCH ON TRANSISTER T4 AND ENERGISE THE OFF HOOK RELAY RL1 . THE CONTECTS OF RELAY RL 1 WILL BECOME CLOSED AND THE TELEPHONE LINE WILL GET CONNECTED TO DTMF DECODER IC 8870, THROUGH  C7 AND C8. RESISTANCE R8 COMES ACROSS THE TELEPHONE LINE AND THAT LOWERS THE VOLTAGE,AND THIS ULTIMATELY SENDS MESSAGES TO EXCHANGE THE USER HAS LIFTED THE HAND SET. AT THE SAME TIME ,MICROCONTROLLER PIN P3.5 WILL TRIGGER THE VOICE IC API8208 WHERE ALL THE MESSAGES ARE STORED. CALLER WILL GET THE MESSAGE”PLEASE INTER THE PASSWARD”THE DEFAULT PASSWORD IS 1-2-3-4. WHEN CALLER  ENTERS THIS 4 DIGIT PASSWORD. IC 8870 DECODES THESE NUMBERS AND BINARY  NUMEBRS ARE AVAILABLE AT Q1 TO Q4 ,PIN NO 11-14  OF DTMF DECODER WHENEVER NEW CODE  COMES PIN NO 15( STB) OF DTMF DECODER WILL PULSE LOW, AND IS CONNECTED TO P1.1 INFORMING MICROCONTROLLER THAT CODES ARE AVAILABLE PLEASE TAKE THEM . MICROCONTROLLER WILL READS THESE CODES ONE BY ONE, THROUGH  ITS  LINE P1.2-P1.5. IF THE CODES MATCHES WITH THE CODES INSIDE THE MICROCONTROLLER, MICROCONTROLLER WILL SEND MESSAGE “ PLEASE ENTER CHANNEL NUMBER”. IF THE PASSWARD ENTERED IS NOT CORRECT. MICROCONTROLLER WILL DISCONNECT THE TELEPHONE LINE WITHOUT ANY REPLY, BY MAKING P1.0 LOW.

IF THE PASSWORD ENTERED IS CORRECT. YOU WILL GET MESSAGE “PLEASE ENTER CHANNEL NUMBER” NOW YOU HAVE TWO CHOICES

1.      YOU CAN GO TO NORMAL ON /OFF MODE

2.      YOU CAN CHANGE YOUR PASSWORD 

NORMAL ON/ OFF MODE   PRESS STAR AND 1( * & 1) NOT TOGETHER ONE BY ONE AFTER THAT PRESS 1&1 TO SWITCH ON IST RELAY.SIMILARLY

ALL RELAYS CAN BE SWITCHED ON AS FOLLOWS

             * & 1   NORMAL ON /OFF MODE

               1& 1   TO SWITCH ON IST RELAY

                2&1  TO SWITCH ON 2^ND RELAY

                7&1  TO SWITCH ON SEVENTH RELAY

                9 & 1 TO SWITCH ON ALL RELAYS

EVERY TIME YOU WILL RECEIVE MESSAGE” REQUESTED CHANNEL IS ON “PRESS CHANELL NO AND 0 SAY  1&0 WILL SWITCH OFF IST RELAY,2&0 WILL SWITCH OFF 2^ND RELAY. 9&0 WILL SWITCH OFF ALL RELAYSYOU WILL RECEIVE “REQUESTED CHANNEL IS OFF’ MESSAGE .IF YOU WANT TO KNOW THE STATUS OF THE PREVIOUSLY ON/ OFF RELAYS.PRESS CHANNEL NUMBER AND 2, SAY 2&2 WILL MESSAGE WHEATHER THE CHANNEL IS ON OR OFF  .IF YOU WANT TO CHANGE THE PASSWORD FOLLOW THESE STEPS

 DIAL THIS NUMBER FROM  OTHER LINE AFTER SIX TO EIGHT  BELLS YOU WILL RECEIVE MESSAGE ” PLEASE ENTER YOUR PASSWORD”. (1,2,3,4)SO YOU ENTER YOUR PASSWORD AND WAIT FOR THE MESSAGE “ PLEASE ENTER THE CHANNEL NUMBER” NOW1.PRESS  * & 2 ( FIRST STAR AND THEN TWO).YOU WILL RECEIVE MESSAGE(“PLEASE ENTER YOUR PASSWORD”.SO YOU MUST ENTER YOUR OLD PASSWORD AGAIN.SAY(1,2,3,4)AFTER THAT YOU WILL RECEIVE MESSAGE ”PLEASE ENTER NEW PASSWORD” SAY YOU ENTERED 4,5,6,7, YOU WILL RECEIVE MESSAGE “YOUR PASSWORD HAS BEEN CHANGED’’ KEEP THE HENDSET BACK ON THE INSTRUMENT FROM WHERE YOU ARE CALLING.NEXT TIME WHEN YOU DIAL ,YOU WILL HAVE TO ENTER YOUR NEW PASSWORD. NEW PASSWORD WILL STAY AS LONG AS POWER IS NOT INTERRUPTED.IF POWER FAILS IT WILL REVERT TO DEFAULT PASSWORD 1,2,3,4,