In this project we can control any electrical appliances through mobile or landline from any part of the country. In this project one base unit is connected to the basic landline in parallel with the land line phone.. When we want to control any electrical appliances through outer phone then first we dial the home number, bell is ringing and after few bell phone is automatic on and switch on the base unit to operate. Now we press the single digital excess code, if the excess code is ok then unit give a tone pulse and switch on the base unit. Now again we press the switch on/off code to on/off any electrical appliances. With the help of this code unit is on and base unit give a acknowledge pulse for on and off separately.
Complete circuit is divided into three parts.
AUTOMATIC ON THROUGH OUTER PHONE
In dtmf decoder circuit we use ic 8870 ic. IC 8870 is a dtmf decoder ic. IC 8870 converts the dual tones to corresponding binary outputs.
DTMF SIGNALLING. Ac register signaling is used in dtmf telephones, here tones rather than make/break pulse are used fro dialing, each dialed digit is uniquely represented by a pair of sine waves tones. These tones ( one from low group for row and another from high group fro column) are sent to the exchange when a digit is dialed by pushing the key, these tone lies within the speech band of 300 to 3400 hz, and are chosen so as to minimize the possibility of any valid frequency pair existing in normal speech simultaneously. Actually, this minimisator is made possible by forming pairs with one tone from the higher group and the other from the lower of frequencies. A valid dtmf signal is the sum of two tones, one from a lower group ( 697-940 Hz) and the other from a a higher group ( 1209-1663 Hz). Each group contains four individual tones. This scheme allows 10 unique combinations. Ten of these code represent digits 1 through 9 and 0. . tones in DTMF dialing are so chose that none of the tones is harmonic of are other tone. Therefore is no change of distortion caused by harmonics.