4. DTMF stands for Dual Tone - Multi Frequency and it is the basis for our telephone system. The touch-tone® phone is technically a DTMF generator that produces DTMF tones as we press it. How does it work?  When we press the buttons on the keypad, a connection is made that generates two tones at the same time.  A "Row" tone and a "Column" tone. These two tones identify the key we pressed to any equipment we are controlling.  If we are using a DTMF keypad to remotely control equipment, the tones can identify what unit we want to control, as well as which unique function we want it to perform.

5. 123697 Hz 456770 Hz 789852 Hz *0#941 Hz 1209 Hz1336 Hz1477 Hz When we press the digit 1 on the keypad, we generate the tones 1209 Hz and 697 Hz. Pressing the digit 2 will generate the tones 1336 Hz and 697 Hz. Sure, the tone 697 is the same for both digits, but it take two tones to make a digit and the decoding equipment knows the difference between the 1209 Hz that would complete the digit 1, and a 1336 Hz that completes a digit 2.

11. The implementation of DTMF decoder involves the detection of the DTMF tones. The integrated DTMF decoder is the device that receives the incoming DTMF data and converts it into a respective 4-bit binary coded decimal (BCD) numbers. The CM8870 is a full DTMF receiver that integrates both band split filter and decoder functions into a single 18-pin DIP. Its filter section uses switched capacitor technology for both the high and low group filters and for dial tone rejection.

13. The latch 74LS74 is essentially a 14 pin IC whose internal connections are shown. These devices contain two independent D-type positive-edge-triggered flip- flops. A low level at the preset or clear inputs sets or resets the outputs regardless of the levels of the other inputs. When preset and clear are inactive (high), data at the D input meeting the setup time requirements are transferred to the outputs on the positive-going edge of the clock pulse.

14. BC547 is an NPN bi-polar junction transistor. A transistor is commonly used to amplify current. A small current at its base controls a larger current at collector & emitter terminals. BC547 is mainly used for amplification and switching purposes. It has a maximum current gain of 800. Its equivalent transistors are BC548 and BC549. The transistor terminals require a fixed DC voltage to operate in the desired region of its characteristic curves. This is known as the biasing. For amplification applications, the transistor is biased such that it is partly on for all input conditions. The input signal at base is amplified and taken at the emitter. BC547 is used in common emitter configuration for amplifiers. The voltage divider is the commonly used biasing mode. For switching applications, transistor is biased so that it remains fully on if there is a signal at its base. In the absence of base signal, it gets completely off.

15. Relays are switches, the terminology applied to switches is also applied to relays; a relay switches one or more poles, each of whose contacts can be thrown by energizing the coil. Normally-open (NO) contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. Normally-closed (NC) contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. Change-over (CO), or double-throw (DT), contacts control two circuits: one normally-open contact and one normally-closed contact with a common terminal. SPDT – Single Pole Double Throw. A common terminal connects to either of two others. Including two for the coil, such a relay has five terminals in total.

21. In making the circuit as the device is connected to a corresponding circuit, it gets activated as soon as the call is made to the circuit in which mobile or equivalent GSM module is present to perform a specific operation as per the relay device is connected. In these as the keypad has 12 different key, it can perform 12 distinct operation as per connected to a relay. It can also made off by pressing 8 The cell phone-based device control with voice acknowledgement is an excellent device to operate any electronic equipment from miles away as the mobile technology is becoming advanced day by day; it is used for much other application as device control. As mobile service is used by everyone these days, this system will be very much useful in rural areas as well the device control can be applied in every field like agriculture, home, factories etc. The use of mobile communication in device control has been thoroughly justified and the previous drawbacks and problems have been overcome.

22. http://www.studymode.com/essays/Dtmf-Remote- Appliance-Control-System-Using-896207.html www.wikipedia.org http://seminarprojects.com/Thread-home-appliance- control-by-mobile-phone-dtmf#ixzz3Vz7KVn6m http://seminarprojects.com/Thread-home-appliance- control-by-mobile-phone-dtmf#ixzz3Vz7KVn6m Electronics for you http://electronicsforu.com/electronicsforu/circuitarchives Principles of Electronics by V.K.Mehta Electronics Materials and Components by Madhuri.A.Joshi Linear integrated circuit by K.R.Borkar