" press 1 for balance
press 2 for internet plans
..."
Aren't these lines familiar? In fact they are. These are the lines of the automated voice which guides you when you call the service number of your mobile operator.
Ever it crossed your mind how they are able to know which button has been pressed?
It is DTMF(Dual Tone Multiple frequency) decoder IC which enables it to judge which button is pressed. The pressing of the button generates tone which is a combination of a high and a low frequency signal. This signal is received at the receiving or the far end where DTMF decoder ic interprets it and generates an encoded output which can be used to switch various appliances or carry out different operation using suitable decoder if required.
Here is the list of the frequency levels for different keys.
This project is a simple demonstration of the decoding feature of DTMF decoder IC CM8870 which is used to control the ON/OFF operation of a lamp remotely.
Components Required
- CM8870PI - DTMF IC
- 7805 - 5v voltage regulator
- Crystal 3.57Mhz
- Capacitor - .1uf(3nos)
- Resistor - 100k(2nos), 300k(1ns), 100ohm(2nos)
- Transistor- 2n2222
- A Cellphone
- 12V relay
Circuit Diagram:
Explanation:
7805 IC has been used to provide a regulated dc supply of 5V to the DTMF ic. Rest of the connection for single ended input connection can be found on the datasheet itself.
Input frequency required is giving through the audio jack of the cellphone. The audio cable has 3 wires- ground, left and right (in mobile earphones 4th wire is for mic). Use any one of the channel and the ground for input to the IC.The output is latched to Q1, Q2, Q3 and Q4 pins. In this project output is directly taken from Q1 which implies that it will turn on for all odd numbers and turn of for all even numbers pressed. The output is given to the base of the transistor configured in switching mode to turn the relay on and off.
I've always been curious about cellphone controlled electronics in the home. It would be most convenient with my disability to be able to turn the smoke alarm off if I accidentally smoke out the kitchen. I am way too short for waving a towel at the smoke alarm.
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